W U Prénom

Fernandez-Fuentes N O'donohue

Bryant D Fernandez- Fuentes

N O'donohue

Evelyne Forano

Harry J Gilbert

Nathalie Lisa

Yvonne Coralline

Julien, Louise, Eleni Pablo Barbara

Alex Yanick

Characterization of an unusual GH10 xylanase from termite gut

Keywords: Xylanase, GH10, CBM, MST, XUS, Biomasse lignocellulosique, Bacteroidetes, métagénomique, bioeconomie Xylanase, GH10, CBM, MST, XUS, lignocellulosic biomass, Bacteroidetes, metagenomic, bioeconomy Microscale thermophoresis (MST), binding studies, fluorescence quenching, fluorescent label, Kd (dissociation constant)

RESUME

Dans le contexte de la bioéconomie, la découverte et la caractérisation des enzymes capables de dégrader la paroi végétale est particulièrement intéressante pour l'utilisation de la biomasse lignocellulosique dans l'industrie. A cet égard, la métagénomique fonctionnelle est un outil puissant pour découvrir de nouvelles enzymes à partir d'écosystèmes microbiens variés, comme l'illustrent les travaux sur le tube digestif du termite Pseudacanthotermes militaris. Cette étude a fourni une mine d'informations et identifié un hypothétique locus d'utilisation du xylane (XUS), codant pour cinq glycosides hydrolases (GH) et une carbohydrate esterase (CE) de Bacteroidales.

Le XUS du métagénome de Pseudacanthotermes militaris contient une xylanase de la famille GH10 qui possède une organisation modulaire complexe dans laquelle la séquence du domaine GH10 est interrompue par une insertion de deux carbohydrate binding modules (CBM). Des travaux préliminaires ont montré que cette enzyme modulaire, désignée Pm25, est active sur xylane. Par conséquent, un des objectifs de cette étude a été la caractérisation détaillée des propriétés biochimiques et catalytiques de Pm25. Le rôle des CBM a également été examiné en quantifiant les interactions protéines-sucres et permettant ainsi une meilleure compréhension du rôle spécifique de ces modules, les résultats obtenus permettent de cerner l'impact de la modularité de Pm25 sur ses propriétés fonctionnelles.

Dans une deuxième partie de l'étude, nous avons entrepris d'étudier la fonction de Pm25 dans le contexte du cluster XUS. Pour ce faire, nous avons étudié les enzymes adjacentes à Pm25 sur le locus, une autre GH10, une GH11, une GH115 et une GH43. La comparaison des paramètres cinétiques et une étude détaillée des produits d'hydrolyse ont été analysés par spectrométrie de masse et ont révélé que la GH10 et la GH11 étaient les enzymes clefs de la dépolymérisation en étant 20 fois plus efficaces que Pm25.

En parallèle, nous avons développé un protocole pour l'utilisation de la micro-thermophorèse (MST) pour quantifier les interactions CBM-sucres, une approche intéressante qui nécessite peut d'échantillon et de ligand contrairement à d'autres méthodes biophysiques.

Dans l'ensemble, cette étude a révélé le rôle important de Pm25 et ses homologues dans les locus d'utilisation des xylanes chez les Bacteroidetes et a permis d'identifié le sens de cette architecture particulière.

LIST OF FIGURES RESUME EN FRANCAIS

LIST OF TABLES BIBLIOGRAPHY

Table A-1. Occurrence of hemicelluloses in primary and secondary cell walls of plants ..... Table A-2 S3. Relative activity (%) of Pm25 and mutants thereof towards polysaccharides. ...

ARTICLE II

Table 1. Biochemical properties and kinetic parameters of Pm12, Pm24 and Pm25. .........

ABBREVIATIONS

Introduction

La découverte et l'optimisation de biocatalyseurs constitue un levier pour le développement de la bioéconomie et particulièrement dans le domaine de la valorisation des agro-ressources dont les polysaccharides de la paroi végétale constituent la ressource de carbone la plus abondante sur terre. La métagénomique est une technique puissante pour explorer l'ensemble des génomes d'un écosystème en s'affranchissant des conditions de cultures des microorganismes [START_REF] Handelsman | Metagenomics : Application of Genomics to Uncultured Microorganisms[END_REF]Madhavan et al., 2017), cette technique a permis un accroissement considérable des séquences disponibles dans les bases de données mais ne garantit pas l'activité des enzymes annotées. La métagénomique fonctionnelle est basée non pas sur l'exploration des séquences des génomes mais sur l'activité des enzymes codées par ces métagénomes et a permis de découvrir de nouvelles enzymes (Bastien et al., 2013;[START_REF] Fernández-Arrojo | Metagenomic era for biocatalyst identification[END_REF]Tasse et al., 2010).

Précédemment au laboratoire une campagne de métagénomique fonctionnelle du métagénome du tube digestif du termite champignonniste Pseudacanthotermes militaris a permis de découvrir 63 nouvelles enzymes dédiées à la modification des sucres (Bastien et al., 2013). Un fragment génomique a été identifié lors du criblage sur xylane, l'annotation de ce fragment a montré qu'il contient plusieurs gènes juxtaposés codant pour des activités xylanolytiques potentielles et appartenant aux familles de glycoside hydrolases GH10, GH11, GH43, GH115 et d'une carbohydrate estérase CE1. L'analyse bio-informatique a montré ces enzymes pourraient faire partie d'un locus d'utilisation des polysaccharides, PUL, puisque les GH sont codées après une paire de gène sucC/sucD rencontrés spécifiquement dans les PUL. De manière générale SusC/SusD sont responsables de la reconnaissance et de l'internalisation des substrats partiellement hydrolysés par les GH codées par les PUL, ce système constitue un paradigme pour l'hydrolyse des polysaccharides végétaux chez les bactéries anaérobies de l'embranchement des Bacteroidetes (Martens et al., 2009) initialement décrit pour l'hydrolyse de l'amidon, il a été étendu aux xylanes on parle alors du couple XusC/XusD (xylan utilisation system). La caractérisation des enzymes du PUL Pm25 (GH10), Pm115 (GH115), Pm12 (GH10), Pm24 (GH11) et Pm21 (GH43) (Fig I -A) et plus particulièrement Pm25, a constitué le coeur de cette thèse.

Pm25 possède une activité xylanase (thèse de G. Arnal), serait une xylanase multi modulaire constituée d'un domaine catalytique de la famille GH10 et de deux domaines non catalytiques ou CBM pour carbohydrate binding domain de la famille 4 de la base de données CAZy (Lombard et al., 2014). La particularité de cette enzyme provient de son organisation puisque les deux CBM4 sont insérés dans la séquence primaire du domaine catalytique GH10. Cette architecture a retenu notre attention et la caractérisation de cette enzyme constitue la première partie du travail présenté dans cette thèse. Les autres glycosides hydrolases codées par ce cluster de gène sont hypothétiquement actives pour la dégradation des xylanes (Fig I -A) soit en dépolymérisant la chaine principale, activité xylanase, soit en hydrolysant les substitutions de cette même chaine, xylosidase, glucuronidase. La caractérisation des enzymes du PUL et les synergies intra et inter-moléculaires sont étudiées dans le chapitre II. Et enfin, microthermophorèse a été utilisée pour quantifier l'affinité du CBM4 de Pm25, cette mise au point a permis la rédaction d'un chapitre dans Methods in Molecular Biology (Wu et al., 2017) qui est présenté dans le chapitre III.

Fig I Présentation schématique et réseaux de séquences similaires (SSN).

(A) Représentation schématique du cluster de gène xylanolytique. (B) Alignement de la séquence de Pm25 avec les séquences des xylanases GH10 caractérisées et dont la structure est connue (identifiées par leur code PDB). (C) Réseau de séquences similaires construit avec 5068 séquences de GH10 non redondantes. Chaque noeud représente des séquences avec 60% d'homologie et les segments reliant deux noeuds représentent les séquences partageant au moins 35% d'homologie.

Part A-Caractérisation d'une xylanase multi modulaire de la famille GH10 issue d'une banque metagénomique du tube digestif de termite.

L'analyse in silico de la séquence multi modulaire de Pm25 a permis d'identifier les frontières des différents modules. La séquence est composée d'un peptide signal (residus 1-32), suivi d'une première partie du domaine GH10 (residus 66-151), de deux CBM4s (CBM4-1, résidus 161 to 321 and CBM4-2, résidus 324 to 486) et terminant par la deuxième partie du domaine catalytique GH10 (résidus 514-753). L'alignement de Pm25 avec les séquences de structure connues montre que l'insertion des CBM4 se situe entre le troisième feuillet beta et la troisième hélice alpha (Fig I -B), ce qui est en accord avec les homologues de Pm25, aussi dénommés XynC-like, décrits dans la littérature (Flint et al., 1997).

Dans un premier temps, une analyse des réseaux d'homologie de séquences, sequence similarity network, SSN, a été réalisé afin de savoir si les homologues de Pm25 parmi la famille des GH10 partageaient les mêmes propriétés. Cette approche permet de facilement représenter des milliers de séquences de façon bien plus lisible qu'un arbre phylogénétique (Gerlt et al., 2015). L'analyse SSN réalisée à partir des 5068 séquences de GH10 non redondantes a montré que toutes les séquences analogues à Pm25 se retrouvent dans le même groupe composé uniquement de séquences de Bacteroidetes (cluster 4, Fig I -C) et ceci est vérifié si les séquences des CBM4 sont retirés de la séquence totale des GH10 indiquant que ce regroupement ne vient pas uniquement de la présence des CBMs. Il est ressorti que toutes les protéines homologues à Pm25 sont codées après des paires susC/susD avec quelques exceptions (4 sur 79), ce qui est en accord avec son implication dans un locus d'utilisation des xylans chez les Bacteroidetes (Dodd et al., 2011), on parle alors de XUS. Afin d'étudier la fonction des différents domaines de Pm25, différentes mutations ou troncatures ont été réalisées et les protéines exprimées (Fig II). La protéine Pm25 a été caractérisée biochimiquement sur différents arabinoxylanes, et l'affinité pour le substrat grâce à un mutant inactif (M1) ou de la protéine sans CBM (M2) a permis de montrer que l'affinité est guidée par les CBM (M3). (Fig IIA, milieu) (A) Gauche: représentation schématique des différentes constructions dérivées de Pm25 avec leurs constantes de dissociations (kd, mg/ml) sur l'arabinoxylane de blé (WAX) et le xylane de hêtre (BX) (panneau milieu). Trx, étiquette thioredoxine. Droite: Affinité sur substrat insoluble des constructions avec et sans CBM (M1/M2) sur son de blé. L'albumine de sérum bovin (BSA) a été utilisée comme control négatif. Fraction non-fixée (ligne 1), lavage (ligne 2) et fraction fixée (ligne 3). Ligne M, marqueur de masse moléculaire. (B) Profil des produits finaux après 14h d'hydrolyse du son de blé (à gauche). X1, xylose ; X2, xylobiose ; X3, xylotriose, X4, xylotetraose. *, p ≤ 0.01.

Part B-Le role des CBMs de Pm25 en présence des enzymes du PUL.

Trois types de synergies peuvent être étudiés lorsque l'on s'intéresse à l'hydrolyse des polysaccharides: la synergie intermoléculaire entre des enzymes aux activités différentes (endo/exo-glucanases), ou entre des activités du meme types (exo/exo-glucanases ou endo/endo-glucanase), et la synergy intramoléculaire entre les différents domaines au sein d'une protéine multi modulaire tel que CBM/ domaine catalytique. Différentes synergies ont pu être observés pour différentes combinaisons ou orientations des CBM et de leur domaines catalytiques [START_REF] Meng | Distinct Roles for Carbohydrate-Binding Modules of Glycoside 10 (GH10) and GH11 Xylanases from Caldicellulosiruptor sp. Strain F32 in Thermostability and Catalytic Efficiency[END_REF]) [START_REF] Tajwar | Impact of orientation of carbohydrate binding modules family 22 and 6 on the catalytic activity of Thermotoga maritima xylanase XynB[END_REF]. Les CBM sont retrouvés couplés à des GH, GT et des toxines et peuvent jouer différents rôles (Boraston et al., 2004;[START_REF] Gilbert | Advances in understanding the molecular basis of plant cell wall polysaccharide recognition by carbohydrate-binding modules[END_REF]Guillén et al., 2010). La fonction la plus décrite des CBM est d'améliorer l'activité des enzymes auxquelles ils sont rattachés en permettant une plus grande proximité entre le l'enzyme et son substrat.

Cependant, dans certains cas, aucun effet du CBM sur l'activité du domaine catalytique n'a pu être observé (Ali et al., 2001;Black et al., 1996;Kellett et al., 1990;Valenzuela et al., 2012;Zhao et al., 2005). Certaines études sont basées sur des substrats modèles, de petite taille comme des oligosaccharides d'un degré de polymérisation, DP, inférieur à sept, ce qui n'est peut-être pas suffisant pour complètement invalidé la contribution du CBM. Au cours de ce deuxième chapitre l'activité des différentes constructions de Pm25 avec ou sans CBM ont été comparées en analysant les produits d'hydrolyse du xylane de hêtre. Les xylo-oligosaccharides XOS ont été analysés par HPLC pour les XOS DP1 à 6 et par spectrométrie de masse pour les XOS de DP 7 à 28. Afin de caractériser la synergie intermoléculaires entre les enzymes du PUL, les enzymes Pm12, Pm21, Pm24, Pm25 et Pm115 ont été produites et caractérisées. Le xylane de hêtre a été hydrolysé avec différents cocktails des enzymes du PUL et les produits d'hydrolyse analysés.

La complémentarité des enzymes du PUL a pu être mise en évidence, néanmoins il n'y a pas de synergie notable. Les résultats montrent une faible contribution des CBM sur le substrat soluble suggérant que les CBM ont peut-être un rôle plus important sur une biomasse complexe.

Part C-Quantification des intéractions CBM-sucre par microthermophorèse.

Les interactions protéinessucre peuvent être caractérisées par différentes techniques telles que les gels d'affinités par électrophorèse (AGE), la microcalorimétrie (ITC) qui renseigne sur les paramètres thermodynamiques de l'interaction ou la spectroscopie de fluorescence, basées sur des interactions avec des substrats solubles, [START_REF] Abbott | Quantitative approaches to the analysis of carbohydrate-binding module function[END_REF]. La microthermophorèse (MST) est une technologie émergeante pour étudier de nombreuses interactions moléculaires avec une grande sensibilité. Parmi les avantages on peut citer les volumes de réactions à l'échelle du microlitre ainsi que la rapidité du test de quelques minutes.

Cette technique a été utilisée pour quantifier l'affinité du CBM3a de Clostridium Thermocellum pour des nano-cristaux de cellulose et du CBM4 de Pm25 pour le xylohexaose.

Apres le marquage et la purification des protéines l'expérience a été optimisée pour les paramètres de mesures et de traitement des données. Une constante de dissociation apparente a pu être déterminée pour le CBM3a et les nano-cristaux de cellulose (0.24 ± 0.05 g/L).

Cependant, du quenching de fluorescence a pu être observé pour le CBM4, ceci a pu être expliqué par la présence d'une lysine potentiellement marquée à proximité du site de fixation du ligand. Dans ce cas, la constante de dissociation du CBM4 pour le xylohexaose a pu être déterminée (2.5 ± 0.3 mM) par la mesure de la fluorescence sans utiliser la thermophorèse démontrant ainsi l'utilité de cette technique pour quantifier les intéractions CBM-sucres.

Référence:

Ali, M.K., Hayashi, H., Karita, S., Goto, M., Kimura, T., Sakka, K., and Ohmiya, K. (2001).

Importance of the carbohydrate-binding module of Clostridium stercorarium Xyn10B to xylan hydrolysis. Biosci. Biotechnol. Biochem. 65, 41-47.

Bastien, G.G., Arnal, G.G., Bozonnet, S., Laguerre, S., Ferreira, F., Fauré, R., Henrissat, B., Lefèvre, F., Robe, P., Bouchez, O., et al. (2013). Mining for hemicellulases in the fungus-

PART A COMPOSITION AND STRUCTURE OF PLANT CELL WALLS (PCWS)

Unlike animal cells, plant cells are surrounded by a plant cell walls (PCWs) that are extracellular composite structures that contain polysaccharides as well as other chemical components. Primarily, PCWs serve as a physical barrier that defends the protoplast from mechanical injuries, osmotic shock and ultraviolet irradiation [START_REF] Katō | Ultrastructure of the Plant Cell Wall: Biochemical Viewpoint[END_REF]. Secondly, like scaffolding, PCWs buttress the cell and act as transmitters of signals following microbial assault [START_REF] Sarkar | Plant cell walls throughout evolution: Towards a molecular understanding of their design principles[END_REF][START_REF] Somerville | Toward a systems approach to understanding plant cell walls[END_REF]. Furthermore, PCWs are of significant economic importance, because it is widely recognized that they constitute the most abundant reservoir of organic carbon on the planet [START_REF] Okkerse | From fossil to green[END_REF]. Finally, PCWs are also of nutritional importance, because they constitute a major source of dietary fiber [START_REF] Selvendran | The plant cell wall as a source of dietary fiber: chemistry and structure[END_REF].

While evolution has conferred many properties to PCWs that are beneficial to the plant, these properties are not always advantageous regarding the economic exploitation of PCWs.

For example, in a biorefinery context, the natural resilience of PCWs towards abiotic and biotic aggressions means that a lot of energy and complex operations are required to extract renewable carbon from PCWs. The overall resistance of PCWs to processing is usually referred to as biomass "recalcitrance" (DeMartini et al., 2013a;[START_REF] Mccann | Biomass recalcitrance: A multi-scale, multifactor, and conversion-specific property[END_REF]. This is a composite property that arises from many factors including the chemical and structural complexity of PCWs.

A.1 Composition of plant cell walls

The chemical composition of PCW is heterogeneous, varying with the developmental stages, tissue types and plant species, which reflect the different needs of plants [START_REF] Somerville | Toward a systems approach to understanding plant cell walls[END_REF]. In general, in lignocellulosic biomass, cellulose, hemicellulose and lignin constitute 30-40%, 20-30%, and 10-20% of the dry weight of PCWs, respectively [START_REF] Davison | Plant Cell Walls: Basics of Structure, Chemistry, Accessibility and the Influence on Conversion[END_REF]Varner et al., 1989;White et al., 2014) (FigA-1and 2). Compositional variation is often linked to different species, for example with lignin being more abundant in softwood [START_REF] Stephen | Other Plant Polysaccharides[END_REF].

A.1.1 Cellulose

In 1838, Anselme Payen discovered and isolated cellulose from green plants (Payen, 1838).

Cellulose is the main constituent of plant cell walls, but is also distributed in bacteria, fungi, algae and even in animals, although the amount of cellulose in the latter e.g. in tunicates is negligible when compared with plants (O'sullivan, 1997). Cellulose is used as an important raw material for paper products, fibers, and biofuels.

Cellulose is a high molecular polymer whose chain is constituted of D-glucose linked by β- ,4-glycosidic bonds (McKendry, 2002). Natural cellulose can have degrees of polymerization (DPs) in the range 5,000 to 15,000 glucose monomer units (O'sullivan, 1997).

1
It is present in PCW as fibrils (about 500 nm in diameter, [START_REF] Chen | Chemical Composition and Structure of Natural Lignocellulose[END_REF])), which are composed of very fine threads (or ribbons) called microfibrils containing approximately 10 3 chains molecular chains of (1→4)-β-D-glucan ranged in parallel, which may vary from 10-25 nm in breadth and can be more than 7 µm long (14,000 D-glulcose units) depending on the species [START_REF] Gibson | The hierarchical structure and mechanics of plant materials[END_REF][START_REF] Somerville | Toward a systems approach to understanding plant cell walls[END_REF][START_REF] Tanner | Plant Carbohydrates II : Extracellular Carbohydrates[END_REF](Fig A -2A). Glucan chains in the microfibrils interact with each other via hydrogen bonds and can form amorphous (irregularly arranged) or crystalline regions (orderly arranged), which when observed by wide-angle X-ray scattering appear as dark and light areas respectively [START_REF] O'sullivan | Cellulose: the structure slowly unravels[END_REF][START_REF] Tanner | Plant Carbohydrates II : Extracellular Carbohydrates[END_REF]. Overall, the properties of cellulose make it play a central structural role in plants [START_REF] Lodish | The Dynamic Plant Cell Wall[END_REF].

A.1.2 Hemicelluloses (Xylan)

The term hemicelluloses was coined by E. Schulze in 1891. This term includes all PCW components that are fairly amenable to extraction and hydrolysis [START_REF] Preece | Studies on hemicelluloses[END_REF].

Hemicelluloses cover a quite large, diverse family of (usually) heterogenous polymers that are nevertheless all based on a mainchain composed of β-linked D-sugars. In general, hemicelluloses are amorphous, branched, single-chain polysaccharides displaying an average length of about 30 nm [START_REF] Maureen | Complexity in the spatial localization and length distribution of plant cell-wall matrix polysaccharides[END_REF]. Hemicelluloses mainchains can contain pentose (D-xylose, L-arabinose), hexoses (D-mannose, D-glucose and D-galactose) and be decorated by a wide variety of sidechain substitutions, including uronic acids (α-D-glucuronic, α-D-4-Omethyl-galacturonic and α-D-galacturonic acids) and L-arabionfuranose (Fig A -2B). D-pentose sugars is more prevalent than D-hexose, while the latter is typically found in softwoods [START_REF] Davison | Plant Cell Walls: Basics of Structure, Chemistry, Accessibility and the Influence on Conversion[END_REF] Xylan is widely distributed among cereal grains and hardwoods, and in lesser quantities in softwood [START_REF] Stephen | Other Plant Polysaccharides[END_REF]. It accounts for one third of renewable organic carbon available on Earth and constitutes the major component of hemicelluloses [START_REF] Prade | Xylanases : from Biology to BioTechnology[END_REF]. The occurrence of homoxylan in higher plants is rather rare [START_REF] Ebringerová | Xylan and xylan derivatives -Biopolymers with valuable properties, 1: Naturally occurring xylans structures, isolation procedures and properties[END_REF], with the majority of xylans being heteroxylans. Heteroxylans (hereafter referred to as xylans) of all higher plants are a diverse group of polysaccharides that shares as a common feature a backbone composed of β-1,4-linked D-xylosyl subunits that can be chemically modified (acetylation) and substituted at different position with various glycosyl residues [START_REF] Aspinall | Constitution of Plant Cell Wall Polysaccharides[END_REF]. The decoration pattern correlates with taxonomy. Acetylation at C-3 and/or C-2 of xylosyl residues occurs in most xylans (especially in dicots), and the acetyl groups are numerous enough to avoid molecular aggregation [START_REF] Katō | Ultrastructure of the Plant Cell Wall: Biochemical Viewpoint[END_REF]. Glucuronoxylans (GX), substituted with α-1,2-linked 4-O-methyl glucuronosyl residues are often found in dicots, while glucuronoarabinoxylans (GAX), displaying decorations at O-2 and/or O-3 with Larabinofuranose are mainly found in monocots [START_REF] Dumon | Progress and future prospects for pentose-specific biocatalysts in biorefining[END_REF]. In grass arabinoxylans, xylosyl residues are mostly substituted at O-3, while dicot arabinoxylans substitutions mostly occur at the O-2 position (Scheller and Ulvskov, 2010). Xylan in commelinid monocots is also characterized by the presence of ferulic acid esters, which are attached to the O-5 of some of the L-arabinosyl groups. This provides the basis for the presence of ferulate dehydrodimers that help with intra-and intermolecular linkages [START_REF] Grabber | Cross-linking of maize walls by ferulate dimerization and incorporation into lignin[END_REF]Scheller and Ulvskov, 2010). Recent studies revealed several precise substitution pattern rules in xylan.

For example, in gymnosperm xylan, every six xylosyl residues there is one methylated glucuronosyl and one arabinofuranosyl decoration, which are located precisely at the sixth and fourth xylosyl residue, respectively [START_REF] Busse-Wicher | Evolution of xylan substitution patterns in gymnosperms and angiosperms: implications for xylan interaction with cellulose[END_REF].

A.1.3 Lignins

Lignins, the major non-polysaccharide polymeric components of PCWs, are generally thought to be amorphous and display high molecular weight. Lignins are a combination of three aromatic subunits, p-hydroxylphenyl (H), guaiacyl (G) and syringyl (S) respectively [START_REF] Reale | Mass spectrometry in the biosynthetic and structural investigation of lignins[END_REF])(Fig A -2C) that are derived from p-coumaryl, coniferyl, and sinapyl alcohol respectively, which in turn are derived (in biosynthesis) from phenylalanine or tyrosine (Varner et al., 1989). Regarding the native in muro structure of lignin, this remains elusive, because all studies of lignins require their extraction and thus modification [START_REF] Buranov | Lignin in straw of herbaceous crops[END_REF][START_REF] Howard | Lignocellulose biotechnology: issues of bioconversion and enzyme production[END_REF][START_REF] You | Applications of of Molecular Methods to Applications to the Elucidation of Lignin Structure the Elucidation of Lignin Structure[END_REF]. However, a recent article has suggested that in muro lignins might not after all be high molecular weight polymers. Instead the authors propose that lignins are actually composed of linear oligomers of variable length that are covalently linked to cellulose and hemicellulose fibers [START_REF] Banoub | A critique on the structural analysis of lignins and application of novel tandem mass spectrometric strategies to determine lignin sequencing[END_REF].

Nevertheless, the major part of our current knowledge being based on the analysis of extracted lignins, the following information summarizes the state of the art.

The average degree of polymerization of lignins is around 13 to 20 (Vanholme et al., 2010).

The composition of lignins varies among taxa. For example, softwood is predominantly made up of G units, whereas that of hardwood is abundant in both G and S units, which is also why the former is more resistant to delignification than the latter [START_REF] Baucher | Lignin: genetic engineering and impact on pulping[END_REF][START_REF] Kishimoto | Influence of syringyl to guaiacyl ratio on the structure of natural and synthetic lignins[END_REF]. Various intermonomeric linkages have been found in lignins, such as ether and carbon-carbon bonds [START_REF] Baucher | Lignin: genetic engineering and impact on pulping[END_REF]. Regarding the role of lignins in PCWs, lignins confer stiffness, strength and hydrophobicity, the latter facilitating water and soluble nutrient transport as well as providing resistance to abiotic and biotic aggressions [START_REF] Boerjan | Lignin biosynthesis[END_REF]Vanholme et al., 2010;[START_REF] Wyman | Bioethanol from Lignocellulosic Biomass[END_REF]. Overall, it is widely considered that lignins constitute one of the major factors responsible for biomass recalcitrance and thus the economically-viable exploitation of biomass for industrial purposes.

A.1.4 Pectin

Pectin was originally identified in apples (in 1790 ) by the French chemist Louis Nicolas Vauquelin, although it was not until 1937 that Schneider and Bock established the basic formula of pectin (Mrak et al., 1962). Pectins are also complex polysaccharides, but compared to hemicelluloses they are more easily solubilized in hot water and dilute acid solutions, partly because their major components are 1,4 -linked α-D-galacturonic acid (GalpA) residues [START_REF] Cosgrove | Growth of the plant cell wall[END_REF]. The structural classes of the pectic polysaccharides include homogalacturonan (HG, ~65%), rhamnogalacturonan I (RG1, ~25%), rhamnogalacturonan II (RG2, ~10%), xylogalacturonan and others [START_REF] Mohnen | Pectin structure and biosynthesis[END_REF][START_REF] Ridley | Pectins: Structure, biosynthesis, and oligogalacturonide-related signaling[END_REF]

(Fig A-2D).
Pectin is thought to be one of the most complex polysaccharide in biosphere [START_REF] Cosgrove | Growth of the plant cell wall[END_REF], which implied its multiplicity functions during plant growth, development, morphogenesis, defense and so on [START_REF] Mohnen | Pectin structure and biosynthesis[END_REF].

A.2 Structural organization of plant cell walls

PCWs are much thicker than the plasma membrane, ranging from 0.1 µm to several micrometers. The PCWs generally consist of three layers: the middle lamella ( or more appropriately "intercellular substance" [START_REF] Charles | On the origin and nature of the middle lamella[END_REF][START_REF] Colvin | Ultrastructure of the Plant Cell Wall: Biophysical Viewpoint[END_REF]), the primary cell wall, and secondary cell wall that may not be necessary for some cells [START_REF] Cosgrove | Growth of the plant cell wall[END_REF]

(Fig A- 3A).
The middle lamella is the outermost layer and also the first layer laid down during cell division [START_REF] Charles | On the origin and nature of the middle lamella[END_REF]. It is initially rich in pectic polysaccharide and serves as an adhesive layer that glues cells together to prevent cell sliding under large stress [START_REF] Cosgrove | Growth of the plant cell wall[END_REF][START_REF] Somerville | Toward a systems approach to understanding plant cell walls[END_REF]. In the case of mature wood cells, the middle lamella is where most lignin deposited [START_REF] Gibson | The hierarchical structure and mechanics of plant materials[END_REF][START_REF] Mathews | Bacterial biodegradation and bioconversion of industrial lignocellulosic streams[END_REF].

The primary cell wall is synthesized during cell expansion [START_REF] Carpita | Structural models of primary cell walls in flowering plants : consistency of molecular structure with the physical properties of the walls during growth[END_REF][START_REF] Dhugga | Maize biomass yield and composition for biofuels[END_REF]. It is composed of pectin compounds, hemicellulose, highly crystalline cellulose microfibrils that are oriented parallel to elongation direction [START_REF] Caffall | The structure, function, and biosynthesis of plant cell wall pectic polysaccharides[END_REF], and glycoproteins. Regarding the distribution of hemicelluloses, in dicots primary cell walls, xyloglucan accounts for 20 to 25% (w/w) [START_REF] Mccann | Biomass recalcitrance: A multi-scale, multifactor, and conversion-specific property[END_REF]. In contrast, in grass primary cell walls, glucuronoarabinoxylan represent 20 to 40% (w/w) [START_REF] Carpita | Structural models of primary cell walls in flowering plants : consistency of molecular structure with the physical properties of the walls during growth[END_REF] (Table A-1). Primary cell walls provide mechanical and elastic strength, cell adhesion and defense response [START_REF] Cosgrove | Growth of the plant cell wall[END_REF]. The composition of primary cell wall varies with time, and higher degree of lignification was found in mature cells than young ones [START_REF] Gibson | The hierarchical structure and mechanics of plant materials[END_REF].

The secondary cell wall is a thick layer located between the plasma membrane and the primary cell wall (e.g. three sub layers in wood cell walls) that is deposited once cells have ceased to grow (Fig A -3A). It is composed of cellulose chains that display a higher molecular weight than those in primary cell walls, a fact that is linked to the structural (reinforcement) role of the secondary cell wall [START_REF] Caffall | The structure, function, and biosynthesis of plant cell wall pectic polysaccharides[END_REF][START_REF] Cosgrove | Growth of the plant cell wall[END_REF]. Moreover, in secondary cell walls, the cellulose chains lie in antiparallel orientation with each other [START_REF] Caffall | The structure, function, and biosynthesis of plant cell wall pectic polysaccharides[END_REF] and hemicelluloses are more abundant than in primary cell walls (Table A-1). In dicots, glucuronoxylan constitutes 20-30% (w/w) of the secondary cell walls, while in grass glucuronoarabinoxylans comprise 40-50% (w/w) of the secondary cell walls. With age, the secondary cell wall becomes highly lignified fortifying the plant [START_REF] Boerjan | Lignin biosynthesis[END_REF][START_REF] Gibson | The hierarchical structure and mechanics of plant materials[END_REF]. Macromolecules may be formed by hypothetical covalent bond (d) between xyloglucan and pectin polysaccharides and between arabinoxylan and ferulic acid esters (A-F-F-A)(e) [START_REF] Cosgrove | Growth of the plant cell wall[END_REF].

A.3 Interaction among the composition of plant cell wall

Various intra-and inter-molecular interactions between cell wall polymers help maintain fiber cohesion and plant cell wall integrity (Fig A -3B) (Collins et al., 2005). Hemicellulose chains can be hydrogen bonded to cellulose fibrils [START_REF] Schädel | Hemicellulose concentration and composition in plant cell walls under extreme carbon source-sink imbalances[END_REF]

(Fig A-3B), a feature
that is thought to provide robustness to the overall structure. Moreover, studies have also shown that covalent cross-linking helps to decrease stiffness, while increasing the extensibility [START_REF] Chanliaud | Mechanical effects of plant cell wall enzymes on cellulose / xyloglucan composites[END_REF]. In recent studies using molecular dynamics simulations, it has been demonstrated that α-1,2-linked decorations in xylan not only enhance the flexibility of the threefold screw conformation of xylan, but also help stabilize xylan binding to the cellulose surface [START_REF] Berglund | Regular Motifs in Xylan Modulate Molecular Flexibility and Interactions with Cellulose Surfaces[END_REF][START_REF] Pereira | Effects of Xylan Side-Chain Substitutions on Xylan-Cellulose Interactions and Implications for Thermal Pretreatment of Cellulosic Biomass[END_REF]. Ferulic acid ester substitutions on xylan can crosslink (forming dehydrodimers) that provide xylan-xylan and xylan-lignin interactions [START_REF] Grabber | How do lignin composition, structure, and cross-linking affect degradability? A review of cell wall model studies[END_REF][START_REF] Grabber | Cross-linking of maize walls by ferulate dimerization and incorporation into lignin[END_REF]. Lignin is always linked to other matrices, especially hemicellulose, via ester and ether bonds [START_REF] Buranov | Lignin in straw of herbaceous crops[END_REF][START_REF] Davison | Plant Cell Walls: Basics of Structure, Chemistry, Accessibility and the Influence on Conversion[END_REF], forming an impenetrable barrier that prevents the penetration of external substances [START_REF] Howard | Lignocellulose biotechnology: issues of bioconversion and enzyme production[END_REF], which explains why the lignin is difficult to exract and isolate.

A.4 Comparison of different plant cell walls

Although many common biochemical and physical features are observed among different PCWs, it is clear that not all share exactly the same chemical composition and organizations (Table A-2). Therefore, understanding how these differences affect PCW structure and stability is a vital step towards the development of processing routes that can better surmount PCW recalcitrance (DeMartini et al., 2013b).

Primary and secondary cell walls are not clearly separated in Charophytes and Bryophytes [START_REF] Sarkar | Plant cell walls throughout evolution: Towards a molecular understanding of their design principles[END_REF]. Charophytes are recognized as one of the earliest plant groups to appear on land. Their PCWs were designed to make them more adapted to terrestrial life, for example (1) the formation of microfibrils that can support their bigger bodies compared to those of more primitive algae; (2) the abundance in uronic acids confers Charophytes with the ability to defend against desiccation and microbial attacks. Bryophyte cell walls are evolved to be more rigid by introducing XG and pectin.

Following the emergence of lignified secondary cell walls in Leptosporangiates, Gymnosperm cell walls became reinforced in cellulose, fucoside XG, HG and RG. Together these confer plants with the ability to withstand the load of their tall woody bodies (Table A- 2). However, the lignin in Leptosporangiates and Gymnosperms cell walls contain only guaiacyl units, while the lignin in angiosperms contain both guaiacyl and syringyl units, a composition that correlates with easier delignification [START_REF] Kishimoto | Influence of syringyl to guaiacyl ratio on the structure of natural and synthetic lignins[END_REF], although different opinions may exist on this point (DeMartini et al., 2013b). Grass cell walls are not particularly well adapted to support tall woody bodies, since they contain low levels of XG and pectins. Instead, the cell walls of grassy species contain higher amounts of mixed-linkage glucans [START_REF] Sarkar | Plant cell walls throughout evolution: Towards a molecular understanding of their design principles[END_REF].

Charophyte

Only mannan, glucuronic acid, mannuronic acid and 3-O-methyl rhamnose have been detected.

--

A.5 Plant cell wall in the context of biofuels

The development of second generation biofuels is considered preferable to first generation fuels, because the former can be produced using non-edible, lignocellulosic biomass (Table A-3) (Rakshit, 2013;[START_REF] Wyman | Bioethanol from Lignocellulosic Biomass[END_REF]. Lignocellulosic biomass can be sourced in many ways, either using so-called energy crops (e.g. switchgrass), wood or agricultural residues (e.g. wheat bran, wheat straw) [START_REF] Saini | Lignocellulosic agriculture wastes as biomass feedstocks for second-generation bioethanol production: concepts and recent developments[END_REF]. Many of these lignocellulosic biomass feedstocks arise from angiosperms, which means that their chemical composition and structural organization are complex and thus their conversion into products such as biofuels is difficult to achieve (Lynd et al., 2017) (Table A-3). As an alternative, it is possible to focus on algal feedstocks, using the primitive cell walls of Charophytes as the source of carbon for 3 rd generation biofuels, such as biodiesel [START_REF] Menetrez | An overview of algae biofuel production and potential environmental impact[END_REF].

A.6 Plant cell wall in the context of diet-microbiota interactions

PCWs from grass (family Poaceae) are the primary source of dietary fibers (i.e. non-starch polysaccharides consumed by human, cow and some insects). Dietary fibers are important, because it is known that they play an important role in the crosstalk between host and gut microbita, a current topic of great interest [START_REF] Barratt | The Gut Microbiota, Food Science, and Human Nutrition: A Timely Marriage[END_REF][START_REF] Cani | Talking microbes: When gut bacteria interact with diet and host organs[END_REF][START_REF] Koropatkin | How glycan metabolism shapes the human gut microbiota[END_REF]. Arabinoxylan, pectin, and XG are classified as soluble dietary fibers, whereas cellulose and lignin are grouped into insoluble dietary fibers [START_REF] Williams | Gut Fermentation of Dietary Fibres : Physico-Chemistry of Plant Cell Walls and Implications for Health[END_REF].

Cereal grains such as wheat and rye are rich in arabinoxylans for human nutrition and health [START_REF] Andersson | Content and molecularweight distribution of dietary fiber components in whole-grain rye flour and bread[END_REF][START_REF] Selvendran | The plant cell wall as a source of dietary fiber: chemistry and structure[END_REF]. A whole-grain diet is generally considered beneficial for human health [START_REF] Gong | Whole cereal grains and potential health effects: Involvement of the gut microbiota[END_REF][START_REF] Topping | Cereal complex carbohydrates and their contribution to human health[END_REF], consistent with the fact that the amount of arabinoxylan in wheat bran is much more than in flour (20 and 2% of dry weight, respectively) [START_REF] Van Der Kamp | Dietary Fibre: New Frontiers for Food and Health[END_REF].

PART B ENZYMATIC DECONSTRUCTION OF LIGNOCELLULOSE

The degradation of lignocellulose embedded in the plant cell wall leads to the recycling of photosynthetically fixed carbon and is therefore of profound biological and industrial importance. Due to the broad range recalcitrance of lignocellulosic biomass, merely one step or one method cannot achieve full degradation [START_REF] Mccann | Biomass recalcitrance: A multi-scale, multifactor, and conversion-specific property[END_REF]. Nevertheless, in this regard, the industrial exploitation of enzymes is advantageous, because they perform specific reactions with low energy costs (temperature) and generate hydrolysis products that are generally innocuous both with regard to fermentation microbes and the environment [START_REF] Alvira | Progress on enzymatic saccharification technologies for biofuels production[END_REF].

B.1 Lignocellulose-degrading enzymes

B.1.1 Classification

Within the International Union of Biochemistry and Molecular Biology (IUB)'s Enzyme Nomenclature and Classification system, lignocellulose-degrading enzymes are classified according to the reaction they catalyze and their substrate specificities (http://www.chem.qmul.ac.uk/iubmb/enzyme/). Since the majority of lignocellulose-degrading enzymes catalyze hydrolysis reactions, these are mostly classified in EC3 [START_REF] Sweeney | Biomass Converting Enzymes as Industrial Biocatalysts for Fuels and Chemicals: Recent Developments[END_REF], although some lignocellulose-degrading enzymes are oxidases or lyases.

An alternative classification system has been proposed by Bernard Henrissat and his colleagues [START_REF] Henrissat | A classification of glycosyl hydrolases based sequence similarities[END_REF]. This scheme, which is focused on carbohydrate-active hydrolases and their associated modules together with conservation of the catalytic residues and catalytic mechanism [START_REF] Henrissat | Updating the sequence-based classification of glycosyl hydrolases[END_REF]. Glycosyl hydrolase families are grouped into 5 clans, GH-A, GH-B, GH-C, GH-D and GH-E. Enzymes that work on the same substrate are sometimes found in different clans. For example, although GH10 and GH11 both act on xylans, GH10 belongs to clan A while GH11 belongs to clan C. Conversely, enzymes belonging to the same family and/or the same clan can act on different substrates. This is exemplified by members of families GH10 and GH53 (both belong to clan A) that act on xylan and galactan respectively. The clan concept demonstrates a broader relationship between GHs, which suggests a more distant common evolutionary ancestor (Pollet et al., 2010). Recently, omics studies have greatly increased the number of CAZy families and enriched some families with a large number of new members. This is the case for GH 5, 13, 30 and 43 families that display a wide variety of substrate specificities, with several specificities being associated with single families.

Consequently, in these large families it has been possible to divide them into subfamilies on the basis of substrate specificity [START_REF] Aspeborg | Evolution, substrate specificity and subfamily classification of glycoside hydrolase family 5 (GH5)[END_REF][START_REF] Mewis | Dividing the Large Glycoside Hydrolase Family 43 into Subfamilies: a Motivation for Detailed Enzyme Characterization[END_REF]Stam et al., 2006). 1 More recently, the CAZy database has included other enzymes that also contribute to the degradation of PCWs. These include polysaccharide lyases and redox enzymes (classed under the term 'Auxiliary Activities') that act in conjunction with CAZy hydrolases.

Taken together, the CAZy classification is more valuable for bioinformatics study than IUB system.

B.1.2 Catalytic mechanism

B.1.2.1 The retaining and reverting mechanism

GHs are enzymes that catalyze the hydrolysis of the glycosidic bonds of glycosides, glycans and glycoconjugates, producing a sugar hemiacetal or hemiketal and the corresponding free aglycon. Whether the anomeric configuration of the final product is identical to that of the reactant or not, provides the basis to classify GHs into two groups known as retaining or inverting GHs [START_REF] Dies | Structures and mechanisms of glycosyl hydrolases[END_REF]Koshland, 1953)(Fig B-1). Catalysis in both cases is mostly due to the presence of two acidic (Asp or Glu) amino acids located in the active site. In retaining enzymes these act as a proton donor and a nucleophile respectively, whereas in inverting enzymes one acts as a general acid and the other as a general base. For any given enzyme, one can distinguish between the two mechanisms by close examination of structural and functional properties (Vuong and Wilson, 2010) and in either case mutation of the two catalytic residues to inert amino acids (Ala, Gly, etc.) essentially deactivates the enzyme [START_REF] Macleod | Mechanistic Consequences of Mutation of Active Site Carboxylates in a Retaining β-1,4-Glycanase from Cellulomonas fimi[END_REF].

In inverting GHs , catalysis occurs via a two-pronged attack on the anomeric center. The catalytic acid residue donates a proton to the anomeric oxygen of the donor substrate, while the catalytic base concomitantly attacks the anomeric carbon from opposite side. This causes anomeric inversion, meaning that for example a β-glycoside donor yields an α-glycoside product. et al., 1996) based on "upward"(base, deprotonated, low pKa) and "downward"(acid, protonated, high pKa) conformation switch of Glu (Wan et al., 2015).

Overall, at the end of the reaction the anomeric configuration of the glycoside product is finally identical to that of the substrate. Therefore, careful analysis of the anomeric configuration of the reaction end products reveals whether any given GH operates via an inverting or a retaining mechanism.

From a structural standpoint, the nature of the retaining reaction means that the two catalytic residues are separated by a distance of ~5.5 Å (mean value calculated for many enzymes), whereas in inverting GHs this distance is larger (~10 Å) [START_REF] Dies | Structures and mechanisms of glycosyl hydrolases[END_REF]. This is because inverting enzymes need to simultaneously accommodate the sugar donor and the incoming water molecule.

Despite the fact that the canonical double displacement mechanism has been experimentally validated [START_REF] Davies | Snapshots along an Enzymatic Reaction Coordinate: Analysis of a Retaining β-Glycoside Hydrolase †[END_REF]Suzuki et al., 2009), this mechanism has been extensively debated and other schemes, for example one involving an oxocarbenium ion intermediate, have been proposed [START_REF] Phillips | The Hen Egg-White Lysozyme Molecule[END_REF] and experimentally supported [START_REF] Hövel | Crystal structure and snapshots along the reaction pathway of a family 51 α-L-arabinofuranosidase[END_REF][START_REF] Tanakas | Transition State Structures for the Hydrolysis of a-D-Glucopyranosyl Fluoride by Retaining and Inverting Reactions of Glycosylases[END_REF]. In fact the main difference between the two mechanistic schemes is the nature of the transition state: either it is a covalent glycosyl-enzyme intermediate (SN2 reaction) or an oxocarbenium ion-like transition state without assistance of nucleophile (SN1 reaction). Other complex transition states and intermediate structures are also mentioned in the literature [START_REF] Sabini | Catalysis and specificity in enzymatic glycoside hydrolysis: a 2,5 B conformation for the glycosyl-enzyme intermediate revealed by the structure of the Bacillus agaradhaerens family 11 xylanase[END_REF].

B.1.2.2 Sugar binding subsite topologies

The structural analysis of GHs generally reveals that the two catalytic residues are usually located somewhere within a pocket, which intimately accommodates the sugar donor substrate. This pocket or subsite is often numbered as the subsite -1, meaning that it is the subsite that accommodates the glycone group (sugar donor) that lies on the non-reducing side of the scissile bond. The subsite accommodating the sugar (or an aglycone group) on the other side of the bond is thus designated subsite +1. In enzymes possessing extended active sites, the other subsites are numbered accordingly with all subsites on the non-reducing side of the bond being negatively numbered [START_REF] Davies | Nomenclature for sugar-binding subsites in glycosyl hydrolases[END_REF]Pell et al., 2004a)(Fig B -2A). In GHs, subsites are generally structured to favor sugar binding via the formation of hydrogen bonds or hydrophobic stacking interactions. Accordingly, they play a crucial role in substrate recognition (Pollet et al., 2010).

Regarding the structural configuration of the active sites in GHs, it is convenient to use the term active site topology. This term refers to the specific 3D conformation and arrangement of the subsites that form an active site, and which specifically accommodate the glycoside substrate. Three common active-site topologies are described as pockets or craters, clefts or grooves and tunnels (Fig B -2). (A) Glycon subsite of Cellvibrio japonicas Xylanase 10C (Pell et al., 2004b). Three types of active site topologies (B) The pocket, PDB: 3GLY, glucoamylase from Aspergillus awamori; (C) The cleft, PDB: 1TML, endoglucanase E2 from Thermononospora fusca; (D) The tunnel, PDB: 3CBH, cellubiohydrolase II from Trichoderma reesei. The red zone is the proposed catalytic residues [START_REF] Dies | Structures and mechanisms of glycosyl hydrolases[END_REF].

Enzymes with "pocket" shaped topology are usually exo-acting and act on non-reducing extremities of saccharides. Radial structural substrate is more preferable than fibrous ones.

Examples of pocket enzymes are β-amylase, β-glucosidase. The enzymes possessing an open "cleft" are more adapted to cleave long polymers to short oligomers, such as endocellulases, endoxylanases. "Tunnel" topology is found in cellobiohydrolases and facilitate the release of product while cleavage, i.e. processivity [START_REF] Dies | Structures and mechanisms of glycosyl hydrolases[END_REF]. [START_REF] Fischer | Cellulases from insects[END_REF] that binds a cellulose glucan chains at any accessible point along its length. Endo-1,4-β-D -glucanases then perform apparently random cleavage and then dissociate from the chain. Archetypical endoglucanases (EG) are grouped in families GH 6, 9, 45 and 58, all of these being inverting enzymes. Other archetypical EGs are found within families GH 5, 7 and 12 which contain enzymes that operate via a retaining mechanism.

B.1.3 Cellulases

Unlike EGs, exocellulases are characterized by an active site that displays tunnel-like topology (Fig B -2C). Consequently, cellulose glucan chains bind at one extremity of the tunnel and are then fed through in a processive manner, with cleavage occurring at regular repeated intervals until substrate dissociation occurs. This leads to the release of short glucooligosaccharides (or cellodextrins), with cellubiose being a typical product [START_REF] Kristensen | Enzymatic hydrolysis of lignocellulosese[END_REF]. The action of exocellulase requires free glucan extremities and thus generally follows the action of EGs, which generate such extremities. There are two types of exocellulases, 1,4β-D-glucan glucanohydrolases (also known as cellodextrinases, such as GH7, EC3.2.1.74) and 1,4-β-D-glucan cellobiohydrolases (cellobiohydrolases, such as GH6, EC 3.2.1.91). The former act on reducing ends of cellulose glucan chains, while the latter that attacks nonreducing ends. The co-occurrence and synergism between endo-and exo-cellulases are prerequisites for high efficient enzymatic systems for biorefineries [START_REF] Sweeney | Biomass Converting Enzymes as Industrial Biocatalysts for Fuels and Chemicals: Recent Developments[END_REF].

A third class of cellulases is β-glucosidases or β-glucoside glucohydrolases (EC 3.2.1.21), which hydrolyze the exocellulase product (i.e. cellobiose) into individual monosaccharides.

The representative families are GH1 and 3. Unlike the majority of lignocellulases, βglucosidases act upon soluble rather than insoluble substrate. In biorefinery applications where high substrate concentrations are desirable (>20% dry matter content) to achieve sufficiently high end product titers, β-glucosidase (especially GH1) is particularly useful to mitigate the inhibition of endo-and exo-cellulases by cellodextins.

Oxidative cellulases, for instance cellobiose dehydrogenase (acceptor), depolymerize cellulose by radical reactions, Finally, cellulose phosphorylases depolymerize cellulose using phosphates instead of water [START_REF] Lynd | Microbial Cellulose Utilization : Fundamentals and Biotechnology[END_REF][START_REF] Sweeney | Biomass Converting Enzymes as Industrial Biocatalysts for Fuels and Chemicals: Recent Developments[END_REF].

The GH9 family GH9, formerly known as cellulase family E, is mainly composed of members that possess endoglucanase (EG) activity and operate by random cleavage of β-1,4 glycosidic bonds. Their role is to reduce the degree of polymerization of the glucan chains and provide substrates for the exo-cellulases. The catalytic core of GH9 EGs displays a (α/α6-fold) and is characterized by the presence of two sequence motifs Asp-Ala-Gly-Asp and Asn-Glu-Val-Ala, which contain the catalytic nucleophile (an Asp residue) and the proton donor (Glu residue) respectively. Studies showed that the mutation on Gly147 to Arg in the GH9 EG from the termite Reticulitemes speratus led to complete loss of enzymatic activity, suggesting that Gly147 might play a significant role in maintaining enzyme activity [START_REF] Ni | Identification of activity related amino acid mutations of a GH9 termite cellulase[END_REF]. More generally, GH9 endo-β -1,4-glucanases are common among different invertebrates such as beetles, flies, cockroaches, and termites [START_REF] Bujang | A phylogenetic study of endo-beta-1,4-glucanase in higher termites[END_REF].

B.1.4 Hemicellulases

The degradation of hemicelluloses has several effects on biomass. First, since hemicelluloses in plant cell walls are in intimate contact with cellulose microfibrils, the action of hemicellulases unmasks the microfibrils and opens up access for cellulases. Second, hemicellulases also complete hydrolyze hemicelluloses making the sugars available for metabolism by microorganisms. Since hemicelluloses are chemically and structurally complex, their degradation relies upon the interplay of a whole range of enzymes (Table B-1) that can be collectively designated using the term hemicelluases [START_REF] Sweeney | Biomass Converting Enzymes as Industrial Biocatalysts for Fuels and Chemicals: Recent Developments[END_REF].

Table B-1 The major hemicellulases and their classification

B.1.4.1 Xylanases

Owing to the heterogeneous structure of xylans, their degradation requires the intervention of a large number of enzymes (Fig B -3). The key enzyme activity responsible for the hydrolysis of heteroxylans is endo that are commonly referred to as xylanases. These are produced by a wide variety of organisms, incuding fungi, bacteria, yeast, marine algae, protozoa, snails, crustaceans, insect, sees, etc [START_REF] Sunna | Xylanolytic enzymes from fungi and bacteria[END_REF]. Xylanases act on the β-1,4 bonds that link internal xylosyl residues in xylan backbones. The action of xylanases is completed by that of 1,, which are commonly referred to as xylosidases. These preferentially hydrolyze xylooligosaccharides (XOS), which are the products of xylanase action. Although it is often assumed that xylosidases preferentially act upon xylobiose, this is not completely true since some xylosidases clearly display preference for longer XOS. However, unlike xylanases, xylosidases are exo-acting enzymes that hydrolyse substrates by removing β-Dxylopryranosyl residues from the non-reducing termini of XOS.

Finally, to be exhaustive, it is noteworthy that some xylanases are endo-1,3-β-xylanases that catalyze the hydrolysis of β-1,3-xylan, which is typically found in marine algal species [START_REF] Dumon | Progress and future prospects for pentose-specific biocatalysts in biorefining[END_REF]. [START_REF] Salis | Industrial Enzymes[END_REF] B. Xylanases are key enzymes, because they depolymerize xylans and generate products that serve as substrates for most of the other hemicellulases (Collins et al., 2005). Most known endoxylanases belong to GH families 10 and 11 (formerly family F and G, respectively, (NR. Gilkes et al., 1991)) (over 4000 gene sequences are known), although xylanases are also found in families 5, 7, 8, 9, 12, 16, 26, 30, 43, 44, 51 and 62 (www.cazy.org). However, it is important to stress that xylanases belonging to families GH 16 and 62 are bifunctional enzymes that possess two distinct catalytic domains. From the perspective of evolution, it is also noteworthy that family 5 is related to the archetypal xylanase family GH10, while family GH12 is related to the equally archetypal xylanase family GH11. Indeed, in the case of the latter pair, it has been suggested that GH11 members possibly result from the evolutionary specialization of family GH12 (Collins et al., 2005). One manifestation of this specialization is the presence in GH11 enzymes of a large flexible loop that probably confers xylan specificity and catalytic potency, which is absent in GH10 enzymes [START_REF] Paës | GH11 xylanases: Structure/function/properties relationships and applications[END_REF][START_REF] Polizeli | Xylanases from fungi: properties and industrial applications[END_REF].

Family GH10 is populated by enzymes of plant, fungal, and bacterial origin, whereas GH11 family arise from fungi and bacteria only (www.cazy.org). Moreover, the majority of bacterial xylanases are GH10 family members, whereas most fungal xylanases belong to GH11 family.

Due to the general relationship between molecular weight (MW) and isoelectric point (pI) [START_REF] Wong | Multiplicity of beta-1,4-xylanase in microorganisms: functions and applications[END_REF], most of the GH 10 xylanases display a MW ≥30 kDa, low pI, while GH11 members generally display a MW <30 kDa and high pI [START_REF] Sunna | Xylanolytic enzymes from fungi and bacteria[END_REF].

Moreover, GH11 enzymes are active across a wide pH range from from 2 to 11, with GH11 subgroups being classified as acidic and alkaline xylanases according to their pH optima [START_REF] Joshi | Hydrogen bonding and catalysis: a novel explanation for how a single amino acid substitution can change the pH optimum of a glycosidase[END_REF][START_REF] Sapag | The endoxylanases from family 11: Computer analysis of protein sequences reveals important structural and phylogenetic relationships[END_REF]. Like enzymes belonging to GH families 1, 2, and 5, the catalytic domain of GH10 endoxylanases is composed of a TIM-barrel fold, unlike that of GH11 counterparts that display a β-jelly roll. Nevertheless, despite these difference, both GH 10 and GH 11 xylanases perform catalysis via a retaining mechanism that requires a catalytic dyad composed of a catalytic nucleophile and a proton donor (Gebler et al., 1992;Withers et al., 1986).

Regarding their substrate specificity, GH10 xylanases are generally considered to be more catalytic versatile than GH11 xylanases, often displaying activity cellulose and aryl β-Dcellubiosides (Tull and [START_REF] Mccarter | Mechanisms of enzymatic glycoside hydrolysis[END_REF]. When acting on branchy xylans, it has been shown that GH10 xylanases require the presence of one or two unsubstituted xylopyranosyl residues between decorated residues, corresponding to subsites -2 and -1 (Biely et al., 1997;Kolenová et al., 2006;Pell et al., 2004a). In the case of GH11 xylanases, it has been shown that these require three unsubstituted consecutive xylopyranosyl residues accommodating from subsites -2 to +1 (Biely et al., 1997;Katapodis et al., 2003). As a consequence, GH10 xylanase can produce smaller hydrolysis products from glucuronoxylan and arabinoxylan than GH11 xylanases (Pollet et al., 2010).

B.1.4.1.2 GH10 enzyme: Topology of catalytic site and its polymorphism

At the time of writing (December 2017), the structures of 29 GH10 xylanases have been resolved and publically released, thus providing precious structure-sequence information .The active site of GH10 xylanases is commonly described by an open cleft in which the substrate can bind. The cleft is lined by a series of amino acid sidechains, with several being aromatic residues. The catalytic acid-base (proton donor) and nucleophile resides are located on loops at the end of β sheets 4 and 7, which explains why GH10 enzymes are assigned to the so-called 4/7 superfamily [START_REF] Jenkins | Beta-glucosidase, betagalactosidase, family A cellulases, family F xylanases and two barley glycanases form a superfamily of enzymes with 8-fold beta/alpha architecture and with two conserved glutamates near the carboxy-terminal ends of beta-strands four[END_REF].

In the glycon region, xylose residues are bound through several hydrogen bonds. In contrast, ligand binding in the aglycon region of the substrate-binding cleft is dominated by hydrophobic interactions between xylopyranose rings and aromatic amino acids (Charnock et al., 1999;[START_REF] Zolotnitsky | Mapping glycoside hydrolase substrate subsites by isothermal titration calorimetry[END_REF].

The crystal structures of GH10 xylanase in complex with oligosaccharides and inhibitors indicate that, in general, the -2, -1, and +1 subsites are highly conserved [START_REF] Charnock | Substrate Specificity in Glycoside Hydrolase Family 10[END_REF][START_REF] Notenboom | Insights into transition state stabilization of the beta-1,4-glycosidase Cex by covalent intermediate accumulation in active site mutants[END_REF][START_REF] White | Crystallographic observation of a covalent catalytic intermediate in a beta-glycosidase[END_REF]. Amino acid residues in these subsites play a key role in subsite recognition and make several interactions between the enzyme and the xylose moieties.

At the -2 subsite, the O-2 of the sugar hydrogen bonding to a tryptophan located in β8, the O-3 to an asparagine in the loop connecting β3 and α3, and the endocyclic oxygen to a lysine in the same loop, too. It has been shown that -2 subsite is prerequisite for cleavage branched xylan (Kolenová et al., 2006). At the -1 subsite the O-3 of xylose makes a hydrogen bond with a highly conserved histidine in β6, whereas O-2 interacts with an asparagine and a glutamate (which acts as the catalytic nucleophile) that located in the end of β4 (Charnock et al., 1999;[START_REF] Harris | Structure of the catalytic core of the family F xylanase from Pseudomonas fluorescens and identification of the xylopentaose-binding sites[END_REF]Pell et al., 2004a).

Generally, xylohexaose was mainly degraded into xylotriose by endoxylanase while

xylopentaose was cleavage into xylobiose and xylotriose. In contrast, xylotetraose can be cleaved into primarily two xylobioses (Charnock et al., 1999;[START_REF] Hong | Two new xylanases with different substrate specificities from the human gut bacterium Bacteroides intestinalis DSM 17393[END_REF][START_REF] Millward-Sadler | Novel cellulose-binding domains, NodB homologues and conserved modular architecture in xylanases from the aerobic soil bacteria Pseudomonas fluorescens subsp. cellulosa and Cellvibrio mixtus[END_REF]Pell et al., 2004a;Sun et al., 2007), or xylose and xylotriose [START_REF] Hong | Two new xylanases with different substrate specificities from the human gut bacterium Bacteroides intestinalis DSM 17393[END_REF][START_REF] Millward-Sadler | Novel cellulose-binding domains, NodB homologues and conserved modular architecture in xylanases from the aerobic soil bacteria Pseudomonas fluorescens subsp. cellulosa and Cellvibrio mixtus[END_REF]Pell et al., 2004b). The different product profile implied that the -3 subsite is not conserved in GH10 xylanases. At the -3 subsite, the aromatic ring of tyrosine stacks against the xylopyranose ring in CjXyn10C, CjXyn10A, Cellvibrio mixtus Xyn10A et al. The mutation of this tyrosine significantly reduced the activity against xylotetraose and xylan, consistent with the greater importance of the tyrosine at -3 subsite (Pell et al., 2004b). Furthermore, the product profile of xylotetraose hydrolysis was exclusively xylobiose when the tyrosine at 340 of CjXyn10C mutated into alanine. It was proposed that the presence of a tyrosine in GH10 enzymes in the equivalent position to Y340 in CjXyn10C is indicative of a high affinity -3 subsite (Pell et al., 2004b). The sequence of each protein was extracted from the PDB file, and the alignment analysis was done by ESPript 3 (Robert and Gouet, 2014). The labels α, β, η, T refer to α-helices, β-strands, 310-helices and π-helices respectively. Grey stars indicate alternative conformations.

B.1.4.1.3 GH10 enzyme: 3D structure-domain organization

The first determined structure of the catalytic domain of a GH10 xylanase was that from Streptomyces lividans xylanases A [START_REF] Derewenda | Crystal structure, at 2.6-Å resolution, of the Streptomyces lividans xylanase A, a member of the F family of beta-1,4-D-glycanases[END_REF]. Since then, numerous studies have shown that GH10 xylanases are characterized by a highly conserved (β/α)8-barrel fold, with only relatively small variations being observed especially in loops and helices surrounding the inner β-strands [START_REF] Schmidt | Xylan binding subsite mapping in the xylanase from Penicillium simplicissimum using xylooligosaccharides as cryo-protectant[END_REF][START_REF] Solomon | Structure-specificity relationships of an intracellular xylanase from Geobacillus stearothermophilus[END_REF]

(Fig B-2).
Additionally, some variation of the shape of the barrel has been observed in GH10 xylanases, with this ranging from nearly circular to quite elliptical (Pollet et al., 2010).

(β/α)8-barrel fold

The canonical (β/α)8-barrel contains about 200 amino acids and is composed of eight units, each of which consists of a β-strand and an α-helix that are connected by a βα-loop. The eight individual units are linked by αβ-loops. The βα loops are important for function, because the actives sites of all TIM-barrel enzymes are located at the C-termini of β strands [START_REF] Wierenga | The TIM-barrel fold: A versatile framework for efficient enzymes[END_REF]. On the other hand, αβ loops are believed to play important roles in maintaining stability of TIM-barrel proteins (Urfer and Kirschner, 1992). The βα loops display variable length, but tend to be longer than αβ loops. The word "barrel" refers to a curved central parallel β -sheet formed by eight β -strands, around the barrel are α -helices. (β/α)8-barrel protein fold is firstly found in the triosephosphate isomerase or TIM, and that's why it is also called TIM barrel fold (Spratt and Pardee, 1975). This fold is one of the most common protein folds known is most often associated with enzymes that are involved in molecular or energy metabolism within the cell [START_REF] Copley | Homology among (βα)8 barrels: implications for the evolution of metabolic pathways[END_REF][START_REF] Nagano | One fold with many functions: The evolutionary relationships between TIM barrel families based on their sequences, structures and functions[END_REF].

It remains an open question as to whether the TIM barrel fold is evolved via convergent evolution or divergent evolution. Understanding its evolutionary history is of significant importance for the prediction of β/α-barrel structures and for the design of enzymes with new functions. Comparing intron positions in pyruvate kinase and triosephosphate isomerase genes, Straus and Gilbert showed that these mapped onto the three-dimensional structures of the corresponding proteins (Straus and Gilbert, 1985), strongly favours the hypothesis of convergent evolution. Farber, Branden et al proposed the existence of a common ancestor, i.e. divergent evolution for TIM barrel fold based on structural similaries [START_REF] Brändén | The TIM barrel-the most frequently occurring folding motif in proteins[END_REF][START_REF] Farber | The evolution of α/β barrel enzymes[END_REF]. This argument was further supported by more structural evidences [START_REF] Nagano | One fold with many functions: The evolutionary relationships between TIM barrel families based on their sequences, structures and functions[END_REF][START_REF] Reardon | The structure of alpha / beta barrel proteins[END_REF] and phylogenic studies [START_REF] Copley | Homology among (βα)8 barrels: implications for the evolution of metabolic pathways[END_REF]. Recent studies were also in support of its divergent origin and showed that many TIM barrel superfamilies probably originated from common ancestors that demonstrate a high degree of catalytic diversity thanks to evolutionarily malleable active site pockets [START_REF] Goldman | The TIM Barrel Architecture Facilitated the Early Evolution of Protein-Mediated Metabolism[END_REF]. Other voice concerning the evolution of TIM barrels is saying that the smallest evolutionary unit could be the half-barrel, (β/α)4 . This is because HisA and HisF, two enzymes involved in histidine biosynthesis, were shown to evolved by twofold gene duplication of a half-barrel ancestor, followed by gene fusion [START_REF] Lang | Structural Evidence for Evolution of the beta /alpha Barrel Scaffold by Gene Duplication and Fusion[END_REF].

While the argument about its evolution origin is going on, enzyme engineers have achieved enzymatic activity migration [START_REF] Saab-Rincón | Evolutionary walk between (β/α) 8 barrels: Catalytic migration from triosephosphate isomerase to thiamin phosphate synthase[END_REF] by combining rational design and random mutagenesis and developed different tools for chimeric protein design [START_REF] Wang | Recurring sequence-structure motifs in (βα)8-barrel proteins and experimental optimization of a chimeric protein designed based on such motifs[END_REF]. Research in the de novo design of a TIM barrel protein guided by geometrical and chemical principles suggests that the potential functional range of the TIM barrel is greater than represented naturally occurring proteins [START_REF] Huang | De novo design of a four-fold symmetric TIM-barrel protein with atomiclevel accuracy[END_REF].

Insertional domain in (β/α)8-barrel fold Formally, domains are defined as stable globular fragments of proteins that may refold autonomously and carry out specific functions (Khosla and Harbury, 2001). Proteins can comprise a single domain or a combination of domains. In multi-domain proteins, the amino acid sequence of domains almost always occur in tandem from N-terminal to C-terminal except 9% exceptions, where one domain (insert) inserted into another domain (parent) that is often in α/β fold (Aroul-Selvam et al., 2004).

TIM barrels can contain one or more insertional domains [START_REF] Nagano | One fold with many functions: The evolutionary relationships between TIM barrel families based on their sequences, structures and functions[END_REF], with these often occurring in the βα3 loop. In pyruvate kinase, the βα3 loop insertion is a ~100 residuedomain, known as the domain B, which barely interacts with the domain A of the protein [START_REF] Larsen | Structure of rabbit muscle pyruvate kinase complexed with Mn2+, K+, and pyruvate[END_REF][START_REF] Mattevi | Crystal structure of Escherichia coli pyruvate kinase type I: molecular basis of the allosteric transition[END_REF]. Similarly, α-amylase also contains a so-called domain B in its βα3 loop. From one amylase to another, this insertional domain displays variable length and sequence [START_REF] Janecek | Alpha-Amylase Family: Molecular Biology and Evolution[END_REF][START_REF] Jespersen | Starch-and glycogen-debranching and branching enzymes: Prediction of structural features of the catalytic (β/α)8-barrel domain and evolutionary relationship to other amylolytic enzymes[END_REF][START_REF] Klein | Structure of cyclodextrin glycosyltransferase refined at 2.0 Å resolution[END_REF][START_REF] Qian | Structure and Molecular Model Refinement of Pig Pancreatic α-Amylase at 2•1 Å Resolution[END_REF], although researchers have reported the fact that there is a relatively conserved aspartate residue near the C-terminus of the domain B (D175 in Taka-amylase A), which is involved in the binding of calcium ions [START_REF] Liu | Crystal structure of a raw-starch-degrading bacterial α -amylase belonging to subfamily 37 of the glycoside hydrolase family GH13[END_REF]. It has been postulated by Buisson that Ca 2+ ions preserves the structural integrity of the active site by linking the catalytic (β/α)8-barrel and the extruding domain B [START_REF] Buisson | Three dimensional structure of porcine pancreatic alpha-amylase at 2.9 A resolution. Role of calcium in structure and activity[END_REF]. In α-amylase, it has been shown that domain B reinforces the conformation of the active site and enhances enzyme thermostability [START_REF] Janecek | Alpha-Amylase Family: Molecular Biology and Evolution[END_REF][START_REF] Liu | Crystal structure of a raw-starch-degrading bacterial α -amylase belonging to subfamily 37 of the glycoside hydrolase family GH13[END_REF]. In the case of susG protein, an αamylase belonging to the starch utilization system (SUS), the domain B is itself a target for insertion, containing a CBM58 (Koropatkin and Smith, 2010). This further modification of the TIM-barrel, plausibly related to the glycan foraging function of members of Bacteroidetes, reveals the tolerance of the TIM barrel architecture to modification and provides a paradigm for the creation of chimeric enzymes. In this respect, further evidence of the relative 'plasticity' of the TIM-barrel is provided by the existence of circular permutations, which provide family GH70 α-glucan acting enzymes [START_REF] Macgregor | A circularly permuted αamylase-type α/β-barrel structure in glucan-synthesizing glucosyltransferases[END_REF].

Finally, beyond the α amylase group, recent studies revealed that a GH10 xylanase also contains inserted CBMs in its βα3 loop (Flint et al., 1997;Zhang et al., 2014). Strikingly, this enzyme is part of polysaccharide utilization loci (PUL, see section B3.2) analogous to SUS and therein might play a role that is comparable to that of SusG. that many retaining xylosidases display substantial transferase activity especially at high substrate concentrations [START_REF] Reilly | Xylanase: structure and function[END_REF][START_REF] Wong | Multiplicity of beta-1,4-xylanase in microorganisms: functions and applications[END_REF], and this activity results in products of higher molecular weight than that of the substrate and the formation of both β-1,3 and β-1,4 bonds [START_REF] Claeyssen | Transfer reactions catalysed by a fungal beta-d-xylosidase:enzymic synthesis of phenl beta-d-xylobioside[END_REF][START_REF] Sunna | Xylanolytic enzymes from fungi and bacteria[END_REF]. Xylosidase activity is inversely related to the chain length of xylooligosaccharides, with xylobiose being the preferred substrate [START_REF] Takenishi | Studies on hemicellulases. IV. Purification and properties of the Beta-xylosidase produced by Aspergillus niger van Tieghem[END_REF] except in some cases [START_REF] Saha | Purification and characterization of an extracellular beta-xylosidase from a newly isolated Fusarium verticillioides[END_REF]. Some xylosidases are strongly inhibited by D-xylose and its homologues [START_REF] Claeyssens | d-Xylose-derivatives with sulfur or nitrogen in the ring: Powerful inhibitors of glycosidase-activities[END_REF][START_REF] Saha | Purification and characterization of an extracellular beta-xylosidase from a newly isolated Fusarium verticillioides[END_REF], while others are not [START_REF] Yan | A xylosetolerant β-xylosidase from Paecilomyces thermophila: Characterization and its co-action with the endogenous xylanase[END_REF][START_REF] Zanoelo | Purification and biochemical properties of a thermostable xylose-tolerant β-D-xylosidase from Scytalidium thermophilum[END_REF].

Xylosidases are found in animal, plant and microbial kingdoms, with many of the latter being of fungal origin (e.g. from Aspergillus, Botryodiploidia, Penicilliu, Pestalotia and Trichoderma). Regarding bacterial xylosidases, those of Bacillus are numerous, while others are produced by plant pathogens, such as Agrobacterium, Corynebacterium and Erwinia [START_REF] Reilly | Xylanase: structure and function[END_REF]. A striking feature of xylosidases is their multiplicity and the fact that they are distributed throughout several GH families, including families GH 30 (contains 8 subfamilies), 39, 43 (contains 37 subfamilies) 51, 52, 116 and 120. Consistent with this, different xylosidases display quite different specificities, being active on from aryl-β-Dxylopyanosides, aryl-α-L-arabinopyranosides, aryl-β-D-glucopyranosides, aryl-β-Dquinopyranosides, xylobiitol, xylotriitol, and L-serine xylopyranoside [START_REF] Reilly | Xylanase: structure and function[END_REF][START_REF] Sunna | Xylanolytic enzymes from fungi and bacteria[END_REF].

B.1.4.3 Accessory xylanolytic enzyme

Owing to the heterogeneity of xylan structure, a mere combination of endoxylanase and xylosidase is unlikely to achieve complete degradation. For this to occur, other so-called accessory (or auxiliary) enzymes are required, notably to release mainchain decorations. For example, α-L-arabinofuranosidases can release α-L-arabinofuranosides from heteroxylan, while various xylan esterases will release (for example) acetic and ferulic acid .

Accessory enzymes are produced by many microorganisms, such as Penicillium capsulatum and Talaromyces emersonii, which possess complete xylan-degrading enzyme systems [START_REF] Saha | Hemicellulose bioconversion[END_REF].

B.1.4.3.1 α-L-Arabinofuranosidase

α-L-Arabinofuranosidases (α-L-arabinofuranoside arabinofuranohydrolase, EC 3.2.1.55), hereafter referred to as arabinofuranosidases of Abfs, release arabinose from arabinosecontaining polysaccharides, such as arabinan, arabinoxylan, gum arabic and arabinogalactan [START_REF] Saha | Alpha-L-arabinofuranosidases: biochemistry, molecular biology and application in biotechnology[END_REF]. Abfs are exo-type enzymes that hydrolyze terminal nonreducing residues linked by 1,3 or 1,5-α-arabinosyl linkages. Beldman summarized the classification of this enzyme group based on substrate specificities: class 1 are not active towards polymers; class 2 are active towards polymers; class 3 are specific for arabinoxylans [START_REF] Beldman | Arabinans and arabinan degrading enzymes[END_REF]. The CAZy database classifies Abfs within 6 GH families: GH2, 3, 43, 51, 54 and 62 on the basis of amino acid sequence similarities. There are only two characterized arabinofuranosidases from GH2 and 3, and these are bifunctional containing another activity such as βgalactosidase and β-xylosidase. All members of GH51 and 54 catalyze hydrolysis with net retention of anomeric configuration, while GH43 members operate via an inverting mechanism. The mechanism of GH62 is not elucidated, although it shares the same tertiary fold as GH43, with both families beloning to clan GH-F [START_REF] Wilkens | GH62 arabinofuranosidases: Structure, function and applications[END_REF]. Only GH62 contains exclusively α-L-arabinofuranosidases and these are of fungal and bacterial origin.

B.1.4.3.2 Hemicellulolytic esterase

Xylan esterases hydrolyze the ester bonds that either link acetyl groups to mainchain xylosyl residues acid (acetylxylan esterase, EC 3.1.1.72) or that link cinnamoyl (e.g. ferulolyl) groups to the O-5 position of arabinosyl side chain decorations in arabinoxylans (ferulic acid esterase, EC . Acetylxylan esterases are grouped within 7 of the 16 carbohydrate esterase familes in CAZy [START_REF] Ryabova | A novel family of hemicellulolytic alpha-glucuronidase[END_REF].

Their role in heteroxylan hydrolysis is vital, since the presence of acetyl groups is known to be a major hindrance to xylanases [START_REF] Salis | Industrial Enzymes[END_REF]. The majority of ferulic acid esterases are classified in CE1. The presence of hydroxycinnamic acids in heteroxylans not only hinders the action of enzymes such as Abfs, but also provides the basis for intermolecular crosslinking between xylan molecules and between xylan and lignin. Ferulyol and p-coumaric acid esterases are therefore precious, because they can disrupt crosslinking and help to solubilize xylan [START_REF] Shallom | Microbial hemicellulases[END_REF].

B.1.4.3.3 α-D-Glucuronidase

Only two CAZy families, GH67 and GH115, harbor α-glucuronidases, implying that this type of enzyme is subject to much less variability when compared to other accessory. One GH4 enzyme has also been described as an α-D-glucuronidase. However, it does not recognize 4-O-methyl-D-glucuronoxylan or derivatives thereof (Suresh et al., 2003). Members of both GH67 and 115 operate via an inverting catalytic mechanism. The first GH67 glucuronidase was described in 1983 [START_REF] Dekker | Bioconversion of hemicellulose: Aspects of hemicellulase production by Trichoderma reesei QM 9414 and enzymic saccharification of hemicellulose[END_REF]. This type of enzyme releases 4-Omethyl-D-glucuronic acid (MeGlcA) or GlcA from oligosaccharide motifs that are derived from glucuronoxylan (aldouronic acids), in which the uronic acid is linked to the nonreducing terminal xylopyranosyl residue. However, GH67 enzymes do not attack aryl αglycosides of GlcA or MeGlcA [START_REF] Ryabova | A novel family of hemicellulolytic alpha-glucuronidase[END_REF]. Nurizzo et al studied the 3D structure and biochemical properties of a GH67 enzyme from P.cellulosa. This revealed that its active site adopts the form of a deep pocket that recognizes both the 4-O-methyl and the carboxylate components [START_REF] Nurizzo | The structural basis for catalysis and specificity of the Pseudomonas cellulosa α-glucuronidase, GlcA67A[END_REF]. The more recent discovery of family GH115 glucuronidases revealed that these enzymes hydrolyze linkages that bind MeGlcA moities to both terminal and internal xylopyranosyl residues of glucuronoxylan and aldouronic acids derivatives generated by the action of xylanases [START_REF] Ryabova | A novel family of hemicellulolytic alpha-glucuronidase[END_REF]Tenkanen and Siikaaho, 2000). The detailed characterization of a multidomain GH115 from Bacteroides ovatus showed that it is a dimeric enzyme that contains a flexible active site cleft housed in the TIM barrel domain [START_REF] Rogowski | Evidence that GH115 alpha-glucuronidase activity, which is required to degrade plant biomass, is dependent on conformational flexibility[END_REF].

B.1.4.4 Synergistic interaction between xylanases and accessory hemicellulases

The chemical complexity of heteroxylans is clearly a major obstacle for enzyme degradation (Kormelink and Voragen, 1993;[START_REF] Lee | Purification and characterization of two endoxylanases from Clostridium acetobutylicum ATCC 824[END_REF]. In degrading arabinoxylans, the introduction of Abfs into xylanase can enhance the performance of the latter [START_REF] Suh | Synergic effects among endo-xylanase, βxylosidase, and α-L-arabinofuranosidase from Bacillus stearothermophilus[END_REF][START_REF] De Vries | Synergy between accessory enzymes from Aspergillus in the degradation of plant cell wall polysaccharides[END_REF] and sometimes is a pre-request when degrading agricultural residues (Tuohy et al., 1993), probably due to the involvement of arabinose in the cross-linking between lignin and hemicellulose [START_REF] Buchmann | Purification and cooperative activity of enzymes constituting the xylan-degrading system of Thermomosospora fusca[END_REF][START_REF] Chesson | Substituent groups linked by alkalilabile bonds to arabinose and xylose residues of legume, grass and cereal straw cell walls and their fate during digestion by rumen microorganisms[END_REF][START_REF] Smith | Occurrence and nature of ferulic acid substitution of cell-wall polysaccharides in graminaceous plants[END_REF].

The mix of acetyl esterase with xylanase is advantageous for the action of both enzymes [START_REF] Biely | Cooperativity of Esterases and Xylanases in the Enzymatic Degradation of Acetyl Xylan[END_REF][START_REF] Blum | Characterization of an acetyl xylan esterase from the anaerobic fungus Orpinomyces sp. strain PC-2[END_REF][START_REF] Suh | Synergic effects among endo-xylanase, βxylosidase, and α-L-arabinofuranosidase from Bacillus stearothermophilus[END_REF]. The release of ferulic acid by ferulic acid esterases rely more on the action of xylanase especially when complex substrate is utilized [START_REF] Faulds | The purification and characterization of 4hydroxy-3-methoxycinnamic (ferulic) acid esterase from Streptomyces olivochromogenes[END_REF]Mackenzie and Bilous, 1988;[START_REF] De Vries | Synergy between accessory enzymes from Aspergillus in the degradation of plant cell wall polysaccharides[END_REF][START_REF] Yu | Release of ferulic acid from oat hulls by Aspergillus ferulic acid esterase and Trichoderma xylanase[END_REF], and different xylanases used may result in different product form [START_REF] Faulds | Synergy between xylanases from glycoside hydrolase family 10 and family 11 and a feruloyl esterase in the release of phenolic acids from cereal arabinoxylan[END_REF].

It was reported that the function of α-glucuronidase requires a low-molecular-weight glucuronoxylan substrate [START_REF] Smith | Alpha-Glucuronidase and other hemicellulase activities of Fibrobacter succinogenes S85 grown on crystalline cellulose or ball-milled barley straw[END_REF]. Xylanases can improve the performance of α-glucuronidase by providing methylglucuronoxyl XOS [START_REF] Shao | Purification and Characterization of the alpha -Glucuronidase from Thermoanaerobacterium sp . Strain JW / SL-YS485 , an Important Enzyme for the Utilization of Substituted Xylans[END_REF][START_REF] De Vries | Synergy between accessory enzymes from Aspergillus in the degradation of plant cell wall polysaccharides[END_REF]. The synergistic interaction between α-glucuronidase and β-xylosidase was also proven [START_REF] Shao | Purification and Characterization of the alpha -Glucuronidase from Thermoanaerobacterium sp . Strain JW / SL-YS485 , an Important Enzyme for the Utilization of Substituted Xylans[END_REF].

B.2 CBMs

Glycoside hydrolases are often inefficient for the degradation of insoluble polysaccharides. This is because it is difficult for GHs to access the substrate at the right position for catalysis [START_REF] Bornscheuer | Enzymatic degradation of (ligno)cellulose[END_REF][START_REF] Salis | Industrial Enzymes[END_REF]. Therefore, a distinct structural feature of many lignocellulose-degrading enzymes is modularity, with the catalytic core being covalently linked to one or more non-catalytic protein domains. Majoritarily, these domains are carbohydrate-binding modules (CBM), although others are fibronectin 3-like modules, dockerins, immunoglobulin-like domains, or functionally unknown "X" domains. Regarding CBMs, these are found as discretely folded units within the multi-modular structures of many polysaccharide-targeting enzymes, such as cellulases and hemicellulases.

B.2.1 Classification

Boraston et.al. manually classified the structures of CBMs from 22 different CBM families into seven 'fold families' based on the conservation of the protein fold. These seven families possess β-sandwich, β-trefoil, cysteine knot, unique, OB, hevein and hevein-like folds respectively (Boraston et al., 2004). However, this system of classification is not useful to predict function. Therefore, an alternative system that considers the topology of the CBM binding site has been proposed. Herein, type A CBMs possess flat or platform-like hydrophobic surfaces composed of aromatic residues that are well-adapted for the binding of crystalline polysaccharides, such as cellulose or chitin. The planar architecture of the binding site is thought to be complementary to the flat surfaces presented by cellulose or chitin crystals [START_REF] Hashimoto | Recent structural studies of carbohydrate-binding modules[END_REF]. Type B CBMs displays a cleft/groove arrangement in which aromatic residues are oriented in such a way that forms twisted or sandwich platforms (Guillén et al., 2010), and interact with internal glycan chains (endo type) including amorphous cellulose or xylan, and also recognize β-1,3-glucans, β-1,3-1,4-mixed linkage glycans, β-1,4-mannan, glucomannan, and galactomannan. Type C CBMs or lectin-like CBMs , lack the extended binding-site grooves of type B CBMs and bind to the termini of glycans (exo type), so it has limited ligands such as mono-, di-, or trisaccharides (Boraston et al., 2004;[START_REF] Bornscheuer | Enzymatic degradation of (ligno)cellulose[END_REF]. Finally, like the carbohydrate-active enzymes, CBMs are also present in the CAZy classification and are currently grouped into 81 families based on amino acid sequence similarity (see http://www.cazy.org/Carbohydrate-Binding-Modules.html). Together, the 81 families represent a wide range of ligand specificities.

Considering the different classifications it is possible to note that type A CBMs are found CAZy CBM families 1, 2, 3, 5, and 10 and all of these bind to insoluble ligands. Type B CBMs are found in CAZY CBM families 4, 6, 15, 17, 20, 22, 27, 28, 29, 34 and 36, while type C CBMs are grouped in CAZY CBM families 9, 13, 32, 47, 66 and 67 (https://www.cazypedia.org/index.php/Carbohydrate-binding_modules).

B.2.2 The mechanism of binding

Lehito et al. has used transmission electron microscopy to probe the location of the CBMbinding site on Valonia crystalline cellulose. They confirmed that both CBM1 and CBM3a bind to the hydrophobic face [START_REF] Lehtiö | The binding specificity and affinity determinants of family 1 and family 3 cellulose binding modules[END_REF]. The study of its 3D crystal structure indicates that the major interaction relies on a planar strip of aromatic residues. Additionally, polar amino acid residues are proposed to anchor the CBM molecule to two other adjacent chains of crystalline cellulose (Tormo et al., 1996). Aromatic residues in three types of CBMs mentioned above play a pivotal role in ligand binding and for type B and C CBMs, the direct hydrogen bonds also play a key role in the affinity and ligand specificity.

The most important conformational element of the majority of CBMs is the beta-sheet, and β-sandwich is the major fold in CBM structures [START_REF] Hashimoto | Recent structural studies of carbohydrate-binding modules[END_REF]. β-sandwich CBMs often bound metal ions that mainly contribute to structural stability [START_REF] Ghosh | Deciphering ligand specificity of a Clostridium thermocellum family 35 carbohydrate binding module (CtCBM35) for gluco-and galacto-Substituted mannans and its calcium induced stability[END_REF][START_REF] Tormo | were produced in house as His-tag fusion proteins. 2.4 Sample preparation materials[END_REF]. Furthermore, the calcium ion in CBM35 [START_REF] Bolam | X4 modules represent a new family of carbohydrate-binding modules that display novel properties[END_REF], CBM36 [START_REF] Jamal-Talabani | Ab initio structure determination and functional characterization of CBM36: A new family of calcium-dependent carbohydrate binding modules[END_REF] was shown to involve in ligand binding. The residues involved in the binding sites have been identified using NMR, site-directed mutagenesis and structural investigations [START_REF] Itoh | Importance of Trp59 and Trp60 in chitin-binding, hydrolytic, and antifungal activities of Streptomyces griseus chitinase C[END_REF][START_REF] Newstead | Galactose recognition by the carbohydrate-binding module of a bacterial sialidase[END_REF][START_REF] Pantoom | The effects of the surfaceexposed residues on the binding and hydrolytic activities of Vibrio carchariae chitinase A[END_REF][START_REF] Rodríguez-Sanoja | A single residue mutation abolishes attachment of the CBM26 starch-binding domain from Lactobacillus amylovorus α-amylase[END_REF].

In general, the importance of aromatic amino acids, especially tryptophan, but also the highly conserved tyrosine residues, is well known. They form stacking interactions with the sugar rings resulting in strong van der Waals interactions, thus stabilizing the complex. In addition, the side chain of other polar amino acid residues may form hydrogen bonds with the sugar ligand, helping to stabilize the interaction. Several studies suggest that orientation of aromatic residues is responsible for the different ligand specificities of the CBM families [START_REF] Miyanaga | The family 42 carbohydrate-binding module of family 54 α-L-arabinofuranosidase specifically binds the arabinofuranose side chain of hemicellulose[END_REF][START_REF] Najmudin | Xyloglucan is recognized by carbohydrate-binding modules that interact with β-glucan chains[END_REF][START_REF] Simpson | The structural basis for the ligand specificity of family 2 carbohydrate-binding modules[END_REF].

B.2.3 The function of CBM in glycoside hydrolases

The relationship between a CBM and a catalytic domain within a single polypeptide is variable depending on the protein under consideration. In some cases, the functional properties of the domains are apparently independent of one another. This has been illustrated by removing CBMs from enzymes and demonstrating that the catalytic domain remains active [START_REF] Gilbert | The N-terminal region of an endoglucanase from Pseudomonas fluorescens subspecies cellulosa constitutes a cellulosebinding domain that is distinct from the catalytic centre[END_REF][START_REF] Gilkes | Precise excision of the cellulose binding domains from two Cellulomonas fimi cellulases by a homologous protease and the effect on catalysis[END_REF] and that the CBM retains its substrate-binding ability [START_REF] Hachem | Carbohydrate-binding modules from a thermostable Rhodothermus marinus xylanase: cloning, expression and binding studies[END_REF][START_REF] Zverlov | The binding pattern of two carbohydrate-binding modules of laminarinase Lam16A from Thermotoga neapolitana: Differences in β-glucan binding within family CBM4[END_REF]. However, there are also numerous examples in the literature that show the contrary.

When catalytic and CBM domains display functional interdependency, this can be expressed to different degrees and be rather substrate dependent. For example, the separation of CBMs from catalytic modules in some enzymes has resulted in significant decreases in the activity of the enzymes on insoluble substrates (Ali et al., 2001;Black et al., 1996;[START_REF] Gilkes | Precise excision of the cellulose binding domains from two Cellulomonas fimi cellulases by a homologous protease and the effect on catalysis[END_REF][START_REF] Puranen | Trichoderma Enzymes for Textile Industries[END_REF][START_REF] Sakka | Characterization of Paenibacillus curdlanolyticus B-6 Xyn10D, a xylanase that contains a family 3 carbohydrate-binding module[END_REF] and soluble polysaccharides [START_REF] Kittur | Fusion of family 2b carbohydrate-binding module increases the catalytic activity of a xylanase from Thermotoga maritima to soluble xylan[END_REF][START_REF] Zverlov | The binding pattern of two carbohydrate-binding modules of laminarinase Lam16A from Thermotoga neapolitana: Differences in β-glucan binding within family CBM4[END_REF]. In other cases removal of CBMs has been shown to have more globally drastic effects on enzyme activity [START_REF] Arai | Characterization of a cellulase containing a family 30 carbohydrate-binding module (CBM) derived from Clostridium thermocellum CelJ: Importance of the CBM to cellulose hydrolysis[END_REF][START_REF] Burstein | Physical association of the catalytic and helper modules of a family-9 glycoside hydrolase is essential for activity[END_REF][START_REF] Gilad | Cell, a noncellulosomal family 9 enzyme from Clostridium thermocellum, is a processive enduglucanase that degrades crystalline cellulose[END_REF]. This is well-exemplified by the removal of a CBM3 from a GH9 cellulase, which resulted in almost complete loss of activity, even when soluble substrates were tested [START_REF] Gal | CelG from Clostridium cellulolyticum: A multidomain endoglucanase acting efficiently on crystalline cellulose[END_REF]. Likewise, the direct participation of a CBM32 in substrate recognition required for catalytic action was also suggested [START_REF] Mizutani | Essential role of a family-32 carbohydrate-binding module in substrate recognition by Clostridium thermocellum mannanase CtMan5A[END_REF]. In the case of the Clostridium stercorarium Xyn10B, an enzyme that is highly active toward barley β-1,3-1,4-glucan as well as β-1,4-xylan, excision of the CBM22s led to loss of the β-1,3-1,4-glucanase activity, but not xylanase activity [START_REF] Araki | Essential role of the family-22 carbohydrate-binding modules for β-1,3-1,4-glucanase activity of Clostridium stercorarium Xyn10B[END_REF]Zhao et al., 2005), which highlights the important role for the CBMs in the determining substrate specificity of cognate enzymes.

Studies have revealed that at high substrate concentration and thus lower water activity, the importance of CBMs for substrate recognition and catalysis diminishes. This is illustrated by a study that showed that at 20% (w/w) substrate concentration, the hydrolytic performance of cellulases devoid of CBMs was similar to that of cellulases with CBMs (Várnai et al., 2013). This phenomenon might explain why in some studies no synergistic effects between catalytic modules and CBMs were observed in assays performed on insoluble substrates, such as certain xylans [START_REF] Hachem | Carbohydrate-binding modules from a thermostable Rhodothermus marinus xylanase: cloning, expression and binding studies[END_REF].

Lytic polysaccharide monooxygenases (LPMOs) also figure among the biomass-degrading enzymes and are now thought to be important components of enzyme arsenals [START_REF] Eijsink | Towards new enzymes for biofuels: lessons from chitinase research[END_REF]. It is interesting to note that some studies have shown that the creation of chimeric LPMOs containing a CBM causes deleterious effects on catalytic activity [START_REF] Crouch | The contribution of non-catalytic carbohydrate binding modules to the activity lytic polysaccharide monooxygenases[END_REF]. These observations suggest that CBMs possess a variety of functions, which are different from one CBM to the next [START_REF] Cuskin | How nature can exploit nonspecific catalytic and carbohydrate binding modules to create enzymatic specificity[END_REF][START_REF] Hervé | Carbohydrate-binding modules promote the enzymatic deconstruction of intact plant cell walls by targeting and proximity effects[END_REF]Montanier et al., 2009). This further implies that it is not straightforward to add a CBM from one enzyme to another and generate beneficial effects.

Finally, it is noteworthy that CBMs, often borne on multimodular (non-enzymatic) proteins, have been identified in several species. Study of these proteins has revealed a wide variety of functions for the CBMs, including toxins, virulence factors or pathogenesis associated proteins. In this case, CBMs are involved in the promotion of tissue destruction and the enhancement of either bacterial spread or pathogenesis in plants and animals (Guillén et al., 2010).

B.2.4 The mechanism of CBM in potentiating catalysis

The mechanism by which CBMs potentiate catalysis remains unclear. Three general mechanisms have been proposed: CBMs (i): create substrate disruption, (ii) produce a proximity effect, (iii) target catalytic modules to substrates.

Substrate disruption has been proposed as an important function for cellulose-specific CBMs. It has been hypothesized that these disrupt the ordered hydrogen-bonding network in crystalline cellulose, making the surface chains accessible to the appended cellulase (Jonathan [START_REF] Knowles | Cellulase families and their genes[END_REF][START_REF] Teeri | Crystalline cellulose degradation: New insight into the function of cellobiohydrolases[END_REF]. This has been supported by biochemical, biophysical and microscopic data that indicate that CBMs mediate changes to the surface structure of cellulose (Din et al., 1994;[START_REF] Simpson | The structural basis for the ligand specificity of family 2 carbohydrate-binding modules[END_REF]Sorimachi et al., 1997;Southall, 1999;Viegas et al., 2008;[START_REF] Wang | A novel function for the cellulose binding module of cellobiohydrolase I[END_REF].

Regarding the proximity effect, studies have revealed that some CBMs enhance the efficiency of appended catalytic domains by mediating prolonged and intimate contact between the respective catalytic domain and its target substrate, thereby improving substrate accessibility (Bolam et al., 1998;[START_REF] Gilbert | The biochemistry and structural biology of plant cell wall deconstruction[END_REF]. Finally, work for example on agarases bearing CBM6 domains has shown that these latter bind specifically to the non-reducing end of agarose chains, recognizing only the first disaccharide repeat. This specific targeting towards the agarose chains apparently directs the appended catalytic domain to the area of the plant cell wall where the polysaccharide is likely to be most amenable to enzymatic hydrolysis [START_REF] Henshaw | Family 6 carbohydrate binding modules in β-agarases display exquisite selectivity for the non-reducing termini of agarose chains[END_REF]. In the context of intact primary and secondary cell wall deconstruction, it has also been shown that CBMs can potentiate the action of a cognate catalytic domain through the recognition of non-substrate polysaccharides. In other words, the targeting function of CBMs can be used in the context of complex, heterogeneous matrices to create strong proximity effects. This explains why cellulose-specific CBMs are appended to many PCW-active glycoside hydrolases that do not possess cellulase activity [START_REF] Hervé | Carbohydrate-binding modules promote the enzymatic deconstruction of intact plant cell walls by targeting and proximity effects[END_REF].

B.2.5 The distribution of CBM in lignocellulose-degrading enzymes

Based on sequence data, more than 60% of cellulases and xylanases identified lack CBMs. As a general rule, enzymes containing CBMs can bind to polymeric substrates. On the contrary, enzymes lacking CBMs are often specific for soluble molecules.

It is interesting to note that a number of glycoside hydrolases contain multiple CBMs, either arranged in tandem [START_REF] Tomme | Characterization of CenC, an enzyme from Cellulomonas fimi with both endo-and exoglucanase activities[END_REF] or at opposing N and C terminal ends [START_REF] Sermsathanaswadi | The family 22 carbohydrate-binding module of bifunctional xylanase/β-glucanase Xyn10E from Paenibacillus curdlanolyticus B-6 has an important role in lignocellulose degradation[END_REF] of the protein sequence, or both (Guillén et al., 2010;[START_REF] Sainz-Polo | Exploring multimodularity in plant cell wall deconstruction: Structural and functional analysis of Xyn10C containing the CBM22-1-CBM22-2 tandem[END_REF]. In the case of multiple CBMs, the question of function is particularly interesting. One might suppose that the CBMs target the same carbohydrate ligand, recognizing different motifs therein. On the other hand, if the enzyme acts on a complex substrate such as PCWs, one might imagine that the CBMs recognize different substrates within a wider polysaccharide amalgam. Interestingly, the study of enzymes that bear multiple CBMs reveals that these latter can belong to the same CBM family (homogeneous multimodularity) or different families (heterogeneous multimodularity) [START_REF] Sainz-Polo | Exploring multimodularity in plant cell wall deconstruction: Structural and functional analysis of Xyn10C containing the CBM22-1-CBM22-2 tandem[END_REF]. In the case of homogeneous multimodularity, this has been linked to multivalency and avidity effects [START_REF] Bolam | Evidence for synergy between family 2b carbohydrate binding modules in Cellulomonas fimi Xylanase 11A[END_REF]Boraston et al., 2002[START_REF] Boraston | A structural and functional analysis of α-glucan recognition by family 25 and 26 carbohydrate-binding modules reveals a conserved mode of starch recognition[END_REF][START_REF] Freelove | A Novel Carbohydrate-binding Protein Is a Component of the Plant Cell Wall-degrading Complex of Piromyces equi[END_REF]. In the case of heterogeneous multimodularity, it has been proposed that this might provide distinct ligand-binding specificities, which is consistent with the case of Clostridium stercorarium Xyn10B mentioned in B.2.3.

B.3 Microbial carbohydrate-degrading systems

Carbohydrate utilization is inextricably linked to the ability of microbes to persist in environments as diverse as fresh and salt water, soil, and animal gastrointestinal tracts. In these environments competition between organisms for a common and potentially temporary and limited pool of nutrients is fierce (Grondin et al., 2017). In this respect, the strategy adopted by many bacterial and fungal species is to produce a multitude of enzymes that enable them to access and metabolize complex substrates such as heteroxylans.

Improved culturing techniques and next-generation sequencing have recently provided access to enormous amounts of genetic information and thus are gradually revealing the secrets of various microbial ecosystems or microbiomes [START_REF] Flint | Polysaccharide utilization by gut bacteria: potential for new insights from genomic analysis[END_REF]Human Microbiome Project Consortium et al., 2012;[START_REF] Mayorga | Temporal metagenomic and metabolomic characterization of fresh perennial ryegrass degradation by rumen bacteria[END_REF][START_REF] Rossmassler | Metagenomic analysis of the microbiota in the highly compartmented hindguts of six wood-or soil-feeding higher termites[END_REF].

However, in the current post-genomic era, the urgent need is now related to functional data. This is required to elucidate how the microorganisms within a microbiome operate in order to best use available carbohydrates within a competitive environment that is certainly characterized by competition and cooperation.

Conventionally, the PCW-degrading systems of different microbial species are differentiated on the basis of whether these are produced by aerobic (mostly fungi) and or anaerobic microbes (mostly bacteria) (Várnai et al., 2013). The majority of PCW-active aerobic organisms, such as Trichoderma sp. and Aspergillus sp., secrete a whole host of carbohydrateactive enzymes into their surrounding environments. These enzymes, often operating with considerable synergy, bring about PCW hydrolysis and facilitate the importation of monosaccharides by the microorganism [START_REF] Tomme | Characterization of CenC, an enzyme from Cellulomonas fimi with both endo-and exoglucanase activities[END_REF][START_REF] Warren | Microbial Hydrolysis of polysaccharides[END_REF]. In contrast, most of anaerobic Gram positive microorganisms produce an array of hydrolytic enzymes that are secreted to and bound at the cell surface within complex enzyme machineries known as cellulosomes, which also contain CBMs (Gilbert, 2007). On the other hand, some anaerobic Gram negative microbes employ glycan utilization systems (Martens et al., 2009), which are known as Sus-like systems, because the paradigm for these is the well-characterized Starch Utilisation System (Sus).

In general terms, to date the most extensively studied plant fiber degrading enzyme systems are those produced by members of the Firmicutes and Bacteroidetes phyla that are present in gut microbiomes (White et al., 2014).

B.3.1 Cellulosome

The initial definition of the cellulosome was " a discrete, cellulose-binding, multienzyme complex for the degradation of cellulosic substrates" [START_REF] Bayer | Organization and distribution of the cellulosome in Clostridium thermocellum[END_REF][START_REF] Bayer | The cellulosomes: multienzyme machines for degradation of plant cell wall polysaccharides[END_REF]. The principle component of the cellulosome is a scaffoldin subunit, which is composed of contains cohesion modules (usually in multiple copies) for incorporation of different enzymes and other cellulosomal components. Enzymes containing the dockerin modules are loaded onto the cellulosome by tenacious cohesion-dockerin interaction. In some cases, two or more types of scaffoldin may be involved in anchoring the cellulosome to the cell surface. In addition, CBM is commonly found in scaffoldin and plays important roles in substrate binding . [START_REF] Bayer | The cellulosomes: multienzyme machines for degradation of plant cell wall polysaccharides[END_REF].

In class-I (dockerin-bearing) primary scaffoldin, enzymes were incorporated by dockerin-cohesin interaction. An adaptor scaffoldin connects the dockerin of the primary scaffoldin and the cohesins of an anchoring scaffoldin.

The discovery of glycan-specific transporter systems has revealed another paradigm for glycan degradation and uptake. Generally, the examination of Firmicutes genomes reveals that these encode fewer carbohydrate-degrading enzymes than members of Bacteroidetes, but possess more ABC transporters (ATP-binding cassette transporters) and PTS (phosphoenolpyruvate-dependent phosphotransfer systems) that transport carbohydrates [START_REF] Mahowald | Characterizing a model human gut microbiota composed of members of its two dominant bacterial phyla[END_REF]. ABC transporter are frequently encoded adjacent to genes encoding glycoside hydrolases, suggesting that these two functions are co-regulated and function together [START_REF] Rey | Dissecting the in vivo metabolic potential of two human gut acetogens[END_REF].

B.3.2 Sus-like systems

The Sus-like systems are only found in Gram negative bacteria of phylum Bacteroidetes.

The difference between Gram positive and negative bacteria lies in the composition of their cell envelope. Gram-negative bacteria display two cell membranes, the inner and the outer membranes, that are separated by the periplasm. The outer membrane consists of phospholipids, lipopolysaccharides, integral membrane proteins, and lipoproteins [START_REF] Bos | Biogenesis of the gram-negative bacterial outer membrane[END_REF]. Bacteroidetes genomes typically contain 200-350 GH and polysaccharide lyases genes, compared with less than 200 in those of Firmicutes (White et al., 2014).

Regarding glucose-based substrates, it has been shown that maltodextrins longer than maltotetraose cannot be used by the maltotriose/maltose uptake systems (Martens et al., 2009). Therefore, one might assume that other mechanisms exist to ensure the uptake of more complex glycans for catabolism. Seminal work by members of Prof. Abigail Salyers's laboratory (Reeves et al., 1997) revealed a cell envelope-associated multiprotein system, which was designated by the acronym Sus. This system enables Bacteroides thetaiotaomicron to bind and degrade starch . Subsequent microbioal genome sequencing projects revealed that derivatives of this paradigm system (Sus-like systems) are highly represented in many other saccharolytic Bacteroidetes species. A key characteristic of these Sus-like systems is the coordinated action of eight gene products, encoded by the adjacent genes susRABCDEFG , which perform functions such as substrate binding proteins and hydrolysis.

The concerted and sequential action of Sus proteins results in a series of glycan recognition, acquisition and degradation operations. Initially, the extracellular starch transits through a polysaccharide capsule to the cell surface upper right inset). SusC is a TonBdependent receptor, membrane-spanning proteins transporting solutes and macromolecules via energy derived from the proton-motive force and the TonB-ExbBD complex [START_REF] Ferguson | TonB-dependent receptors-structural perspectives[END_REF]. It is suggested that SusD ligand recognition is driven by recognition of the starch helical backbone rather than by the stereochemistry of the individual glucosyl units (Martens et al., 2009) and that the combined effect of SusE, SusC and SusD accounts for over 80% of the affinity for starch. The remaining affinity can be attributed to SusF (Shipman et al., 2000). The postulated role for SusG, is that of a typical GH13 α-amylase that cleaves internal α-1,4 bonds between glucosyl units thus depolymerizing starch and producing maltodextrins (larger than maltotriose). Then according to current knowledge, the latter are transported into the periplasmic compartment by SusC/SusD (Martens et al., 2009). The localization of SusCDEFG on the outer membrane has been confirmed by immunohistochemical studies (Shipman et al., 2000). In the periplasm, the oligosaccharides produced by SusG are further degraded by SusA and SusB prior to final transport to the cytosol. SusA possesses neopullulanase activity, while SusB is an α-glucosidase. SusR, spans both the periplasm and cytoplasm and is constitutively expressed and responsible for transcriptional activation of seven sus genes [START_REF] D'elia | Effect of regulatory protein levels on utilization of starch by Bacteroides thetaiotaomicron[END_REF]. The B. thetaiotaomicron genome contains 101 homologs of susC / susD pairs, suggesting that the similar glycan catabolism strategy may expand to other nutrients [START_REF] Xu | Evolution of Symbiotic Bacteria in the Distal Human Intestine[END_REF]. A PUL (polysaccharide utilization loci), first described in [START_REF] Bjursell | Functional genomic and metabolic studies of the adaptations of a prominent adult human gut symbiont, Bacteroides thetaiotaomicron, to the suckling period[END_REF], is a set of physcially linked genes organized around a susCD gene pair [START_REF] Terrapon | Automatic prediction of polysaccharide utilization loci in Bacteroidetes species[END_REF]. Except susCD, a PUL includes a group of enzymes related to one specific substrate degradation [START_REF] Bjursell | Functional genomic and metabolic studies of the adaptations of a prominent adult human gut symbiont, Bacteroides thetaiotaomicron, to the suckling period[END_REF][START_REF] Martens | Mucosal Glycan Foraging Enhances Fitness and Transmission of a Saccharolytic Human Gut Bacterial Symbiont[END_REF][START_REF] Xu | A Genomic View of the Human-Bacteroides thetaiotaomicron Symbiosis[END_REF]. More generally, Sus-like complexes appear as a paradigm for glycan uptake in bacteria belonging to the phylum Bacteroidetes.

The characterized /predicted PULs have been summarized in CAZy database (http://www.cazy.org/PULDB/).

Of the xylan-degrading Bacteroidetes described to date, the xylanolytic system of the rumen bacterium Prevotella bryantii B14 is the best studied (Dodd et al., 2011;[START_REF] Gasparic | A xylan hydrolase gene cluster in Prevotella ruminicola B ( 1 ) 4 : sequence relationships , synergistic interactions , and oxygen sensitivity of a novel enzyme with exoxylanase and beta-( 1 , 4 ) -xylosidase activities . A Xylan Hydrolase Gene Cluster i[END_REF]. This bacterium has been shown to efficiently utilize water-soluble, but not waterinsoluble xylans [START_REF] Miyazaki | Degradation and Utilization of Xylans by the Rumen AnaerobePrevotella bryantii(formerlyP. ruminicolasubsp.brevis) B14[END_REF]. The xylan-degradation genes in the same cluster were shown to be induced at the transcriptional level to medium to large-sized xylan fragments via the HTCS regulator, XynR [START_REF] Miyazaki | Involvement of the Multidomain Regulatory Protein XynR in Positive Control of Xylanase Gene Expression in the Ruminal Anaerobe Prevotella bryantii B14[END_REF]. This HTCS regulator is also conserved in the genomes of all other xylanolytic Bacteroidetes that contain Xus clusters.

Subsequent transcriptomic analysis has shown that this cluster represents part of a large PUL that includes 12 genes that are highly expressed (up to 400-fold induction) during growth on wheat arabinoxylan (Dodd et al., 2010(Dodd et al., , 2011)). These include two pairs of susCD homologues and also an unusual GH10 enzyme (Xyn10C) in which the catalytic module is interrupted by an internal CBM module (Flint et al., 1997). The function of this enzyme has not been established, although it has been suggested that it plays a role similar to SusG in performing limited cell surface cleavage of polysaccharides to produce fragments that can enter the periplasm (Dodd et al., 2010). In vitro biochemical characterization of Xyn10C-like protein from B.ovatus was performed and showed moderate activity towards xylan (Wang et al., 2016;Zhang et al., 2014). Additional enzymes like arabinofuranosidase, α-glucuronidase, acetyl xylan esterase encoded either by the large PUL or by additional genes encoded elsewhere on the genome that are also highly induced on xylan (Dodd et al., 2010).

More recently, a more complex model of xylan degradation by Bacteroide ovatus was proposed (Rogowski et al., 2015). Different xylan degradation strategies are employed according to the complexity of xylan. It showed that the xylan utilization loci where there is xyn10C are dedicated to breakdown relateively simple xylans, such as wheat arabinoxylan instead of corn glucuronoarabinoxylan. What is more, this paper supported the Xyn10C is a surface enzyme and suggested that it may contain an extended substrate bind cleft due to the insertion of two CBMs.

PART C BIOMASS UTILIZATION SYSTEM OF TERMITE

C.1 Introduction to termite

Termites are superabundant in the tropic terrestrial environment and are widely spread from humid forests to the savanna and even arid areas. Termites are notorious for the wood damage they cause, therefore are menaces for crops and houses. On the other hand, their excellence in lignocellulose decomposition can be applied into biomass utilization.

C1.1 Classification of termite

Termites are members of the Animalia kingdom and belong to the of Arthropoda phylum.

They form part of the Insecta class the Blattodea order and the Isoptera infraorder. Termites are generally classified into seven families (Table C-1). There is agreement among systematists that Mastotermitidae, a single extant species restricted to northern Australia, is the basal lineage among extant termites [START_REF] Eggleton | An introduction to termites: Biology, taxonomy and functional morphology[END_REF]. Termitidae is the largest family, consisting of at least seven sub-families, namely Macrotermitinae, Sphaerotermitinae, Foraminitermitinae, Apicotermitinae, Termitinae, Syntermitinae and Nasutitermitinae [START_REF] Lo | Termite phylogenetics and co-cladogenesis with symbionts[END_REF]. In term of species, the number is over three thousand combing the described and under described [START_REF] Eggleton | An introduction to termites: Biology, taxonomy and functional morphology[END_REF].

Termites can be grouped into lower and higher termites based on the presence or absence of flagellated protistan symbionts in the hindgut. Devoid of symbiotic protozoa, Termitidae are higher termites while the remaining six families are referred to as "lower termites". Higher termites represent over 80% of all termite species [START_REF] Yoon | Cellulase gene expression profiles in termites according to habitat and diet[END_REF]. Compared to lower termite, higher termite have diversified habitats and diets [START_REF] Brune | Symbiotic digestion of lignocellulose in termite guts[END_REF].

Termites of the subfamily Macrotermitinae, which appear to have originated in Africa [START_REF] Aanen | Fungus-growing termites originated in African rain forest[END_REF], are broadly known as termites that live in a complex symbiotic relationship with fungi of the genus Termitomyces [START_REF] Hinze | Polyethism in food processing and social organisation in the nest of Macrotermes bellicosus (Isoptera, Termitidae)[END_REF]. They are abundant in savannah ecosystems and directly mineralize up to 20% of carbon (Ohkuma, 2003). One of the specie, P. militaris, causes significant damage to sugar cane crops in Kenya, Takasdal, and Uganda (Valentin Dibangou, 2012).

Table C-1 Extant families of termites [START_REF] Lo | Termite phylogenetics and co-cladogenesis with symbionts[END_REF] Termites are eusocial insects and an individual separated from the colony will die [START_REF] Eggleton | An introduction to termites: Biology, taxonomy and functional morphology[END_REF]. Based on cast structure, termites can be classified into alates, queens, kings (reproduction), workers (construction), soldiers (defence), and immature forms (fortification).

This obligate association with the colony is mainly because each caste lacks some elements that are present in solitary insects, which lead to the proposition that a termite colony is a superorganism [START_REF] Eggleton | An introduction to termites: Biology, taxonomy and functional morphology[END_REF].

C.1.2 Anatomy

A normal termite has a head, thorax and abdomen. The termite gut generally consists of the foregut (stomatodeum), midgut (mesenteron), and hindgut (proctodeum). Both the foregut and hindgut are ectodermal and covered by cuticle whereas the midgut is of endodermal origin and its epithelial surface can be accessed by microorganisms [START_REF] Brune | The Gut Microbiota of Termites: Digesting the Diversity in the Light of Ecology and Evolution[END_REF]. The ingested lignocellulose underwent mechanical grinding in the foregut, which is composed of an enlarged anterior segment ("the crop") and a posterior segment ("the gizzard"). The endogenous lignocellulases were mostly secreted into the columnar midgut [START_REF] Eggleton | An introduction to termites: Biology, taxonomy and functional morphology[END_REF].

The hindgut absorbing gut fluid is more developed and compartmentalized in higher termites than in lower termites (Hongoh, 2011)(FigC-1cd). The hindgut from higher termite is further subdivided into P1 (ileum), P2 (enteric valve), P3 (paunch), P4 (colon) and P5 (rectum) segments (Watanabe and Tokuda, 2010)(FigC-1d). The elongated P3 compartment, is the major site of symbiosis and where oxygen content is steeply shifted from anaerobic (P1-3) to aerobic (P4-5) [START_REF] Brune | The termite gut microflora as an oxygen sink: Microelectrode determination of oxygen and pH gradients in guts of lower and higher termites[END_REF]. The hindguts in lower termites are relatively short, so as to that of Macrotermitinae that possesses "fungal gardens" (C1.4). The pH of foregut can be slightly acidic while that of midgut is circumneutral [START_REF] Brune | pH profiles of the extremely alkaline hindguts of soilfeeding termites (Isoptera: Termitidae) determined with microelectrodes[END_REF]. Regarding the hindgut, the P1, P2 and P3 are more alkaline than P4 and P5, with the highest pH over 12 (Bignell, 2010).

C.1.4 Digestive strategies

Termite is one of the most efficient lignocellulose-degrading systems (Sun et al., 2014). In terms of hemicellulose, up to 87% of it in the components of lignocellulose ingested can be dissimilated, which are more efficient than in human (up to 72%) (Ohkuma, 2003;[START_REF] Slavin | Neutral detergent fiber, hemicellulose and cellulose digestibility in human subjects[END_REF]. It has dual enzyme sources, one is from endogenous enzymes, the other is from the microbial inhabitants of the gut compartments [START_REF] Hongoh | Toward the functional analysis of uncultivable, symbiotic microorganisms in the termite gut[END_REF][START_REF] Nakashima | Dual cellulose-digesting system of the wood-feeding termite, Coptotermes formosanus Shiraki[END_REF][START_REF] Watanabe | Animal cellulases[END_REF][START_REF] Watanabe | Cellulolytic systems in insects[END_REF], which plays a major role in lignocellulose degradation [START_REF] Breznak | Role of microorganisms in the digestion of lignocellulose by termites[END_REF]Veivers et al., 2006).

In spite of the utilization of enzyme systems, the digestive strategies of termite also include the delicate procedures for degradation. The fungus-growing termites employed a series of procedures to achieve the pretreatment and almost complete degradation of lignocellulose [START_REF] Hinze | Polyethism in food processing and social organisation in the nest of Macrotermes bellicosus (Isoptera, Termitidae)[END_REF]Ohkuma, 2003). The plant litters collected by old workers are firstly pretreated by young workers via mandibles and gizzards and turned to be small fragments (10-20 µm in diameter) [START_REF] Brune | Symbiotic digestion of lignocellulose in termite guts[END_REF]. The resulting fecal pellets form combs allowing the growth of the symbiotic fungi, which decreases the lignin content therein [START_REF] Hyodo | Role of the mutualistic fungus in lignin degradation in the fungus-growing termite Macrotermes gilvus (Isoptera; Macrotermitinae)[END_REF]. The mature fungus nodules are then dissimilated by old workers to a great extent as final feces are rarely found [START_REF] Liu | Metagenomic Insights into Metabolic Capacities of the Gut Microbiota in a Fungus-Cultivating Termite (Odontotermes yunnanensis)[END_REF]. In all other higher termites, symbiotic digestion is independent of fungal symbionts. Instead, they rely on different community of bacteria that adapted with the host phylogeny and diet [START_REF] Brune | Symbiotic digestion of lignocellulose in termite guts[END_REF].

In lower termites, over 90% of lignocellulose can be assimilated benefiting from employing dual enzyme systems (Ohkuma, 2003). The biomass are partially degraded by endogenous cellulases in salivary gland, foregut and midgut, and then be endocytosed and fermented by the symbiotic systems, i.e., protists and bacteria in hindgut. The symbiotic bacteria play major roles in producing acetate from precursors [START_REF] Hogan | The site of cellulose breakdown in higher termites (Nasutitermes walkeri and Nasutitermes exitiosus)[END_REF]. Without the symbiotic system, the termites can't grow on lignocellulose [START_REF] Todaka | Phylogenetic analysis of cellulolytic enzyme genes from representative lineages of termites and a related cockroach[END_REF]. In addition, the combined host/symbiont effect is synergistic in glucose liberation and apparently additive in pentose release [START_REF] Scharf | Multiple levels of synergistic collaboration in termite lignocellulose digestion[END_REF].

C.2 Termite microbial enzymes

The termite gut is a rich reservoir of diverse microorganisms/mutualistic symbionts. The gut microbial community contains all three domains of organisms: Archaea, Bacteria, and Eukaryotes (protists) (Hongoh et al., 2008a). Relatively few microbes are found in the foregut and midgut, whereas the hindgut microbiota is densely populated [START_REF] Hongoh | Toward the functional analysis of uncultivable, symbiotic microorganisms in the termite gut[END_REF][START_REF] Köhler | High-resolution analysis of gut environment and bacterial microbiota reveals functional compartmentation of the gut in wood-feeding higher termites (Nasutitermes spp.)[END_REF]. In some cases no 16SrRNA genes were found in P3 lumen of Nasutitermes species (Warnecke et al., 2007). A better understanding of termite microbiota is facilitated by "omic" research, which has been reviewed by Scharf (Scharf, 2015a).

C.2.1 Omics studies in termite gut

Using classical microbiology it is impossible to characterize the genomes of most bacterial species, because 99.9% of microorganisms in environmental niches cannot be cultivated using standard laboratory techniques (Madhavan et al., 2017). However, the advent of metagenomics has furnished a way to surmount this problem thus provides a way to probe the genetic resources of complex microbial communities [START_REF] Ferrer | Metagenomics for Mining New Genetic Resources of Microbial Communities[END_REF].

As the term metagenomics suggests, the approach involves the isolation and characterization of the genomic patrimony of whole microbial communities. To exploit the information contained in so-called metagenomic libraries, which contain the fragmented metagenome of a given microbiota, one of several strategies can be deployed. First, to establish a repertory of the species present, it is possible to sequence 16s RNA genes using universal primer sets.

Second, one can go beyond this using high throughput sequencing to sequence all DNA. This allows large genome fragments and even whole genomes to be reconstructed and proteinencoding genes to be identified. Finally, to target specific functions, one can deploy a functional screening strategy. This involves the confrontation of individual metagenomic library clones with a functional screen (e.g. an enzyme assay to detect cellulase activities).

The advantage of this strategy is that positive clones can be fully sequenced and the protein responsible for the detected activity identified, thus establishing a link between sequence and function (Tasse et al., 2010).

C.2.2 Functional screening of metagenomic libraries

To perform functional metagenomics, DNA is isolated directly from a target sample, such as termite gut and then fragmented. The metagenomic libraries are constructed by the cloning of fragmented genomic DNA in various lengths using appropriate expression vectors, such as plasmids (<10 kb), cosmids (25-35 kb), fosmids (25-40 kb), or bacterial artificial chromosomes (100-200 kb) (Uchiyama and Miyazaki, 2009) and then are transformed to a surrogate host, such as Escherichia.coli, for expression of the desired genes. Instead of directly sequencing the libraries, they are firstly screened against different polysaccharides (usually chromogenic substrates). Positive clones then undergo another screening to pinpoint the glycosidic linkage specificity within clones that were able to target the same polysaccharide in the primary screens prior to in-depth pyrosequencing for putative enzymes annotation (Tasse et al., 2010). The main advantage of functional metagenomics is that it avoids unnecessary sequence analysis, especially when clones do not contain genes of interest (e.g. lignocellulase-encoding genes) (Hongoh et al., 2008a[START_REF] Hongoh | Genome of an endosymbiont coupling N2 fixation to cellulolysis within protist cells in termite gut[END_REF]. However, the drawbacks of functional screening is low success (hit) rate (Uchiyama and Miyazaki, 2009). This is linked to the fact that some genes may not be expressed in the host cell (perhaps because of poor promoter recognition, translational inefficiency, misfolding of proteins, defective posttranslational modification of desired proteins, etc), or that they might not be detected in the screen due to low activity or an inappropriate choice of substrate [START_REF] Lam | Current and future resources for functional metagenomics[END_REF]Madhavan et al., 2017).

Regarding the use of metagenomics for bioprospection in termites, many omic studies have been performed (c.f. Michael E. Scharf 2015). An article published in 2007 reported the results of a metagenomics study conducted on the gut microbiota of Nasutitermes corniger and revealed detailed taxonomic and functional data (Warnecke et al., 2007). This watershed study by Warnecke demonstrated that metagenomics is a powerful approach to probe the functionalities of termite microbiota. Importantly, this study established the fact that the gut microbiota of N. corniger is a rich reservoir of carbohydrate-active enzymes, since this study alone revealed 700 genes, or gene modules, that display homology to glycoside hydrolases (GHs), with more than 100 of these being putative cellulose or hemicellulose-active enzymes.

Since then, the termite gut has been a popular resource for probing new enzymes [START_REF] Tormo | were produced in house as His-tag fusion proteins. 2.4 Sample preparation materials[END_REF][START_REF] Brennan | Unusual microbial xylanases from insect guts[END_REF][START_REF] Liu | Microbiome of fungus-growing termites: a new reservoir for lignocellulase genes[END_REF][START_REF] Liu | Metagenomic Insights into Metabolic Capacities of the Gut Microbiota in a Fungus-Cultivating Termite (Odontotermes yunnanensis)[END_REF]. A huge quantity of enzymes aside, omics approacesh have also revelaled a variety of enzymes, such as cellulase, hemicellulase, ligninase [START_REF] Geib | Lignin degradation in woodfeeding insects[END_REF] and laccase [START_REF] Beloqui | Novel polyphenol oxidase mined from a metagenome expression library of bovine rumen: Biochemical properties, structural analysis, and phylogenetic relationships[END_REF].

C.2.3 Composition of bacterial groups

In higher termites, the microbial population housed in the hindgut is almost exclusively responsible for enzymatic degradation of lignocellulose. Regarding bacterial diversity in termite guts, for Reticulitermes speratus it was estimated that up to 740 phylotypes are present [START_REF] Hongoh | Molecular analysis of bacterial microbiota in the gut of the termite Reticulitermes speratus (Isoptera; Rhinotermitidae)[END_REF], which indicates that termite hindguts are extremely rich from a biodiversity standpoint. However, the community structure is typically uneven, with just a few numerically predominant phylotypes, associated with a large number of rarer phylotypes.

The dominant phyla in termite gut is quite dependent upon the host species, an observation that has led scientists to suggest that termite gut bacteria have coevolved with termites [START_REF] Hongoh | Intra-and Interspecific Comparisons of Bacterial Diversity and Community Structure Support Coevolution of Gut Microbiota and Termite Host Intra-and Interspecific Comparisons of Bacterial Diversity and Community Structure Support Coevolution of Gut Micro[END_REF]. To illustrate this point, it is noteworthy that the gut of Amitermes wheeleri (Termitidae family, subfamily Amitermitinae), a termite that forages on herbivore dung containing undigested fibrous plant biomass, is characterized by a predominance of Firmicutes and Spirochaetes. In contrast, in N. corniger (Termitidae family, subfamily Nasutitermitinae), Spirochaetes and Fibrobacteres dominate [START_REF] He | Comparative Metagenomic and Metatranscriptomic Analysis of Hindgut Paunch Microbiota in Wood-and Dung-Feeding Higher Termites[END_REF]. Likewise, in the termites Trinervitermes trinervoides (Termitidae family, Nasutitermitinae subfamily) and Pseudacanthotermes militaris (Termitidae family, subfamily Macrotermitinae) the dominant phyla are Firmicutes and Bacteroidetes (Bastien et al., 2013;[START_REF] Sanyika | Luminal hindgut bacterial diversities of the grass and sugarcane feeding termite Trinervitermes trinervoides[END_REF].

From a general standpoint, six major bacterial phyla are represented across higher and lower termites. These are Bacteroidetes, Firmicutes, Spirochaetes, Proteobacteria, Fibrobacteres, and Elusimicrobia [START_REF] Brune | Symbiotic digestion of lignocellulose in termite guts[END_REF]. Among them, Bacteroidetes are highly abundant in fungusgrowing termites, with clade Bacteroidale cluster V exclusively found in termites and cockroaches [START_REF] Brune | The Gut Microbiota of Termites: Digesting the Diversity in the Light of Ecology and Evolution[END_REF][START_REF] Noda | Complex coevolutionary history of symbiotic Bacteroidales bacteria of various protists in the gut of termites[END_REF][START_REF] Schauer | The bacterial community in the gut of the cockroach Shelfordella lateralis reflects the close evolutionary relatedness of cockroaches and termites[END_REF].

C.2.4 CAZyme of bacterial enzymes

The expression and distribution of lignocellulases in termite gut vary with the termite castes and developmental stages [START_REF] Fujita | Differences in cellulose digestive systems among castes in two termite lineages[END_REF][START_REF] Ni | Lignocellulose-degrading enzymes from termites and their symbiotic microbiota[END_REF]. Among 45 GH families in the hindgut microbial communities of Nasustitermes sp., 7 GH families (5,8,9,44,45,51 and 74) are putative EGs, whereas GH1 and GH3 might be acting as BGs (Warnecke et al., 2007). CAZyme bioprospecting in the gut of Coptotermes gestroi generated more than 12,000 ORFs with predicted functions related to carbohydrate metabolism, 587 of them encoding lignocellulases distributed in 18 GH families [START_REF] Do | Mining biomass-degrading genes through Illumina-based de novo sequencing and metagenomic analysis of free-living bacteria in the gut of the lower termite Coptotermes gestroi harvested in Vietnam[END_REF]. Potential CBHs or components of cellulosomes has not been identified from metagenomic studies [START_REF] Ni | Lignocellulose-degrading enzymes from termites and their symbiotic microbiota[END_REF]. It is worth noting that horizontal gene transfer is an important process in gut bacterial evolution and adaptation, and key enzymes probably evolved from a common ancestor and were conserved among the evolution notably concerning CAZymes production [START_REF] Ricard | Horizontal gene transfer from bacteria to rumen ciliates indicates adaptation to their anaerobic, carbohydratesrich environment[END_REF]. So far, the production of recombinant CAZymes of termite gut microbiotal origin has been reported in the literature. These are from 1 GH1, 27 GH5s, 12 GH9, 1 GH45, 1 GH10, 1 GH11, 1GH8 (Scharf, 2015b).

C.3 Endogenous CAZymes of termite

Initially, insects were thought to fully depend on the lignocellulases from symbiotic microbes until endogenous cellulases were found from orders Dictyoptera, Orthoptera, and Coleoptera [START_REF] Oppert | Prospecting for cellulolytic activity in insect digestive fluids[END_REF]. The first case was discovered in 1998 in the termite Reticulitermes speratus, which retained its ability to feed on wood even when its gut fauna were ablated [START_REF] Watanabe | A cellulase gene of termite origin[END_REF]. The endogenous EGs of termites showed high homology, implying that the endogenous genes were present before the establishment of host/symbionts effect [START_REF] Nakashima | Dual cellulose-digesting system of the wood-feeding termite, Coptotermes formosanus Shiraki[END_REF]. Compared to lower termites, the endogenous lignocellulases are more important for higher termite, in which case, the endogeneous cellulolytic activity were demonstrated to meet its metabolic requirements [START_REF] Zhou | Correlation of cellulase gene expression and cellulolytic activity throughout the gut of the termite Reticulitermes flavipes[END_REF] [START_REF] Ni | Lignocellulose-degrading enzymes from termites and their symbiotic microbiota[END_REF].

Compared to vast genomic data of termite gut bacteria, only two termite genome sequences are currently available (Scharf, 2015a). Mining endogenous termite enzymes relies mostly on separation from the host and metagenomic studies [START_REF] Husseneder | Transcriptome profiling of female alates and egg-laying queens of the Formosan subterranean termite[END_REF]. Compared to the CAZymes of gut bacteria, the endogenous termite enzymes are less diverse. All endogenous EGs are affiliated with the family GH9, while almost all endogenous BGs belongs to GH1 [START_REF] Ni | Lignocellulose-degrading enzymes from termites and their symbiotic microbiota[END_REF][START_REF] Watanabe | Cellulolytic systems in insects[END_REF].

C.4 The potential application in biomass degrading

Wood-chewing insects are miniature factories converting lignocellulose into sugars, which were used to sustain life rather than produce target products such as biofuels. The potential application of the digestive processes/strategies of termite in biomass degradation is striking as up to 99% of cellulose and 87% of hemicellulose can be saccharified [START_REF] Fischer | Cellulases from insects[END_REF]Ohkuma, 2003).

Being fascinated by the humble termite's ability to turn wood into energy for life, scientists are focusing on how to advance biochemical biomass processing in industry. Ke et al studied the chewing stage of termite, and proved that the termite mechanical chewing process combines with specific biological pretreatment on biomass, resulting in vitro in increased enzymatic cellulose digestibility [START_REF] Ke | Advanced biorefinery in lower termiteeffect of combined pretreatment during the chewing process[END_REF]. The mimicking of this process could make the pretreatment of biomass more efficient in industrial biomass conversion.

PART D INDUSTRIAL APPLICATIONS OF LIGNOCELLULOSIC ENZYMES

D.1 Biomass and biofuels

D.1.1 Defining the term Biomass in the context of the bioeconomy

World energy needs are currently met mainly by the use of fossil resources, in the form of coal, gas and oil. These resources result from processes that took place over millions of years and in specific circumstances, and which led to the fossilization of plant-based organic matter.

Fossil resources constitute massive reservoirs of carbon, which when used by human for energy purposes leads to the release of CO2 into the atmosphere. For well-known reasons the continued use of fossil resources is considered to be unsustainable and ultimately dangerous for the geoclimatic equilibrium on Earth.

The term biomass is widely used to describe organic matter produced by plants via photosynthesis. Moreover, with increased interest in the use of plant-based resources for industrial processes, the term biomass is often used to specifically describe lignocellulosic matter that is composed of cellulose, lignin and hemicelluloses [START_REF] Howard | Lignocellulose biotechnology: issues of bioconversion and enzyme production[END_REF](Table D1). This biomass is the major component of tress, herbaceous plants/grasses, aquatic plants and organic waste, such as manure. This biomass has always been a major source of energy for mankind and is presently estimated to contribute approximately 10-14% of the world's energy supply [START_REF] Mckendry | Energy production from biomass (part 1): Overview of biomass[END_REF].

Increasing interest in the use of biomass as a substitute raw material for oil, gas and coal has sparked interest in plants such as switchgrass and poplar trees, non-comestible species that are renowned for fast growth, high biomass yields, their capability to grow on marginal land and their relatively low use of water and nutrients [START_REF] Sarkar | Plant cell walls throughout evolution: Towards a molecular understanding of their design principles[END_REF]. These and other abundant biomass resources are now widely considered to be important raw materials for a future fossil-free economy, which is now designated by the term bioeconomy.

Table D-1: A wide variety of biomass resources for conversion into bioproducts [START_REF] Howard | Lignocellulose biotechnology: issues of bioconversion and enzyme production[END_REF].

D.1.2 Defining the term Biofuels

Using suitable chemical and/or biological operations, biomass can be converted into a whole range of chemicals including energy vectors such as methane, ethanol, butanol and fatty acid methyl esters (FAMEs). During the latter part of the 20 th century, much focus was put on the production of FAMEs and ethanol from biomass, with major R&D programs being financed in both the USA and Europe.

Ethanol and FAMEs are of particular interest to industry, because these energy vectors are liquid at ambient temperature and can be used in standard (albeit with minor adjustments) combustion engines. So-called 1 st generation biofuels are either ethanol or FAME-based liquid fuels that are currently used in vehicle fleets in Europe, the USA and Brazil. They are referred to as 1 st generation, because they were the first biofuels to be commercially produced using technologies that convert either vegetable oils (e.g. sunflower or rapeseed oil) into FAMEs, or sucrose (sugar cane or sugar beet) and starch-based (Maize and wheat grain) feedstocks into ethanol. Although the production of 1 st generation biofuels is commercially viable, concerns related to competition with the food chain have led to a recent decrease (at least in Europe) in their desirability and an increased ambition to transit to 2 nd generation biofuels.

Second generation biofuels (especially ethanol) are ones that are produced from noncomestible biomass, such as lignocellulosic biomass, which is highly abundant. Ethanol (10%, v/v) has been used in gasohol or oxygenated fuels since the 1980s in the USA [START_REF] Howard | Lignocellulose biotechnology: issues of bioconversion and enzyme production[END_REF]. Several studies have shown that 2 nd generation biofuels could be more sustainable (i.e.

better carbon life cycle performance and less competition with food) and that they can be made from a wide variety of biomass feedstocks, including coproducts from agriculture, forestry and industry and dedicated lignocellulosic crops. Nevertheless, as mentioned previously, lignocellulosic biomass is recalcitrant and thus less readily convertible into products of interest. Therefore, despite extensive research during the 20 th century, the commercial production of 2 nd generation biofuels remains to be achieved on a wide scale, even though some industrial-scale units operate in the USA.

D.2 Plant biomass biorefinery

D.2.1 The concept of biorefinery

Biorefining is an integrated facility that sustainably converts biomass into a range of valuable chemicals and power [START_REF] Cherubini | The biorefinery concept : Using biomass instead of oil for producing energy and chemicals[END_REF].

D.2.2 Biorefinery processes

Ethanol as a transportation fuel is currently being produced over 100 billion liters per year form the readily extractable sugars of corn and sugar cane [START_REF] Lopes | Ethanol production in Brazil: a bridge between science and industry[END_REF][START_REF] Sarkar | Plant cell walls throughout evolution: Towards a molecular understanding of their design principles[END_REF]. The production of bioethanol from lignocellulosic biomass requires the implementation of several steps: (1) production and pre-harvest of feedstock; (2) harvest and post-harvest logistics, including storage, pre-processing, handling and transportation to the bioethanol plant gate; (3) conversion to bioethanol [START_REF] Gnansounou | Production and use of lignocellulosic bioethanol in Europe: Current situation and perspectives[END_REF][START_REF] Lei | Hydrolysis of lignocellulosic biomass to sugars[END_REF]. Two main production routes have been identified to procure second generation bioethanol. The first using thermochemical operations, while the second relies heavily on biochemical processes. Much R&D focus has been put on the biochemical route because it is thought that progress in biotechnology will drive strong innovation and offer greater opportunities to achieve overall sustainability. Although the biochemical route is actually a general term that can be applied to a wide variety of processes, most of these are characterized by some key steps.

The biochemical conversion of biomass usually starts with some form of mechanical process (e.g. chopping, milling etc) that aims to reduce particle size and increase surface area and accessibility (Merino and Cherry, 2007). This is followed by what is known as pretreatment.

This is a critical step whose aim is to partially dismantle the macroscopic structure of biomass, increasing accessibility to enzymes and removing or modifying components, such as lignin and perhaps some hemicelluloses [START_REF] Öhgren | Effect of hemicellulose and lignin removal on enzymatic hydrolysis of steam pretreated corn stover[END_REF][START_REF] Wyman | BIOMASS ETHANOL: Technical Progress, Opportunities, and Commercial Challenges[END_REF]. Usually, the product of pretreatment is a cellulose-enriched biomass pulp. Pretreatment is followed by enzymatic deconstruction [START_REF] Alvira | Progress on enzymatic saccharification technologies for biofuels production[END_REF]. This step employs enzyme cocktails that breakdown down the polysaccharides components into sugars that are then suitable for fermentation by ethanologenic strains (most frequently Saccharomyces cerevisiae) [START_REF] Van Zyl | Consolidated bioprocessing for bioethanol production using saccharomyces cerevisiae[END_REF]. The final step is distillation, which furnishes the final product [START_REF] Lopes | Ethanol production in Brazil: a bridge between science and industry[END_REF].

D.2.3 Potential bioproducts and their applications

In the current petro-economy, over 75% of organic chemicals are produced from five primary oil-based chemicals (petro-platform molecules), which are ethylene, propylene, benzene, toluene and xylene. These are used to synthesize all other chemicals that are required by industry [START_REF] Howard | Lignocellulose biotechnology: issues of bioconversion and enzyme production[END_REF]. Except biofuels (D1.2), plant biomass biorefinery could also bring chemical and material products such as vanillin (4-hydroxy-3methoxybenzaldehyde), one of the most widely used and important flavouring materials (Zhang et al., 2016) and bio-platform molecules, the carbon-intermediate chemicals (building blocks) in producing value-added chemicals [START_REF] Cherubini | The biorefinery concept : Using biomass instead of oil for producing energy and chemicals[END_REF]. In comparison to petroplatform molecules, the bio-platform molecules are more variable, the top 12 of them are four carbon 1,4-diacids (succinic), 2,5-furan dicarboxylic aicd, 3-Hydroxy propionic acid, aspartic acid, glucaric acid, glutamic acid, itaconic acid, levulinic acid, 3-hydroxyburtyrolactone, glycerol, sorbitol, xylitol/arabinitol [START_REF] Werpy | Top Value Added Chemicals from Biomass[END_REF]. The added value products derived from succinate include 1,4-butanediol, tetrahydrofuran, γ-butyrolactone, adipic acid, nmethypyrrolidone and linear aliphatic esters [START_REF] Zeikus | Biotechnology of succinic acid production and markets for derived industrial products[END_REF].

D.2.4 Challenges in biorefinery

The recalcitrance of biomass, enzyme cost, low yields of fermentable sugars and undesired by-products are challenging issues in plant biomass biorefinery [START_REF] Mccann | Biomass recalcitrance: A multi-scale, multifactor, and conversion-specific property[END_REF]Merino and Cherry, 2007;[START_REF] Sarkar | Plant cell walls throughout evolution: Towards a molecular understanding of their design principles[END_REF].

Recent research addressing these challenges found that the introduction of hemicellulases can enhance ethanol production yields by decreasing competitive inhibition of cellulases by XOS and by providing a better accessibility of the cellulases to their substrates [START_REF] Zhang | Xylo-oligosaccharides are competitive inhibitors of cellobiohydrolase I from Thermoascus aurantiacus[END_REF][START_REF] Zhang | Xylanase supplementation on enzymatic saccharification of dilute acid pretreated poplars at different severities[END_REF]. To make enzyme deconstruction more economicallyviable, the following approaches have been considered: (1) novel enzymes bioprospecting; (2) strain and enzyme engineering; (3) optimizing production and operations related factors such as substrate, culturing conditions, enzymes recycling [START_REF] Howard | Lignocellulose biotechnology: issues of bioconversion and enzyme production[END_REF][START_REF] Knauf | Lignocellulosic biomass processing : A perspective[END_REF]. A new route to reduce this cost is consolidated bioprocessing (CBP)featuring cellulase production, cellulose hydrolysis and fermentation in one step using cellulolytic bacteria [START_REF] Lynd | Consolidated bioprocessing of cellulosic biomass: An update[END_REF]. In a recent work, Yarrowia lipolytica, a yeast strain that can produce biodiesel precursors, was engineered to metabolize cellulose, providing a new host platform for CBP [START_REF] Guo | Conferring cellulose-degrading ability to Yarrowia lipolytica to facilitate a consolidated bioprocessing approach[END_REF]. Investigating the multimodularity of a Xyn10C-like protein found in termite gut

ABSTRACT:

The functional screening of a the Pseudacantotermes militaris termite gut metagenomic library revealed an array of multi-modular enzymes, including a novel GH10 encoding sequence, Pm25. Sequence analysis showed details of the unusual domain organization of this enzyme. It consists of one catalytic domain, which is intercalated by two Carbohydrate Binding Modules (CBMs). Homologue of this enzyme sharing the same architecture, Xyn10C, was vastly distributed in different xylan utilization loci found in gut Bacteroidetes, which indicated its importance in the glycan foraging for the gut microbiota. In an effort to understand its unusual multi-modularity, detailed characterization of Pm25, including the catalytic domain GH10 separated from the CBM4, and CBM domain was performed. Here we show that the Pm25 is dedicated to the degradation of xylan. The binding results showed that the catalytic domain and CBMs act independently, and the tandem CBM4s act synergistically with each other and assist the release of D-xylose towards complex polysaccharide but not soluble polysaccharide. These findings contribute to a better understanding of the potential role of Xyn10C-like proteins in xylan utilization system of gut bacteria.

INTRODUCTION:

Xylan is the most abundant hemicellulose present in cell walls of higher plants, especially cereal grains and hardwoods [START_REF] Stephen | Other Plant Polysaccharides[END_REF]. Usually, xylan main chains are composed of β- from the reducing extremity (Merino et al., 2007).

According to the Pfam database (http://pfam.xfam.org/), 20-30% of β-D-xylanases are multidomain proteins, comprising catalytic domains associated with accessory, or helper domains such as carbohydrate binding modules (CBMs). These latter have been attributed various roles, including the ability to target specific regions in substrates (Bolam et al., 1998), disrupt polysaccharide structure (Din et al., 1994) or anchor enzymes to bacterial surfaces (Montanier et al., 2009). In the CAZy database (www.cazy.org, Lombard et al. 2014), carbohydratedegrading enzymes and CBMs are grouped into families that are defined on the basis of primary amino acid similarities and, by inference, structural relatedness (Cantarel et al., 2009). β-Dxylanases are mainly found in glycoside hydrolase (GH) families 5, 8, 10 and 11, with family 5, 10 and 11 members performing catalysis via a retaining mechanism (Collins et al., 2005).

The canonical 3D structure of GH5 and 10 families is a TIM barrel -(β/α) 8, which is the most commonly known (2077 occurrences) protein fold in the Protein Data Bank (PDB). Other xylan-degrading enzymes are found in GH families, such as GH43, 51, 62 (α-Larabinofuranosidase activities) and GH115 (α-glucuronidase activity). Regarding CBMs, these are grouped into 81 CAZy families, 15 of which contain members that bind to xylan and display β-sheet architectures.

Bioinformatics reveal that 60 and 80 % of prokaryotic and eukaryotic proteins respectively are formed from multiple domains, which are defined as the structural, functional or evolutionary units of proteins (Aroul-Selvam et al., 2004). In multi-domain proteins, individual domains can be regarded as biological equivalents of components in complex devices, whose parts can be interchanged. For protein engineers, multi-domain architectures are particularly interesting, because they provide clues about how new biocatalysts can be made. Mostly, domains in proteins are sequentially organized, with one domain following another one.

However, 9% of domain combinations are discontinuous, with one domain being inserted into another one.

Termites are wood-feeding animals that are considered to be an abundant source of biomass-degrading enzymes (Ohkuma, 2003). Although termites do not actually produce many endogenous biomass-degrading enzymes, their guts contain microorganisms that are responsible for their ability to capture nutrients and energy from biomass [START_REF] Brune | Woodworker's digest[END_REF][START_REF] Scharf | Termite digestomes as sources for novel lignocellulases[END_REF]. Over the last decade, this fact has underpinned numerous metagenomics studies that have aimed to characterize the enzyme arsenals of termite gut microbiomes and detect promising enzymes for industrial use (Bastien et al., 2013;[START_REF] Otani | Identifying the core microbial community in the gut of fungus-growing termites[END_REF][START_REF] Su | Comparative Gut Microbiomes of Four Species Representing the Higher and the Lower Termites[END_REF]Warnecke et al., 2007).

Detailed studies of gut ecosystems have revealed that gut symbionts deploy different strategies to capture nutrients. While members of the Firmicutes (Gram-positive) phylum deploy vast arsenals of extracellular enzymes (Gilbert, 2007), those of the Bacteroidetes (Gram-negative), the dominant phylum in many animal digestive systems (Bignell, 2010;[START_REF] Bryant | Bacteroides ruminicola n. sp. and Succinimonas amylolytica; the new genus and species; species of succinic acid-producing anaerobic bacteria of the bovine rumen[END_REF][START_REF] Chassard | Characterization of the xylan-degrading microbial community from human faeces[END_REF][START_REF] Dehority | Characterization of Several Bovine Rumen Bacteria Isolated with a Xylan Medium[END_REF][START_REF] Kudo | Termite-microbe symbiotic system and its efficient degradation of lignocellulose[END_REF], possess glycan utilization systems. The paradigm for this type of system is provided by the well-studied

Starch Utilization System (Sus). In Sus-like systems, several proteins encoded by a gene cluster (known as a polysaccharide utilization loci or PUL) act in a coordinated manner, binding and hydrolyzing complex sugars (Martens et al., 2009). Many PULs have been described, including a xylan utilization system (Xus) that is composed of two outermembrane polysaccharide-binding proteins (XusB and XusD), two transporter proteins (XusA and XusC) and two outer-membrane proteins (XusE and Xyn10C) (Dodd et al., 2011). Each of these proteins is expressed from a cluster of tandem genes that are organized as follows:

xus A/B/C/D (or sometimes only xus C/D), followed by xus E and xyn10C, the latter encoding a CBM-containing GH10 β-D-xylanase (Dodd, Mackie, and Cann 2011). According to previous data, the CBMs in Xyn10C are inserted into the polypeptide sequence of the GH10 catalytic domain, between structural elements β3 and α3 (Flint et al., 1997). The expression of Xyn10C was shown to be induced by xylan (Despres et al., 2016) and so were the other xylanases XynA and XynB, the most effective inducer of which is demonstrated to be with a degree of polymerization (DP) similar to the hydrolysates of Xyn10C (around 35) [START_REF] Miyazaki | Medium-to large-sized xylooligosaccharides are responsible for xylanase induction in Prevotella bryantii B14[END_REF], consistent with the hypothesis that Xyn10C might serve as a functional homologue of the Bacteroides thetaiotaomicron VPI-5482 SusG protein, initiating xylan metabolism through extracellular hydrolysis of polymeric substrates (Dodd et al., 2011). In this regard, it has been proposed that Xyn10C could be used as a functional marker of xylan degradation in the human gut (Despres et al., 2016;Dodd et al., 2011;Rogowski et al., 2015).

The potential roles and distributions of the Xyn10C have recently attracted considerable attention, but have not yet been fully described (Despres et al., 2016;Flint et al., 1997;Rogowski et al., 2015;Zhang et al., 2014).

Previously, a putative xus locus assigned to the genus Bacteroides was identified in a metagenomic library from the microbiome of a fungus-growing termite, Pseudacanthotermes militaris (Bastien et al., 2013). This xus is composed of eight different ORFs encoding putative XusC/D-like proteins, GH10|CBM4, GH115, GH11, a putative transporter protein, GH10 and GH43 (Fig. 1A). The GH10|CBM4, designated hereafter as Pm25, presents an insertional modular structure, homologous to Xyn10C protein (Flint et al., 1997;[START_REF] Mcnulty | Effects of Diet on Resource Utilization by a Model Human Gut Microbiota Containing Bacteroides cellulosilyticus WH2, a Symbiont with an Extensive Glycobiome[END_REF]Zhang et al., 2014;Rogowski et al., 2011;Despres et al., 2016). Here we describe the characterization of Pm25 and discuss its activity with respect to its unusual multi-domain organization. pNPX4) were all purchased from Megazyme (Bray, Ireland). Cellulose nanocrystals from cotton linters were prepared as previously described (Martinez et al., 2015).

MATERIAL AND METHODS:

Materials

Oligonucleotide primers were purchased from Eurogentec (Liège, Belgium) (Table S1).

Sequence analysis

The Genbank accession number for the clone containing Pm25 is HF548280.1, and the protein ID for Pm25 is CCO21036.1. Putative signal peptide sequence analysis was performed using SignalP 4.1 server [START_REF] Petersen | SignalP 4.0: discriminating signal peptides from transmembrane regions[END_REF]. The domain annotation of Pm25

Construction of sequence similarity networks (SSNs)

Amino acid sequences of GH10 family members were extracted using InterPro (https://www.ebi.ac.uk/interpro/) on May 17, 2017, and the entry number is IPR001000. To remove redundant sequences, the 8339 sequences were winnowed to 5068 by a sequence identity cut-off of 0.95 [START_REF] Li | Cd-hit: A fast program for clustering and comparing large sets of protein or nucleotide sequences[END_REF] and length cut-off of 250 (Gerlt et al., 2015).

SSNs were constructed using the Enzyme Function Initiative-Enzyme similarity Tool (EFI-EST) (Gerlt et al., 2015) and visualized using Cytoscape 3.4 [START_REF] Shannon | Cytoscape: A software Environment for integrated models of biomolecular interaction networks[END_REF]. The proteins in one node share over 60% identity and the edge between two nodes indicates that they share over 35% identity (the e-value threshold is10 -47 ).

Expression, purification and site-directed mutagenesis of wild type

Pm25 and its truncations

Cloning of the plasmid pDEST17 containing the Pm25-encoding sequence was described in detail elsewhere ( G. Arnal et al. manuscript in preparation, (Vincentelli and Romier, 2013).

All mutants were constructed using the QuikChange Site-Directed Mutagenesis Kit (Strategene, La Jolla, CA, USA) and pairs of complementary oligonucleotide primers (Table S1). The M6 construct, which corresponds to Pm25 deprived of its CBMs, was obtained by gene synthesis. The mutation of E546 to A in M6 yielded M7, while M8 and M9 were constructed by ligating the sequences encoding CBM4-1 and CBM4-2 respectively to pET28a (+) expression vector. Likewise, the construct M10 is the pET32a (+) expression vector containing the sequence encoding both CBM4-1 and CBM4-2 cloned in frame with the thioredoxin tag using the In-Fusion cloning kit (Clontech, Takara, Shiga, Japan). Wild type Pm25 and the mutants M1, 2, 3, 4 and 5 (Fig. 4A) were expressed in Escherichia coli Rosetta TM (DE3) pLysS grown in ZYP medium at 25°C overnight. Constructs M6 to M10

were transformed into E. coli Tuner TM (DE3) and cultured 2 hours at 37°C until the OD600 reached 0.6. At this point, IPTG (200 µM final concentration) was added and growth was pursued at 16°C overnight. Cell pellets were collected by centrifugation, washed and lyzed using sonication (Fisherbrand TM Q700; tip diameter, 13mm; output, 40W) and the clarified cell lysate was applied to TALON® metal affinity resin. After elution, protein purity was estimated by SDS/PAGE to be 95%. Protein concentrations were determined by measuring absorbance at 280 nm and applying the Lambert-Beer equation. Theoretical molar extinction coefficients were calculated using ProtParam online software (Gasteiger et al., 2005).

Determination of pH and temperature optima

The apparent optimal pH of Pm25 was determined in the pH range 3.0 to 11.0, measuring the enzyme activity (35 mg/L final enzyme concentration) on 1% (w/v) beechwood GX at 37°C. The buffers used were 50 mM citrate buffer for pH 3.0 to 6.0, 50 mM phosphate buffer for pH 6.0 to 8.0, 20 mM bicine buffer for pH 8.0 to 9.0 and 20 mM glycine-NaOH for pH 9.0 to 11.0. Xylanase activity was determined by measuring the release of reducing sugars using the 3,5-dinitrosalicylic acid (DNS) assay (Arnal et al., 2014;Miller, 1959). Reactions were performed in triplicate at 37°C in the different buffers from pH 3.0 to 11.0, containing bovine serum albumin (BSA, 1mg/ml). At regular intervals (0, 3, 6, 9, 12, 15, 18, 21, and 24 min), 100 µl of the reaction were removed and added to 100 µl of DNS and kept on ice until all samples were ready. After, all samples were heated at 95°C for 10 min and cooled on ice, before adding 1 ml of deionized water and recording the absorbance at 540 nm using a spectrophotometer. A D-xylose series (0 to 1 mg/ml) was used to prepare a standard curve.

The apparent optimal temperature was determined over the range 21 to 90°C in 50 mM phosphate buffer (pH 7.5). Thermostability was monitored by pre-incubating the enzyme in the absence of substrate in 50 mM phosphate buffer (pH 7.5) at 45, 50, 55 and 60°C from 0 to

Enzyme specificity and kinetics

The final concentration of Pm25 used to degrade beechwood GX and other soluble substrates is 35 and 7 mg/L, respectively. Regarding the measurement of reaction kinetics on polysaccharides, initial rates (the concentration of D-Xyl equivalent released in mg/ml/min) were determined using a range of substrate concentrations (0.5 to 40 g/L beechwood GX and 0.25 to 10 g/L for other soluble substrates) under optimal conditions. The DNS assay was used to monitor reducing sugar release as described earlier. The kinetic parameters (kcat and apparent KM) were calculated using non-linear regression in SigmaPlot (Systat Software, San Jose, CA). One unit of xylanase activity was defined as the amount of enzyme that catalyses the release of 1 µmol of D-Xyl equivalents per min.

To study the hydrolysis of XOS (X4-X6), reactions were performed using various XOS concentrations (0.05 to 0.8 mM) and the optimal reactions conditions. Assays began upon the addition of enzyme, its final concentration being fixed to account for the nature of the substrate. Accordingly, 2.60, 0.26 and 2.60 µM enzyme was used for X4, X5, and X6 respectively. At regular intervals (0, 5,10,15,20,30,40,50, and 60 min) aliquots were removed and immediately heated at 95°C for 10 min to stop the reaction. The hydrolyzed products were then analyzed by high-performance anionic-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) using an ICS 3000 dual (Dionex, France) equipped with Carbo-Pac PA-100 guard and analytical column (2*50mm and 2*250mm, respectively) as described before (Arnal et al., 2014). 10 µl of sample were injected, and separation was achieved by applying a gradient of 0 to 85 mM sodium acetate, 150 mM NaOH from 0 to 30 min, isocratic elution with 500 mM sodium acetate 150 mM NaOH from 30 to 33 min, then re-equilibrate the column with 50 mM sodium acetate, 150mM NaOH for another 10 min at a flow rate of 0.25 ml/min. Calibration was achieved using D-Xyl and XOS (X2, X3, X4, X5 and X6) at concentrations from 5 to 50 µM. Plotting hydrolysis rate (µM/min) versus oligosaccharide substrate concentration (µM) yielded a linear relationship meaning that the catalytic constant kcat/KM could be calculated from the slope k using Eq. ( 1) where [E] is the final concentration of enzyme.

k = kcat/KM*[E] (1)
All experiments were performed in triplicate, and reported values are the means of three experiments.

Determination of subsite affinities

The binding affinities of glycon subsites was calculated using Eq. ( 2) [START_REF] Matsui | Subsite structure of Saccharomycopsis alpha-amylase secreted from Saccharomyces cerevisiae[END_REF][START_REF] Song | Impact of an N-terminal extension on the stability and activity of the GH11 xylanase from Thermobacillus xylanilyticus[END_REF].

𝐴 -𝑖 = 𝑅𝑇 4183 ln ( 𝑘 𝑐𝑎𝑡 𝐾 𝑀 ) (𝑝𝑁𝑃𝑋 𝑖 ) ( 𝑘 𝑐𝑎𝑡 𝐾 𝑀 ) (𝑝𝑁𝑃𝑋 𝑖-1 ) (𝑘𝑐𝑎𝑙 • 𝑚𝑜𝑙 -1 ) (2)
where A-i is the subsite affinity at -i subsite, kcat/KM of pNPXi is the performance constant for pNP-labeled XOS with a DP of i (where i is a whole number) and R is the universal gas constant (8.314 J mol -1 K -1 ).

To determine the catalytic parameters of reactions containing pNP-XOS, the final concentration of Pm25 used was 6, 27 and 136 nM for pNPX4, pNPX3, and pNPX2 respectively.

Similarly, the final concentration of M6 was 10 nM for pNPX4 and pNPX3, and 54 nM for pNPX2. The concentration range of substrate was 0.025, 0.05 and 0.1 mM for pNPX3 and pNPX4, and 0.5, 2, and 5 mM for pNPX2. All experiments were performed in duplicate. The plot of hydrolysis rate against pNP substrate concentration was linear, which indicated that substrate concentration was far below KM. Therefore, the kcat/KM of reactions containing aryl βxylosides was determined under optimum condition using equation ( 3) (Charnock et al., 1999;[START_REF] Matsui | Subsite structure of Saccharomycopsis alpha-amylase secreted from Saccharomyces cerevisiae[END_REF]. Briefly, the substrate concentrations at the beginning of the reaction ([S0])

and at specific times ([St]) were fitted to equation (3) where k = (kcat/KM) [Enzyme]. And

[Enzyme] is the final concentration of enzyme.

𝑘 = ln[𝑆 0 ]/ [𝑆 𝑡 ] (3)
The molar extinction coefficient of pNP (15 570 M -1 cm -1 ) was determined experimentally by measuring the absorbance at 404 nm for a standard curve ranging from 0 to 0.12 mM of pNP at pH 7.5 and 50°C.

Affinity gel electrophoresis (AGE)

The binding of CBM4-1 and CBM4-2 to soluble polysaccharides was evaluated by affinity gel electrophoresis, using 7.5% (w/v) acrylamide gels containing various amounts of polysaccharide (for the concentration range of RAX refer to Table S2). AD-, ED-and LVWAX samples, and beechwood GX were used at 0.006 to 0.06% (w/v), while other polysaccharides were used at 0.5% (w/v). Pure protein (6 µg) was migrated (10 mA/gel for about 1 h at room temperature) on gels in 25 mM Tris, 250 mM glycine buffer, pH 8.3. BSA (15 µg) was also included in the experiment as a negative, non-interacting control. Proteins were visualized by Coomassie Blue staining. The dissociation constant Kd was calculated as previously described [START_REF] Takeo | Affinity electrophoresis: Principles and applications[END_REF]. In Eq ( 4), ( 4) R0 is the relative protein migration distance compared to that of BSA in the control gel (without ligand). The variable r is the relative protein migration distance compared to BSA in ligand-containing gels. Rc is the relative protein migration distance of complex between protein and ligand. c is the concentration of ligand. When Eq (4) was plotted taking 1/ (R0 -r) as the ordinate and 1/c as the abscissa, a straight line was obtained. The intercept of the line on the abscissa provided a negative reciprocal value of the dissociation constant (-1/K). All experiments were performed in triplicate, and reported values are the means of three experiments.

MicroScale electrophoresis (MST)

MST is a novel biophysical technology that is employed to study interactions between biomolecules (Wu et al., 2017). To fluorescently label the amine groups of exposed lysines (Fig. 1D) lying in the vicinity of the ligand binding clefts in M7, 8 and 9, 100 µl of pure proteins (20 µM) were treated with the reagents in the protein labeling kit (RED-NHS),

following the manufacturer's instructions. The labeled proteins were recovered and purified using TALON® metal affinity resin and the concentration of the labelled proteins was estimated using SDS/PAGE and serial dilutions of a protein solution of known concentration.

A total of 16 dilutions (350 to 11 mM) of a solution of X6 containing either 0.075 µM of M7, 0.13 µM of M8 or 0.07 µM M9, in 50 mM phosphate buffer pH 7, 0.05% Pluronic ® F-127 were loaded on to 16 standard capillaries. The initial fluorescence of all 16 samples was obtained by performing a capillary scan with LED power of 25% for M8, and 20% for M7 and M9. The dissociation constant Kd was calculated by selecting the tab "Initial Fluorescence Analysis Set " in the Affinity Analysis software (Jerabek-Willemsen et al., 2014). To perform the SDS-denaturation (SD)-test, 10 µl of samples 1-3 and 14-16 were mixed with 10 µl of 4% SDS, 40 mM DTT after 10 min centrifugation at 15,000 g, followed by 5 min incubation of the mixture at 95°C to denature the protein. Then the samples were loaded into the capillaries to measure their fluorescence intensity.

0 0 1 1 (1 ) c K R r R R c    

Solid depletion assay

The ability of inactivated M1, 7, 8 and 9 to bind wheat bran was investigated by incubating 100 µg of protein with 4 mg of wheat bran in 200 µl of reaction buffer (50mM sodium phosphate, pH 7). Reactions were performed in 0.2 ml PCR tubes, and incubated at 10°C for 2h with agitation in an Eppendorf Thermomixer™ R at 1400 rpm. For each reaction, the supernatant containing the unbound enzyme fraction was recovered after centrifugation using a bench top microcentrifuge. The pelleted substrate was washed 3 times with reaction buffer.

Finally, 20 µl of Laemmli sample buffer were added to the pellet and heated at 95°C for 10 min to denature the protein (bound fraction). All the fractions were verified by SDS-PAGE.

BSA was used as a negative control.

Hydrolysis of wheat arabinoxylan and wheat bran

The product profile were generated by Pm25, M3, 4, 5 and 6 on either LVWAX (0.5% (w/v)) or wheat bran (20 mg/ml of wheat bran pre-hydrated for 12 h at 37°C, 1400 rpm using the Eppendorf Thermomixer™ R). The enzymes (final concentration 0.5 µM) were incubated with the respective substrate in 50 mM phosphate buffer (pH 7.5) and 1 mg/ml of BSA.

Enzymatic reactions were incubated at 37°C for either 24 h for LVWAX or 14 h for wheat

RESULTS:

Bioinformatic analysis of the Pm25-encoding gene sequence

Analysis using SignalP 4.1 revealed that the first 32 amino acids of Pm25 probably constitute a signal peptide. Moreover, further analysis of the sequence revealed that the catalytic domain of Pm25 (amino acid residues 66 to 151, and 514 to 753) contains two putative tandem CBM4s (CBM4-1, residues 161 to 321 and CBM4-2, residues 324 to 486) that constitute insertional domains . Alignment of the amino acid sequence of Pm25 with those of other GH10 family members for which structural data is available revealed that CBM4-1 and CBM4-2 are inserted between β3 and α3 (Fig. 1B). Moreover, this alignment allowed the identification of E546 and E663 to be putative catalytic acid/base and nucleophile, respectively. Alignment of the amino acid sequences of CBM4- this similarity and previous reports, it was possible to assign possible ligand binding role to residues Y213, Q216, and Y257 in CBM4-1, and Y378, Q381, and Y422 in CBM4-2 [START_REF] Alahuhta | The unique binding mode of cellulosomal CBM4 from clostridium thermocellum cellobiohydrolase A[END_REF][START_REF] Alahuhta | Structure of CBM4 from Clostridium thermocellum cellulase K[END_REF]Boraston et al., 2002b;[START_REF] Fisher | Neutron Crystallographic Studies Reveal Hydrogen Bond and Water-Mediated Interactions between a Carbohydrate-Binding Module and Its Bound Carbohydrate Ligand[END_REF][START_REF] Kormos | Binding Site Analysis of Cellulose Binding Domain CBD N1 from Endoglucanse C of Cellulomonas fimi by Site-Directed Mutagenesis[END_REF][START_REF] Schantz | Structural basis for carbohydrate-binding specificity-A comparative assessment of two engineered carbohydrate-binding modules[END_REF] (Fig. 1C).

SSN analysis

To identify Pm25 homologs and study their distribution, a SSN was created for all GH10 sequences (5068 non-redundant sequences) present in the InterPro database (Fig. 2). Archaea, Eukaryota and Bacteria contribute 1, 33, and 65 % respectively of GH10s, the remaining 1% being unassigned sequences. Bacterial GH10s belong mainly to four bacterial phyla, Actinobacteria (20%), Firmicutes (13%), Bacteroidetes (11%), and Proteobacteria (11%).

The SSN showed five main clusters, numbered following each cluster size (Fig. 2). 61% and 35% of Eukaryotic GH10s are grouped in cluster 1 and 2, respectively. Archaeal GH10s are mostly gathered together in cluster 2 closer to Eukaryotic ones. Although bacterial sequences are present in all the 5 clusters, most of GH10s from Actinobacteria (92%), Firmicutes (83%), and Proteobacteria (75%) are grouped in cluster 1 while 45% of GH10s from Bacteroidetes are grouped in cluster 1 and 30% are clustered in cluster 4. Interestingly, cluster 4 apart from one unclassified sequences (UniprotKB: A0A0H5Q2Q6), comprises only Bacteroidetes sequences including Pm25. In this cluster that is composed of 79 nodes, 48 of them display two insertional CBMs, rather like Pm25, and 29 contain just one insertional CBM. Regarding the two remaining nodes in the cluster, one lacks GH10a (i.e. the N terminal part of GH10, UniprotKB: W7XZD0) and the other corresponds to a single GH10 catalytic domain (e.g.

UniprotKB: A0A1H3ZQ12). Furthermore, all the GH10s with insertional architecture based on InterPro domain classification were found in the cluster 4. In complement, a separate SSN analysis of the 589 bacterial non-redondant CBM4s revealed a high level of scattering, no group formed containing CBM4s in Pm25 probably due to poor sequence similarity (data not-shown), reinforcing that the cluster 4 (Fig. 2) is grouped based on the GH10 catalytic domain.

Regarding the genetic context of the Bacteroidetes GH10-encoding sequences (in cluster 4), 73 out of the 79 nodes are consecutive to hypothetical susC-susD-(unk) gene clusters, meaning that these are likely to be PUL components. The search for CBM4-containing GH10 enzymes in the extended PULDB (http://www.cazy.org/PULDB/) further confirmed that Pm25 homologs are always located consecutive to susC-susD(-unk) clusters.

Optimal pH and temperature

The optimal pH of Pm25 was determined using beechwood GX as the substrate. The purified enzyme retained greater than 80% of its activity in the pH range from 4.5 to 9.0 (Fig. 3A). The activity was also measured at temperatures from 50 to 75°C, with maximum activity being observed at 60°C (Fig. 3B). However, since Pm25 was rather unstable at 60°C, thermostability was measured at both pH 7.5 and pH 9 (Fig. 3C and 3D respectively). Pm25 remained fully active for over 24 hours at either 50 °C pH 7.5 or 45 °C pH 9. Overall, optimal conditions for routine Pm25 assays were defined as pH 7.5 and 50°C.

Enzyme assays and Kinetic Analysis

Determining the hydrolytic activity of Pm25 on various substrates revealed that it is 3 to 4fold more active on arabinoxylans than beechwood GX, indicating that the latter is the least suitable substrate among those tested (Table 1). When comparing the relative activities of Pm25 on different arabinoxylans, no significant differences were detected, although RAX, which possess more O-3 substitutions [START_REF] Comino | Characterisation of soluble and insoluble cell wall fractions from rye, wheat and hull-less barley endosperm flours[END_REF], qualifies as the best substrate tested. Testing the activity of the inactive mutants (M1 and 2, Fig. 4) on different substrates revealed that they both displayed much lower (2 to 3 orders of magnitude) activity when compared to Pm25 (Table S3).

Hydrolysis product analysis with XOSs

HPAEC-PAD analysis of reactions containing different XOS and Pm25 revealed that the hydrolysis of X6 (after 60 min) produced a mixture of X2, X3 and X4 in a ratio of 1:2:1 (Fig. 5A). Likewise, the hydrolysis of X5 led to the production of X2 and X3 as major products (Fig. 5B), while the hydrolysis of X4 yielded X1 and X3 (Fig. 5C). Moreover the activity of Pm25 was directly correlated with the DP of the XOS used, with higher DP XOS leading to higher activity (Table 1). This suggests that the Pm25 substrate binding cleft is quite large and can accommodate at least six xylosyl residues. To further study binding cleft subsite interactions, reactions were performed using Pm25 and different aryl-β-xylosides. Accordingly, the efficiency constant (kcat/KM) obtained when using pNPX4 as substrate was approximately 2.3-fold greater than that measured when using pNPX3 (Table 1). These data provided the basis to calculate the binding affinity at -4 subsite, revealing a value of 0.53 kcal/mol. Likewise, the -3 subsite binding affinity is 2.76 kcal/mol, which correlates with the fact that the kcat/KM value for the hydrolysis of pNPX3 is over 70-fold higher than that measured for the hydrolysis of pNPX2 (Table 1). Furthermore, the use of construct M6 (i.e.

Pm25 devoid of CBMs) (Fig. 4) to hydrolyze aryl-β-xylosides revealed similar affinities at subsites -4 and -3 (0.75 and 2.69 kcal/mol, respectively), inferring that the two CBM4 domains do not influence catalysis when using these substrates.

Measuring CBM ligand specificity

To probe the function of CBM4-1 and -2, various constructions M1, M7, M8, M9 and M10

were designed and expressed (Fig. 4) and affinity gel electrophoresis experiments were performed. Since the results of bioinformatics analysis putatively assign CBM4-1 and -2 to CBM family 4, initial tests were performed on xylan and glucan, before performing complementary tests with other polysaccharides (Table 2). All of the proteins displayed strong affinity with the different xylans tested. However, very low or no affinity was detected for βglucan and xyloglucan and none of the proteins bound to arabinan, nanocellulose and galactomannan (Table 2). Importantly, in the presence of different xylans the Kd of M10 (CBM4-1 and -2 in tandem) was much lower than that of M8 (CBM4-1 alone) and M9 (CBM4-2 alone), suggesting that there might be cooperativity between the two CBMs.

Comparing the behavior of the inactive mutants M1 and M7 with that of M10 revealed that the affinity of the inactive Pm25 M1 for xylans was similar to that exhibited by the M10, CBM4-1/-2 tandem, whereas the Kd values obtained for the inactive enzyme devoid of CBMs (i.e. M7) were significantly higher. This suggests that xylan binding is largely driven by the two CBM4 domains. Comparing M8 and M9 revealed that M9 systematically exhibited lower (1.4 to 1.8 times) Kd values for tests involving xylans, inferring that its binding ability is stronger (Table 2). However, the results obtained with other polysaccharides indicate that CBM4-1 weakly binds to xyloglucan, while M9 (CBM4-2 alone) does not (Fig. S1). Overall, the results suggest that binding specificities of CBM4-1 and -2 are not identical.

To further probe the ligand binding ability of the two Pm25-associated CBM4 domains, different residues putatively involved in ligand binding were mutated. Accordingly, the binding abilities of both CBM4-1|Y213A and CBM4-2|Y378A were lost, confirming that these residues play essential roles in the CBM-ligand interaction (Fig. 6). The binding abilities of CBM4-1|Y257A and CBM4-2|Y422A were also diminished, but ligand interactions were still observable, indicating that these tyrosines play less critical roles compared to Y213 and Y378 respectively (Fig. 6A andB). Likewise, other mutants such as CBM4-1|Q216A and CBM4-1|W259A, and CBM4-2|Q381A and CBM4-2|W424A also diminished to some extent ligand binding, but the mutation of asparagines (N218, N261 in CBM4-1 and N383 in CBM4-2) had no apparent effect on binding althought these residues are close to the essential ones

The determination of Kd values for the interaction of M7, M8 and M9 with X6 was achieved using MST. The sigmoidal titration curves were used to calculate Kd values (Fig. S2). The Kd values obtained when using M7, 8 and 9 were 1.5 ±0.2 mM, 4.7 ± 0.4 mM, and 1.8 ± 0.1 mM, respectively. Recalling that M7 is devoid of CBM domains, it is noteworthy that this construction displayed the highest binding ability, with M9 (i.e. CBM4-2) exhibiting a similar ligand binding ability.

Finally, substrate depletion experiments performed using M1, 7, 8 and 9 on wheat bran revealed that M8 and M9 (CBM4-1 and CBM4-2 respectively) exhibited binding ability.

Similarly, M1 (i.e. Pm25|E546A) was also able to bind to wheat bran despite its catalytic impotency (Fig. S3). However, this was not the case for M7 (i.e. the inactivated catalytic domain alone) clearly demonstrating the role of the CBM in the binding.

Analysis of polysaccharide and wheat bran hydrolysis

HPAEC-PAD of soluble polysaccharide hydrolysis mediated by Pm25 and its variants M5 and M6 failed to reveal significant differences in D-Xyl and XOS release (Fig. 7 andS4). This infers that the CBM4 domains do not enhance degradation of soluble polysaccharides, since M6 is devoid of CBM4 domains. Performing a similar analysis using wheat bran as the substrate revealed that (after 14h incubation) D-Xyl and XOS release by Pm25 was approximately twice that of the variants M3, 4, 5 and 6 (Fig. 8). Significantly, in this experiment the consequences of the point mutations CBM4-1|Y213A and CBM4-1|Y378A

were approximately equivalent to those produced by the ablation of the two CBM4 domains.

DISCUSSION:

Unlike the vast majority of multi-modular enzymes that display a sequential arrangement of their modules, the enzyme described herein is characterized by a discontinuous organization that involves the insertion of two CBM domains into one GH10 xylanase domain. In this regard, it is significant that the SSN analysis performed using the amino acid sequence of M6 replacing Pm25 located the sequence within the same cluster, even though the CBMs were omitted. This infers that the Pm25 GH10 domain forms part of a distinct group and implies that the intercalated GH10 arrangement is robust from an evolutionary standpoint. Moreover, the biochemical data described herein demonstrate that, despite its discontinuous organization, Pm25 is a fully functional xylanase.

The first Pm25 analog was identified in a rumen-based member of the Bacteroidetes phylum (Flint et al., 1997). More have since been found in human gut bacteria (Despres et al., 2016;Dodd et al., 2010;Rogowski et al., 2015), with Pm25 being the first described in termite gut.

Several studies have revealed the importance of Pm25-like GH10 in xylan utilization systems (Despres et al., 2016;Flint et al., 1997;Rogowski et al., 2015;Zhang et al., 2014). Using SSN analysis, we have shown that Pm25-like xylanases are exclusively linked to Bacteroidetes and are almost always (73 out 79 cases studied) adjacent to a susC-susD-(unk) cluster. This evidence of strong conservation is consistent with the fact that in their native host the genes encoding Pm25 homologs are highly induced/expressed during growth on xylan (Despres et al., 2016;Dodd et al., 2010). Taken together, one can conclude that Xyn10C-like proteins are essential for xylan utilization by members of the Bacteroidetes phylum in the gut ecosystem.

The in vivo function of Pm25 homologs in gut Bacteroidetes has not yet been fully established,

although it has been suggested that it may be a functional homolog of SusG (Dodd et al., 2011).

SusG is a cell surface-bound GH13 α-amylase that catalyzes the initial cleavage of polysaccharides (Shipman et al., 1999). In our study, we also predicted that Pm25 bears a Nterminal signal peptide that might direct it to the cell surface, consistent with a proposal that was previously made for a Pm25 homolog (Rogowski et al., 2015). Moreover, SusG displays negligible activity compared to periplasmic α-amylases (Shipman et al., 1999), an observation that is consistent with our findings (unpublished). Indeed, when compared to other xylanases [START_REF] Chakdar | Bacterial xylanases: biology to biotechnology[END_REF]Pell et al., 2004a), both Pm25 and its alike display quite poor catalytic efficiency towards polysaccharides (Wang et al., 2016a;Zhang et al., 2014) and oligosaccharides (Rogowski et al., 2015). This trend is also observed in other polysaccharide degrading systems, such as mannan utilization loci from members of the Bacteroidetes phylum (Bågenholm et al., 2016) and the xylan degrading system in Proteobacteria (Pell et al., 2004b) phylum. The underlying reason for such low activity no doubt reflects its function. SusG-like proteins probably play a carbohydrate surveillance function, while highly active intracellular enzymes are charged with complete oligosaccharide breakdown prior to sugar metabolism. This clever and "selfish" strategy ensures that readily metabolizable sugars are not released into the environment where they can be used by other bacteria that lack specific glycan utilization machinery [START_REF] Martens | The devil lies in the details: How variations in polysaccharide fine-structure impact the physiology and evolution of gut microbes[END_REF].

Remarkably, we found that Pm25 remains active over a broad pH range, maintaining more than 80% of its maximum activity at pH 9.0. This observation correlates well with results obtained for the Pm25 homologs BiXyn10C and BiXyn10A, which were identified in the human gut microbiome (Wang et al., 2016a). Accounting for the fact that alkaline-stable xylanases are sought after for use in applications such as paper pulp biobleaching, Pm25 might constitute a useful starting point for enzyme engineering aimed at improving its hydrolytic properties.

So far, we have been unable to obtain structural data pertaining to Pm25 and none is available for its closest homologs. Therefore, at this stage it is tricky to speculate on the exact topology and molecular determinants of its active site. Nevertheless to gain some understanding we have examined similarities with the family GH10 xylanase CjXyn10C, which displays approximately 30% identity with Pm25 and whose structure is known (PDB: 1US3). Like Pm25, CjXyn10C exhibits rather poor activity on XOS ascribed to weak substrate binding in subsite -2 (Pell et al., 2004b). Unlike most other GH10 enzymes, in CjXyn10C subsite -2 contains G295 in the place of E, whose sidechain can hydrogen bond to the substrate According to sequence alignment, Pm25 also lacked the vital E residue in subsite -2, an observation that might explain its poor ability to hydrolyze X4 (Charnock et al., 1999;Pell et al., 2004b). Therefore, the -3 subsite with rather strong affinity value (2.76 kcal/mol) compared to others (Charnock et al., 1999) could probably be involved in the glycone subsite in the degradation of X4 in order to in complement with the poor -2 subsite. Taken together, a hypothetical subsite mapping of the active site of Pm25 with XOS is proposed for Pm25 (Fig. 5D).

The two CBM4s that are inserted into Pm25 clearly contribute to the binding and degradation of complex biomass. Our results reveal that this is especially true when both CBMs are functional and suggest that binding of large ligands involves a cooperativity phenomenon (Fig 8). However, some Pm25 homologs only contain one CBM domain. Similarly, database searching reveals that other Pm25 homologs contain two members of CBM family 22 in place of the CBM4 domains. Recently a multi-domain GH10 discovered from Bacteroides clarus in chicken caecum was integrated into UniProtKB (accession number A0A1Y3YZ70) and shown to incorporate three CBM4s inserted into GH10 amino acid sequences. These exceptional enzymes are also found in putative XULs from gut Bacteroidetes isolate (http://www.cazy.org/PULDB/, [START_REF] Mcnulty | Effects of Diet on Resource Utilization by a Model Human Gut Microbiota Containing Bacteroides cellulosilyticus WH2, a Symbiont with an Extensive Glycobiome[END_REF]. This infers that the SusG-assimilated functions can be fulfilled by enzymes that are not configured in an identical way. Moreover, it also confirms that the TIM-barrel fold in the GH10 family is quite accommodating in terms of insertions at the β3/α3 loop.

Apparently, unlike many highly active periplasmic endoglucanases, such as SusA (Shipman et al., 1999) and CjXyn10D (Pell et al., 2004b), extracellular enzymes such as SusG, CjXyn10A

and CjXyn10C are generally appended to CBMs (Pell et al., 2004b). Therefore, it is of interest to discuss the reason for this. The CBM58 in SusG (Koropatkin et al., 2010) and the CBM4s in Pm25 appear to improve the ability of the enzymes to hydrolyze insoluble substrates (Fig 8), while CBM15 in CjXyn10C does not play an important role in catalysis, irrespective of whether the substrate is soluble or not (Pell et al., 2004b). However, our data suggest that the affinity of Pm25 for soluble substrates was mostly derived from the binding ability of the CBMs (Table 2). In light of this observation, we propose that CBMs in membrane-associated enzymes might temporarily withhold soluble oligosaccharides before their importation into the cell. The implication of this is that the function of the CBM4 domains would be relatively independent of that of the GH10 domain. In this regard, it is noteworthy that the first structure of a SusG protein has been reported (Koropatkin et al., 2010). This structure reveals that the CBM58 domain is inserted into the B domain of the GH13 α-amylase domain. However, from a spatial point of view it is remarkable that the CBM58 does not form hydrogen-bonds with the catalytic domain, an observation that argues in favor of an independent function. Regarding Pm25, evidence for an independent function of the CBM4 domains is provided by the fact that the xylan-degrading profile of Pm25 wild type was almost identical to that of the CBM deleted version, M6 (Fig 7) and the fact that the xylan binding affinity of CBMs was relatively unaltered when the CBM domains were separated from the GH10 domain (Table 2). Finally, it is also useful to recall that the affinity values determined for subsites -4 and -3 of Pm25 and M6 were also nearly identical. Therefore, we believe that the catalytic center of Pm25 and the binding surfaces of the CBM4 domains are disconnected, an organization that corresponds to independent functions and contributes to low enzyme reaction rates (Khosla et al., 2001).

In conclusion, focusing on a termite gut-derived enzyme, we have provided further insight into the properties and function of Xyn10C-like enzymes that form part of core xylan utilization systems. This system seems to be rather efficient in term of evolution since it is conserved in termite gut, rumen, and human gut. Therefore the role of the CBM insertion is an interesting question. In this respect, we have thoroughly succeeded in characterizing the enzyme and shown that the CBM4 domains can be successfully excised without loss of catalytic function. Regarding the enzyme's substrate specificity, although it is difficult to speculate on the group of polysaccharides that might be preferential substrates in the termite gut environment, we have shown that it is better adapted for the hydrolysis of arabinoxylans than glucuronoxylans, which is consistent with the fact that the host termite feeds on crops such as sugarcane, rather than wood.

Table S1. Primes used in this study.

F GTGGGATGTGCTGAACGCGCCCATGAACGAGAAC R GTTCTCGTTCATGGGCGCGTTCAGCACATCCCAC Pm25|E663tga F GGCAAACTCATCAAGGTGACGTGACTCGATATTGCCATATCCAC R GTGGATATGGCAATATCGAGTCACGTCACCTTGATGAGTTTGCC Pm25|tga663A F GCAAACTCATCAAGGTGACGGCGCTCGATATTGCCATATCC R GGATATGGCAATATCGAGCGCCGTCACCTTGATGAGTTTGC Pm25|H634A F GCATCGGCACGCAAATGGCGCTCAACCTCAACTGG R CCAGTTGAGGTTGAGCGCCATTTGCGTGCCGATGC CBM4-1|Y213A F GGCTCGACCGGAAAAGCGGACGCACAACTCAACTCTCC R GGAGAGTTGAGTTGTGCGTCCGCTTTTCCGGTCGAGCC CBM4-1|Q216A F CGGAAAATACGACGCAGCGCTCAACTCTCCGGCG R CGCCGGAGAGTTGAGCGCTGCGTCGTATTTTCCG CBM4-1|N218A F TACGACGCACAACTCGCGTCTCCGGCGTTCCCG R CGGGAACGCCGGAGACGCGAGTTGTGCGTCGTA CBM4-1|Y257A F CGGAACTGGGTAATCAAGCGCCATGGGTGAACGGC R GCCGTTCACCCATGGCGCTTGATTACCCAGTTCCG CBM4-1|W259A F GGTAATCAATATCCAGCCGTGAACGGCGCAGAACTGGC R GCCAGTTCTGCGCCGTTCACGGCTGGATATTGATTACC CBM4-1|N261A F CAATATCCATGGGTGGCGGGCGCAGAACTGGCCG R CGGCCAGTTCTGCGCCCGCCACCCATGGATATTG CBM4-2|Y378A F GGTGCGGCTGGCAAAGCGGACCGCCAACTCAAC R GTTGAGTTGGCGGTCCGCTTTGCCAGCCGCACC CBM4-2|Q381A F GGCAAATACGACCGCGCGCTCAACAGCCCGGCATTTCC R GGAAATGCCGGGCTGTTGAGCGCGCGGTCGTATTTGCC CBM4-2|N383A F GCAAATACGACCGCCAACTCGCGAGCCCGGCATTTCCG R CGGAAATGCCGGGCTCGCGAGTTGGCGGTCGTATTTGC CBM4-2|Y422A F GAACTCGGCAACCAGGCGCCGTGGGGAAGCGGC R GCCGCTTCCCCACGGCGCCTGGTTGCCGAGTTC CBM4-2|W424A F GGCAACCAGTATCCGGCCGGAAGCGGCGGCGAG R CTCGCCGCCGCTTCCGGCCGGATACTGGTTGCC primers for in- fusion clone Pm25_CBM4- 1_L167-L320 F CAGCCATATGGCTAGCTTGTGGACGGAAGTTTCGAAG R GGTGGTGGTGCTCGATTAGAGGTCTGTGATCTCGACG Pm25_CBM4- 2_D332-D484 F CAGCCATATGGCTAGCCTTGTGGACGGCAATTTCGAAGGCGGAG R GGTGGTGGTGCTCGAGTTATTAGAGGTCGGTGACCTCAACGGCG TC Pm25_CBM4- 1+2_L167-D484 F GACGACGACGACAAGCTTGTGGACGGAAGTTTCGAAGAGGGGA TG R GTGGTGGTGCTCGAGGTCGGTGACCTCAACGGCGTCGATC PET32a_linearizatio n F CTCGAGCACCACCACCACCACCAC R CTTGTCGTCGTCGTCGCCAGAACCAGAACCGGCCAGGTTAG

INTRODUCTION:

The biodegradation of plant cell walls is of considerable interest to scientists and technologists alike. This is because plant cell walls constitute a highly abundant reservoir of renewable carbon in the form of lignocellulose. This composite carbon-rich material is principally composed of cellulose, lignin and heteroxylans, these latter constituting the most widespread and abundant form of hemicellulose.

Xylanases are carbohydrate-active enzymes that hydrolyze heteroxylans, bringing about their depolymerization. According to the classification of carbohydrate-active enzymes (the CAZy database, Lombard et al., 2014), the two archetypal xylanase families are GH10 and GH11, families that are composed of both bacterial and fungal origins whereas a majority of GH10s are from bacteria. Typically, GH10 xylanases display high molecular mass, low pI, multiple functions and accessory domains (40%), whilst GH11 members generally display lower molecular mass, higher pI, a unique function and are less likely to be associated with other domains (20%) (NR. Gilkes et al., 1991;[START_REF] Sunna | Xylanolytic enzymes from fungi and bacteria[END_REF]. Similar to enzymes within families GH1, 2, and 5, the catalytic domain of GH10 xylanases display a TIM-barrel fold. This is in contrast to GH11 xylanases, which are characterized by a β-jelly roll structure. Nevertheless, despite these differences, both GH10 and GH11 xylanases perform catalysis via a retaining mechanism that requires a catalytic dyad composed of a catalytic nucleophile and a proton donor (Gebler et al., 1992;Withers et al., 1986).

Regarding xylooligosaccharide (XOS), GH10 generally display good aptitude for the hydrolysis of xylotriose i.e. DP 3 (Pell et al., 2004). In contrast, members of family GH11 are generally much less active on such XOS, requiring a minimal size of DP4 (xylotetraose) (Pollet et al., 2010). When acting on branched heteroxylans, substrate recognition and binding by members of GH10 requires the presence of one consecutive unsubstituted xylopyranosyl residues at substite -1 and at substite -2, it can tolerate the O-3 linked L-arabinofuranoside (Biely et al., 1997;Kolenová et al., 2006;Pell et al., 2004). In contrast, GH11 family members require three consecutive unsubstituted xylopyranosyl residues, which are accommodated in subsites -2 to +1 (Biely et al., 1997;Katapodis et al., 2003). Consequently, GH10 xylanase can produce smaller hydrolysis products from glucuronoxylan and arabinoxylan than GH11 xylanases (Pollet et al., 2010), and generally performed better in biomass deconstruction than GH11 xylanase [START_REF] Hu | Why does GH10 xylanase have better performance than GH11 xylanase for the deconstruction of pretreated biomass?[END_REF]. In addition, GH10

xylanase is more favorable to associate with carbohydrate binding module (CBM) constituting multi-domain proteins compared to GH11 xylanase, with 18% and 3% respectively according to Pfam database (http://pfam.xfam.org/). CBM has been recognized as one of the helper agents enhancing performance of lignocellulases although it is devoid of hydrolytic properties (Kim et al., 2014).

In order to completely degrade xylan, xylanases operate in cooperation with other socalled accessory enzymes. Prominent among these are α-L-arabinofuranosidases (e.g. from families GH43 and GH51), α-D-glucuronidases (e.g. from families GH67 and GH115), acetylxylan esterases , feruloyl esterases (e.g. from family CE1). All of these enzymes remove main-chain decorations, while β-D-xylosidases act on XOS released by xylanases, converting these into xylose. The concerted action between xylanase and accessory enzymes is characteristic of xylanolytic systems and thus such enzymes are widespread in fungi and bacteria [START_REF] Coughlan | beta-1,4-D-xylan-degrading enzyme systems: biochemistry, molecular biology and applications[END_REF][START_REF] Filho | The xylan-degrading enzyme systems of Penicillium capsulatum and Talaromyces emersonii[END_REF][START_REF] Sunna | Xylanolytic enzymes from fungi and bacteria[END_REF][START_REF] Weaver | Genetic analysis of a locus on the Bacteroides ovatus chromosome which contains xylan utilization genes[END_REF].

Bioprospection within microbial xylanolytic systems can be conveniently performed using functional metagenomics, a technique that can be used to probe microbiologicallycomplex environmental samples without the need for prior cultivation of individual species (Bastien et al., 2013;Tasse et al., 2010). Using this type of approach it is possible to simultaneously pinpoint individual genes encoding xylanolytic enzyme components and probe their local genomic context. This provides a means to discover functional gene clusters, such as polysaccharide utilization loci which are characteristic of the bacterial phylum Bacteroidetes (Grondin et al., 2017).

The extraction of monomeric sugars (saccharification) from lignocellulosic biomass is a fundamental step in the production of 2 nd generation biofuels. To achieve this step, a series of unit operations are deployed, which include the hydrolysis of extracted polysaccharides using highly complex enzyme cocktails. These contain a wide range of cellulose-active enzymes and many accessory enzymes, including xylanases. The latter are beneficial because it has been shown that the presence of xylanases and other xylanolytic enzymes not only leads to the release of xylose, but also enhances cellulase action and thus accentuates glucose production, while reducing enzyme loadings [START_REF] Dondelinger | Contrasted enzymatic cocktails reveal the importance of cellulases and hemicellulases activity ratios for the hydrolysis of cellulose in presence of xylans[END_REF][START_REF] Gao | Hemicellulases and auxiliary enzymes for improved conversion of lignocellulosic biomass to monosaccharides[END_REF][START_REF] Tabka | Enzymatic saccharification of wheat straw for bioethanol production by a combined cellulase xylanase and feruloyl esterase treatment[END_REF]. For this reason, the search for efficient xylanolytic enzymes and cocktails containing both main-chain cleaving (MCEs) and side-chain cleaving enzymes (SCEs) has been intensive (Kormelink and Voragen, 1993;[START_REF] Sørensen | Enzymatic hydrolysis of wheat arabinoxylan by a recombinant "minimal" enzyme cocktail containing β-xylosidase and novel endo-1,4-β-xylanase and α-L-arabinofuranosidase activities[END_REF][START_REF] Suh | Synergic effects among endo-xylanase, βxylosidase, and α-L-arabinofuranosidase from Bacillus stearothermophilus[END_REF][START_REF] Vardakou | Synergy between enzymes involved in the degradation of insoluble wheat flour arabinoxylan[END_REF][START_REF] De Vries | Synergy between accessory enzymes from Aspergillus in the degradation of plant cell wall polysaccharides[END_REF].

Within xylanolytic cocktails the different enzymes interact in a variety of ways. When

MCEs or SCEs act in synergy between themselves, this can be described as homosynergy, whereas the concerted action of both MCEs and SCEs is referred to as heterosynergy.

Sometimes, MCEs require the prior action of debranching enzymes (Khandke et al., 1989), in other cases MCE action is required as a first step to release branched oligosaccharides that constitute preferential targets for some SCEs (Mackenzie and Bilous, 1988;[START_REF] Poutanen | An acetyl esterase of Trichoderma reesei and its role in the hydrolysis of acetyl xylans[END_REF][START_REF] Puls | α-Glucuronidase in two microbial xylanolytic systems[END_REF]. In this way, the sequential and simultaneous action of multiple enzymes can favorably modulate the catalytic efficiency of individual enzyme components (Poutanen et al., 1991). On the other hand, the concomitant action of cocktails components can also lead to negative effects, which are known as antisynergy. This last phenomenon can occur when the action of one enzyme prevents or hampers the action of another [START_REF] Coughlan | beta-1,4-D-xylan-degrading enzyme systems: biochemistry, molecular biology and applications[END_REF]. Finally, considering single enzymes, intramolecular synergy is achieved through the functional interplay between catalytic domains and associated CBMs (Boraston et al., 2004;Din et al., 1994). Moreover, CBMs could also be involved in synergism between exo-lytic enzymes, so called exo-exo synergy in cellulolytic enzymes (Badino et al., 2017).

Enzymes encoded by PUL systems are interesting, because they form part of finelytuned catabolic machinery in which strong enzyme synergy provides the basis for efficient polysaccharide catabolism (Arnal et al., 2014;Bågenholm et al., 2016;Wang et al., 2016a).

Therefore, PULs might be considered as interesting paradigms for the creation of enzyme cocktails, especially since it has been suggested that the synergy between PUL enzyme components is greater than that between enzymes from different species [START_REF] Gilbert | A comparison of two xylanases from the thermophilic fungi Thielavia terrestris and Thermoascus crustaceus[END_REF][START_REF] Tauzin | Functional characterization of a gene locus from an uncultured gut Bacteroides conferring xylo-oligosaccharides utilization to Escherichia coli[END_REF].

This study focuses on a putative PUL that contains two GH10 family members (Pm12 and Pm25), one GH11 (Pm22), one GH115 (Pm115) and one GH43 (Pm21) (Bastien et al., 2013) (Fig1). The Pm115 is a putative α-L-glucuronidase that removes the methylated glucuronic acids in side chains. The Pm21 could have both β-xylosidase and α-Larabinofuranosidase, as its amino acid sequence shares 62% identity with CoXyl43, a βxylosidase/α-arabinofuranosidase isolated from a compost metagenome [START_REF] Matsuzawa | Crystal structure of metagenomic β-xylosidase/ α-l-arabinofuranosidase activated by calcium[END_REF]. In addition, Xus C and Xus D are located upstream in the cluster, and one hypothetical transporter protein is between Pm22 and Pm12. Among the three xylanases, Pm22 and Pm25 are multi-domain xylanases. The C-terminal sequence of Pm22 is annotated as GH11 (Pm24)

while N-terminal sequence is poorly annotated and currently unknown. Pm25 is a Xyn10C-like enzyme carrying two CBM4s, which corresponds to a ubiquitous PUL feature that has attracted a lot of attention recently and has been the subject of several studies (Despres et al., 2016;Dodd et al., 2011;Flint et al., 1997;Rogowski et al., 2015;Zhang et al., 2014;Wu, in preparation). It is believed that Xyn10C-like proteins, which contain CBMs, provide early stage hydrolysis of polysaccharides, producing oligosaccharides that are then withheld by the CBMs thus avoiding diffusion. Subsequently, the products of Xyn10C-like proteins are further processed and imported into the bacterial cell by other PUL components (White et al., 2014). In this study we have investigated the role of the Pm25 CBM, notably with the aim to understand how it modulates the release of short-to-medium sized oligosaccharides (DP 1 to 28) when Pm25 operates in synergy with the other PUL enzymes.

MATERIAL AND METHODS:

Protein expression and purification. Cells were collected by centrifugation at 7,000 × g for 20 min and disrupted by sonication.

The purification step was done by applying cell lysates to Talon metal affinity resin (Clontech, USA) following the manufacturer's instructions. All purified proteins were conserved at 4°C in 50 mM phosphate buffer (pH 7.0) after dialysis. The expression and purification of Pm 25, Pm25Y213AY378A and Pm 25ΔCBMs were as described before (H.

Wu, to be published).

Biochemical characterization of PUL enzymes

Beechwood glucuronoxylan (BGX) and wheat arabinoxylan (WAX, low viscosity)

were purchased from Megazyme International (Bray, Ireland), while and aldopentauronic acid (UXXR and XUXXR) were kindly provided by Magazyme. Xylanase activity was estimated by quantifying the reducing sugars released over time using a dinitrosalicylic acid (DNS)based assay (Miller, 1959). Working in triplicate, reactions were initiated by the addition of 90 µl of enzyme (Pm12 and Pm24, 0.2 µM) into 810 µl of BGX (20 mg/ml) in appropriate buffer containing bovine serum albumin (BSA; 1 mg/ml) and were incubated for 18 min at 37 °C. 100 µl of reaction was removed every 3 min and boiled with 100 µl of DNS for 10 min at 95°C, and absorbance was measured at 540 nm in a plate reader after adding 1 ml of MilliQwater (Arnal et al., 2014). The amount of reducing sugar liberated was determined via a standard curve using D-xylose (X1).

The apparent optimal pH for activity of the Pm12, Pm24 was determined by measuring the enzyme activity on BGX in the pH range from 4.0 to 10.0. Buffers used were, Phosphate-citrate buffer pH range 4.0 to 7.0, citric acid (50 mM)-Na2HPO4 (100 mM) (pH according to buffer reference center, Sigma-Aldrich protocol), trizma buffer (50 mM, pH 7.0 to 9.0), glycine-NaOH buffer (50 mM, pH 9.0 to 10.0). The optimum temperature was determined by incubating enzymes at optimum pH over a temperature range of 30-65 ºC.

Thermal stability was evaluated by incubating the enzyme solution at a certain temperature for different durations (0, 0.5, 1, 4, 8, and 12 h) before the residual activity was assayed at the same temperature. One unit (IU) was defined as the quantity of enzyme in mg required to liberate 1 µmol of D-xylose equivalent per minute under optimum condition.

Enzyme kinetics parameters were calculated using BGX (0.4 to 20 mg/ml) and WAX (0.5 to 16 mg/ml) as substrates at the optimum pH and temperature. Apparent Km and Vmax values were determined by fitting Michaelis-Menten curves to one-site saturation under ligand-binding equation using Sigma Plot software version 11.0.

HPAEC-PAD and MALDI-TOF analysis of hydrolyzed products

BGX was chosen for this analysis as it provides the substrate for Pm115. Hydrolysis b Values indicate that the relative enzyme activity was > 80% of the activity observed at optimum conditions. Values in parentheses are the optimal conditions defined for biochemical characterization.

RESULTS AND DISCUSSION:

Biochemical characterization of PUL enzymes

All the glycoside hydrolases from the PUL were produced and purified to homogeneity. The physiochemical parameters for Pm12 and Pm24 were determined (Table 1). The pH optimum was different for each enzyme, with Pm25 showing the widest pH optimum range and Pm12 the narrowest. It is noteworthy that the activity of Pm24 is also stable over a wide range, from pH 5.0 to 9.0 (Table 1). Regarding temperature, all three enzymes achived highest activities in the range 60-65°C. However, enzyme stabilities, measured over a 24h-period were lower, at

Study the cooperation of MCE (Pm12 and Pm24) and SCE (Pm21 and Pm115) in PUL enzymes

In order to investigate synergy or complementarity between PUL components, four different core enzyme combinations, core2, Pm21@core3, Pm115@core3, and core4 respectively, were compared (Fig 3 and4 The populations of medium-sized products after treatment of core2 (A), Pm115@core3 (B), Pm21@core3 (C) and core4 (D) were accessed by MALDI-TOF. The number of methylated glucuronic acids in the products were counted after the treatment of core2(E) , Pm115@core3 (F), Pm21@core3 (G) and core4 (H). which agrees with the antisynergy of CBM and GH10 domain, observed in Fig2. The similar profile of X1-6 was also observed for reactions containing core2, Pm115@core3 and Pm21@core3 (data now shown). Regarding the longer XOS in the products, introduction of either Pm25 or Pm25ΔCBMs into core2 accelerated the appearance of diverse products (at 1 min) that were further hydrolyzed formining a stable product profile after 5 min, compared to 10 min when core2 alone was employed (Fig 4A,Fig 6A and B). Except for some minor fluctuations over the first 5 min, the product profile and substitution pattern of both Pm25 and Pm25ΔCBMs were almost identical from 10 to 15 min (Fig6), this also being the case for using other core enzymes, Pm115@core3, Pm21@core3 and core4 (data not shown).

Comparison of the product profile of Pm25 and Pm25ΔCBMs in the context of PUL enzymes

CONCLUSION:

Characterization of the xylanases forming the putative Pm-derived PUL (Fig1) has provided the basis to define optimal conditions for testing of enzyme combinations. Overall, our results reveal that individually both Pm12 and Pm24 are more active xylanases than Pm25 (Table1), and indeed these enzymes figure among the most active xylanases reported (Pell et al., 2004). This disparity between the activities of the three enzymes suggests different functional roles within the PUL context. Basically, Pm12 and Pm24 appear to be the key depolymerizing enzymes GH10 and GH11, while Pm21 plays important roles in xylose liberation and Pm115 provides sidechain-cleaving activities (Fig 3 and4).

A load of researches proved that the CBM is not essential for hydrolytic activity of the enzyme towards soluble substrates (Ali et al., 2001;Black et al., 1996;Kellett et al., 1990;Valenzuela et al., 2012;Zhao et al., 2005). However these studies focused on only specific activity and product profile of short oligosaccharides, which is not compelling for the conclusion. Previously, the study of Pm25 revealed that the absence of its CBM4 domains did not radically alter its kinetic parameters or the product profile, when monitoring low DP (DP ≤ 6) products (see article I). The data obtained in this study confirmed this, showing that substrate depolymerization is not promoted by the presence of the CBM4 domains, which in fact produce a sort of antisynergy effect (Fig 2). Furthermore, other PUL enzymes, Pm12, Pm24, Pm21 and Pm115 were used to make core enzymes and synergistic effect was observed when Pm115 was added into Pm21 (Fig 3 and4). Again, similar product (DP 1-28) and substitution pattern were observed between wild type and mutants of Pm25 when acting with other PUL enzymes (Fig 5 and6).

All together these results suggest that, as one of the PUL compoments, the activity of Pm25 somehow complements other PUL components (Despres et al., 2016) but its contribution might be quite specific and/or subtle. However, further studies are required to confirm this by varying the substrates, concentration of enzymes/CBMs and substrates, and the order of addition, which may affect the cooperative action (Badino et al., 2017).

Quantifying CBM carbohydrate interactions using Microscale Thermophoresis

Haiyang Wu

INTRODUCTION

Protein-carbohydrate interactions are involved in a myriad of important biological functions.

Therefore many techniques are used in order to quantify and characterize those interactions, such as Isothermal Titration Calorimetry (ITC) which allows characterization of thermodynamics during interaction process, Affinity Gel Electrophoresis (AGE) using soluble polysaccharides, solid state depletion assay using insoluble polysaccharides, UV difference and fluorescence spectroscopy [START_REF] Abbott | Quantitative approaches to the analysis of carbohydrate-binding module function[END_REF].

Thermophoresis, or thermodiffusion / Soret effect, was first described by Carl Ludwig in 1856. This technique uncovered the different responses of particles in a mobile phase towards a gradient of temperature. The miniaturized thermophoresis, namely MST was developed in the last decade to monitor the interactions and calculate the thermodynamic constants among biomolecules ranging in size from 10 1 to 10 7 Dalton in only few microliters.

The description of thermophoresis in a frame of a local thermodynamic equilibrium allows one to quantify the thermophoretic behavior based on the charge, size and hydration shell of biomolecules (Duhr andBraun, 2006a, 2006b). Basically, the MST instrument contains an infra-red (IR) laser which induces a microscopic temperature gradient (e.g. 1°K/10 µm) in a glass capillary, a high-power LED (light-emitting diode) as source of light excitation, a fluorescence detector, and a sample tray of 16 capillaries [START_REF] Baaske | Optical thermophoresis for quantifying the buffer dependence of aptamer binding[END_REF]Jerabek-Willemsen et al., 2014). Briefly, the labeled molecule is mixed with a decreasing concentration of non-fluorescent ligand and each mixture is loaded into one of the 16 glass capillaries. Initial fluorescence is measured as a base line in each capillary and the IR laser is switched on. The IR laser beam is focused into the mid-point of capillary and strongly absorbed by aqueous solution resulting in a temperature gradient from the laser spot to the outer region. Simultaneously, a dichroic mirror allows fluorescence from labeled molecule to be continuously monitored, while complexes migrate in the capillaries. The fluorescence in the laser spot area decreases or increases during thermophoresis, at a velocity related to the concentration of ligand, due to change in size, charge or hydration shell of the complex. Each capillary is scanned in minutes one after another. The constant of dissociation Kd is deduced from the binding curve derived by plotting the normalized fluorescence (FNorm) at a given time against each ligand concentration (L) [START_REF] Baaske | Optical thermophoresis for quantifying the buffer dependence of aptamer binding[END_REF]Duhr et al., 2006b).

Depending on the molecules investigated, the source of fluorescence could be intrinsic (e.g. indole side chain of tryptophan), coming from attached dyes (e.g. Fluorescein, Alexa546) or from fluorescent protein (e.g. GFP, YFP). The fluorescence excitation/detection spectrum range would lead to choose appropriate apparatus. Several labeling strategies could be employed to target proteins by attaching for instance specific fluorophore molecule to the ε-amine group of lysine, thiol group of cysteine, fusion protein (such as GFP), or even unnatural amino acids.

Here we present the use of the MST for the quantification of CBM/ligand interactions. The CBMs used in our protocol are the CBM3a from Clostridium Thermocellum (Tormo et al., 1996) and a newly identified CBM4 from termite gut microbiota (Bastien et al., 2013). They display affinity for insoluble crystalline cellulose and soluble xylan respectively. Both CBMs were labeled to primary amine group of lysine using an adapted protocol from the protein labeling kit provided by NanoTemper and then MST experiments were performed using Monolith NT. 115 instrument.

MATERIALS

All solutions were prepared using deionized water dH2O and analytical grade reagents.

Buffers were prepared and stored at room temperature. Protein samples were kept on ice until use.

CBM4, the highest concentration of xylohexaose was 150 mM (see Note 16). Both ligands were diluted using the dilution buffer.

Assay setup

A titration series of 16 dilutions of the ligand is prepared in 0. Mix by pipetting up and down three times.

Data Setting

CBM3a

1. After performing the capillary scan and MST experiment, the initial fluorescence intensity was checked (Fig. 1a). The variation of fluorescence is less than ± 10% and the MST signal could be analyzed (see Note 22).

2. Select the tab "MST Analysis Set" in the MO.Affinity Analysis software. The fluorescence ratio before and after switching on the laser is calculated and plotted against the ligand concentration. A sigmoidal dose-response curve is obtained (see Note 23).

Choose Kd model, enter the concentration of CBM3a in the same unit used as for the ligand.

3. Choose the ∆FNorm as y-axis (Fig. 1b). The sigmoidal curve needs to have (1) a clear baseline and a clear saturation (at least 3 points respectively), ( 2) an amplitude/noise > 3 (see Note 24).

4. Then the Kd value is determined for CBM3a. MO.Affinity Analysis software could also evaluate the consistency among repeated experiments by simply including all the repeated experiments that have been done in the exact same conditions into the same analysis set, and errors bars will automatically appeared in the dose response curve.

5. The Kd of CBM3a towards cellulose nanocrystals deduced from the binding curve is 0.24 ± 0.05 g/L. Regarding the complexity of cellulose nanocrystals, Kd would be considered as an apparent Kd.

CBM4

1. The initial fluorescence shown in Fig. 2a 3. The Kd of CBM4 towards xylohexaose derived from the binding curve was 2.5 ± 0.3 10 3 µM.

The two proteins were labeled on the lysine. However unlike CBM3a binding site (Fig. 1c) quenching fluorescence within CBM4 could be explained by the presence of a putative labelled lysine close to the binding site (Fig. 2c).

NOTES

1. After purifying the protein by IMAC, determining the protein concentration was not possible by UV absorption because of imidazole. In this case, to avoid dialysis dilution, we performed sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) with labeled protein and a serial diluted protein as standards, and then quantify the labeled protein by the quantifying tool in the software of Image Lab 5.2.1.

2. The buffer of the labeling kit is an amine free buffer at pH 8. Our protein storage buffer was 50 mM phosphate buffer pH 7.0. It is also an amine free buffer. Labeling efficiency using NHS-ester group is optimal at pH 8 but could also be performed at pH 7, not below. The Na2HPO4•12H2O can be dissolved faster if the dH2O is warmed up to about 50°C. If the binding event is calcium dependent, HEPES buffer is recommended instead of the phosphate buffer.

3. Dealing with costly ligand such as xylohexaose, it is better to start with a rather high concentration (in our case 300 mM) for the very first trial before knowing the Kd to make sure the CBM can be saturated.

The Gravity Flow Column B for purification provided in Monolith NT™ Protein Labeling

Kit is a dextran-based gel filtration column. CBM3a and CBM4 displayed affinity for dextran, and were retained by the desalting column. Therefore, vivapure metal chelate mini spin columns were selected because they are made of cellulose free based membrane. Furthermore it allows the use of small volume of IMAC resins in the purification step.

5. The TALON Metal Affinity Resin is made with Sepharose CL-6B, an agarose based polysaccharide polymer material, which has no affinity with the CBMs studied.

6. There are three different types of capillaries for NT.115, standard treated capillaries, hydrophobic capillaries and premium coated capillaries. The premium coated one is the best to tackle with samples that have significant adsorption to surfaces, although it is the most expensive. We recommend trying with the standard treated capillaries first using buffer supplemented with additives such as BSA or detergent if the protein sticks to the inner surfaces of the capillary. If the adsorption is still a problem, then try with other capillaries.

7. 20 µl of each sample is usually prepared for MST experiment, thus a 0.2 mL individual tube is enough. Bigger reaction tubes are not recommended since the large surface area to volume ratio can increase adsorption to the tube wall and loss of protein. Besides, the 0.2 mL strip tubes are also recommended for their convenient manipulation especially in the serial dilution step.

8. A red channel is always recommended as it allows to measure in complex bioliquids (cell lysate, serum) and no autofluorescence of a molecule in this channel has been observed. 9. A refrigerated and temperature regulated centrifuge is recommended for the centrifuge of protein samples to maintain their stability.

10. We recommend preparing a little bit more than 100 µl (e.g. 120 µl) for accurate pipetting afterwards.

11. It is possible to aliquot dye in DMSO and store at -20°C.

12. In our case, as the pH of the labeling buffer is not optimal we used a higher concentration of dye to improve the labeling efficiency which was 3.5 fold of the protein concentration.

13. The maximum volume of the column is 600 µl, and adding 500 µl of dH2O into the column does not leave space for the air. Therefore, leaving the cap open is recommended to ensure that the wash buffer is completely removed.

14. Buffers containing detergent (0.05% pluronic acid, 0.05% tween-20), BSA (0.05 -0.5 mg/mL), casein and others can be tested. A modified pH or ionic strength can also improve the performance.

15. Concentration of the CBM3a was expressed in g/L to be in accordance to the unit of its substrate concentration, cellulose nanocrystals.

16. The highest concentration of xylohexaose in the reaction is 150 mM therefore 300 mM stock solution is required as the ligand will be diluted twice when adding the same volume of protein. We recommend to mix the solution by pipetting up and down carefully and not to vortex in order to avoid bubbles. 17. Vortex will result in a possible adsorption of protein to the inner wall of the tube and considering the very low concentration of protein, even small adsorption would result in a significant decreasing of protein concentration and so to a non-optimal assay.

18. The protein and buffer additives will be diluted twice after mixing with the same volume of ligand keep this in mind when designing the experiment.

19. Stock solution of both protein and ligand required to be centrifuged at full speed 5 min before mixing to avoid aggregates in capillaries.

20. Avoid touching the capillary in the middle where the optical measurement will be performed because it could modify the fluorescence.

GENERAL CONCLUSION AND FUTURE WORK

The plant cell wall polysaccharides are of industrial relevance since they represent the most abundant carbon source on earth. PCW polysacharide valorization is an important target of the bioeconomy which implies developing biocatalysts in order to degrade this highly recalcitrant material in milde conditions and high yields. After cellulases, xylanases are important key player in this field targeting the hemicellulose part of the PCW enhancing the cellulose hydrolysis by reducing the inhibitors and realizing pentose sugars which can be further valorized. Because PCW polysaccharides are also an important part of our diet, xylanase are used in a wide range of industrial applications such as the pulp and paper industry, detergents, in food and feed or bakery. Idealy, enzyme in the deconstruction of plant cell wall shoud be highly effective, thermostable, alkaline/acid tolerable, synergistically work with other enzymes to result in low enzyme loading depending on the process. This doctoral thesis mainly focuses on the characterization of a GH10 xylanase, Pm25 which was identified from a xylan utilization system XUS in the metagenome of the termite gut Pseudacantotermes militaris.

Pm25 is a typical GH10 encountered only in XUS displaying a peculiar sequence organization where the GH10 catalytic module is inserted by two CBM4. The role of the enzyme in the XUS is still not clear and the objective of this thesis was to characterize the biochemical and catalytic properties of Pm25 and to interrogate the role of the CBM4 in the function.

How has this thesis contributed to the enzymology in terms of biorefinery?

A detailed biochemical characterization of Pm25 was performed. The physiochemical parameters showed Pm25 could act in a broad pH range (pH 5 to 9), which is of industrial interests.

Despite the pH range, compared with other GH10 xylanases in literature, Pm25 and its homologues are over tens and hundreds times lower in catalytic efficiency both with polysaccharides and oligosaccharides (Despres et al., 2016).

In the XUS fragment, Pm25 is surrounded by other xylanases, therefore the activity of Pm25 in the context of the XUS was investigated. The results showed that the other GH10 xylanase,

Pm12, was among one of the most active GH10 xylanases [START_REF] Mirande | Characterization of Xyn10A, a highly active xylanase from the human gut bacterium Bacteroides xylanisolvens XB1A[END_REF]Pell et al., 2004). Although both Pm25 and Pm12 are located in the same gene cluster, their putative cellular location is different, the former is anchored onto the cell-membrane and the latter is located in the periplasm based on the model for XUS in xylanolytic Bacteroidetes (Dodd et al., 2011). It was proposed that the Pm25 homologues are responsible for "limited" degradation of glycan at the cell surface before importing into the periplasm to avoid diffusing the XOS products that may be uptaken by other bacteria. Thus, Pm25 and its homologues seem to be "designed" as an inefficient enzyme in producing the final products constituting a "selfish strategy" for the bacteria in the ecosystem. Combing the experimental data and the putative model for xylan utilization system, we suggest that the periplasmic enzymes are more favorable for biorefinery than the suface enzymes from the PUL of Bacteroidetes.

Regarding the domain organization, Pm25 and its homologues are not the only ones showing insertional architecture. In fact, 9% of domain combinations are discontinuous, with one domain being inserted into another one (Aroul-Selvam et al. 2004). Inspired by naturallyoccurring domain insertion, researchers have created chimeric enzymes (Kaneko et al. 2004;Q. Wang and Xia 2007). To achieve this, the insertion site needs to be carefully selected in order to obtain correct folding of the chimeric protein (Saadat 2017). We suggest that Pm25 could be a convenient template for enzyme engineering work. Using the β3/α3 loop as a docking site, insertional constructions could be created combining GH10 xylanase with other catalytic and/or substrate binding functions. More generally, considering that the β3/α3 loop constitutes a docking site for insertions in GH10 (i.e. the Pm25-like enzymes) and also in other TIM barrel enzymes [START_REF] Nagano | One fold with many functions: The evolutionary relationships between TIM barrel families based on their sequences, structures and functions[END_REF] it is likely that this loop is an insertional hot spot that has been used several times throughout evolution. This could lead to the developpement of industrially relevant enzymes and would certainly required ingeneering on the GH10 part to improve its catalytic efficiency.

Like expansins and auxiliary activity family 9 proteins, CBMs are recognized as synergistic proteins and enhanced the catalysis by the proximity effect, targeting and disruptive function (Kim et al., 2014). In a recent study, CBM in one cellobiohydrolase was suggested to mediate the synergy with another cellobiohydrolase by its targeting role (Badino et al., 2017).

Therefore, it is reasonable to hypothesize that the internal CBMs in Pm25 may play a role in orchestrating the synergistic action of PUL enzymes. To better understand this, the product profile of wild type and the CBM domain abrogated Pm25 in combination with other PUL enzymes were analysed using beechwood xylan as a substrate. However, under the tested conditions, no striking difference could be observed. In plant biomass based biorefinery, the substrates are much more complex than beechwood xylan used in this study, which may highlight the potential role of CBMs in mediating synergistic reactions another explanation is that they recognized a very specific motif in the xylan that has not yet been discovered.

How has this thesis contributed to the glycan forgaging study in gut microbiota?

The starch utilization system (SUS) along with polysaccharide utilization loci (PUL) is a paradigm for complex glycan catabolism in Bacteroidetes, one of the dominant gut-associated bacterial phyla (Martens et al., 2009). Bioinformatic study of Pm25 showed that its homologes were mostly distributed in putative PULs. Xyn10C (Pm25 homologue) is proved to be the one of the most induced/expressed proteins when Bacteroides species are growing on xylan (Despres et al., 2016;Zhang et al., 2014), which indicated its important role in xylan degradation. Xyn10C in xylan utilization system is equivalent to SusG as they are the only out membrane glycoside hydrolases in each system Bibliography). The combined in vivo and in vitro experiments have revealed that SusG is important for starch degrading activity and starch binding affinity (Reeves et al., 1997;Shipman et al., 1999Shipman et al., , 2000)). In terms of activity, SusG is essential for bacterial growth on starch but has very low activity (compared to SusA); regarding the binding affinity, SusG is not important for starch binding but may prevent the degraded starch molecule from diffusing in the environment until imported in the periplasm. Therefore, the primary role of SusG is to degrade the starch on the cell surface. The in vitro biochemical characterization of Pm25 and its mutants achieved in this thesis strengthened the similarity between Xyn10C and SusG, such as low activity and independent domain function (Koropatkin and Smith, 2010), which helped to deduce the comparable role of Xyn10C in vivo, degrading xylan at cell surface and producing XOS that can be used for feeding target species of Bacteroidetes. In this case, Xyn10C, and all other SusG like proteins have potential values in generating useful oligosaccharides, such as prebiotics. Overall, enzyme systems from gut microbiota degrading xylan arouses great interest in nutritionist as knowledge in this area could better understand the gut ecology and personalize human gut health.

FigIFigure 1 .

 1 Fig I Présentation schématique et réseaux de séquences similaires (SSN). ............................ Fig II Comparaison des différents variants de Pm25. ..............................................................

Figure 2 .

 2 Figure 2. Taxonomic distribution of GH10 xylanase. .........................................................

Figure 3 .

 3 Figure 3. Optimal physiochemical parameters. ...................................................................

Figure 4 .

 4 Figure 4. Mutational and truncation constructs. ..................................................................

Figure 5 .

 5 Figure 5. Progressive degradation of 0.2mM of XOS, (A) X6, (B) X5, (C) X4 by Pm25. .

Figure 6 .

 6 Figure 6. Affinity gel electrophoresis of different CBM mutations in Pm25. .....................

Figure 7 .

 7 Figure 7. Hydrolysis of LVWAX by Pm25 and its mutants. ...............................................

Figure 8 .

 8 Figure 8. Hydrolytic patterns of Pm25 (WT) and mutants towards wheat bran. .................

Figure S1 .

 S1 Figure S1. Affinity gel electrophoresis of CBMs in Pm25 towards 0.5% (w/v) xyloglucan. ................................................................................................................................................

Figure S2 .

 S2 Figure S2. Measurement of Kd based on ligand induced effect. .........................................

Figure S3 .

 S3 Figure S3. Binding of inactive Pm25 and its truncated derivatives to wheat bran. .............

Figure S4 .Fig 1 .

 S41 Figure S4. The time-dependent degradation of LVWAX by (A) M3 and (B) M4. .............

Fig 2 .Fig 3 .

 23 Fig 2. The product (DP from 1 to 28) profile of by Pm25 and its mutants.......................... Fig 3. The product (DP from 1 to 6) profile of different combinations of core enzymes. ...

Fig 4 .Fig 5 .

 45 Fig 4. The product (DP from 5 to 28) profile of by different combinations of core enzymes. ................................................................................................................................................ Fig 5. The product profile (DP from 1 to 6) of Pm25 and mutants in combination with core4. ................................................................................................................................................ Fig 6. The product (DP from 5 to 28) pattern of Pm25 and Pm25ΔCBMs in combination with core2. .............................................................................................................................. Fig S1. Hydrolysis products of UXXR and XUXXR (Megazyme) by Pm115. ..................

Table 1 .

 1 . Comparison of composition of plant cell wall in different groups of kingdom Plantae ...................................................................................................................................... Table A-3. Major agricultural wastes and their worldwide availability, bioethanol production potential and compositions ....................................................................................................... Table B-1 The major hemicellulases and their classification ................................................ Table C-1 Extant families of termites .................................................................................... Table D-1: A wide variety of biomass resources for conversion into bioproducts ............... ARTICLE I Kinetic parameters of Pm25. ................................................................................. Table 2. Binding of the recombinant proteins to soluble substrates as determined in affinity gel electrophoresis. ................................................................................................................. Table S1. Primes used in this study. .................................................................................... Table S2. The concentration series of RAX used for different constructs. ......................... Table

  . Des différences importantes ont été constatées entre la construction M3 (CBMs) et M4 / M5 (chaque CBM séparé) indiquant une coopérativité entre les deux CBM4s (Fig IIA, milieu). Enfin des tests d'affinité sur substrat insoluble (solid depletion assay) ont confirmé le rôle des CBMs pour l'affinité avec la biomasse complexe telle que le son de blé (Fig IIA, panneau droit). Les constructions de Pm25 dont les acides aminés importants pour la fixation au ligand ont été mutés et ont montré les mêmes profils de produits que la construction sans CBM, de plus les quantités de xylooligosaccharides libérés ont été plus importantes pour Pm25 sauvage confirmant ainsi les rôles des CBM dans l'hydrolyse de la biomasse complexe (Fig IIB).

Fig

  Fig II Comparaison des différents variants de Pm25.(A) Gauche: représentation schématique des différentes constructions dérivées de

Fig A- 1 .

 1 Fig A-1. Chemical structure of the predominant building blocks of plant cell walls[START_REF] Sarkar | Plant cell walls throughout evolution: Towards a molecular understanding of their design principles[END_REF]

Fig A- 2 .

 2 Fig A-2. Structural and chemical representation of cellulose (A)(Baucher et al., 2003), hemicellulose (B) , lignin (C) (Vanholme et al., 2010) and pectin (D) (Cosgrove, 2005).

Fig A- 3 .

 3 Fig A-3. The structure organization of plant cell wall. (A) Typical structural organization in wood cell wall (Sticklen, 2008). (B) The interaction model in primary cell wall of dicots. a. hemicellulose bind (a) to the surfaces or entrapped into (Mccann et al., 1990)(b) of cellulose microfibrils and tighten two adjacent microfibrils by hemicellulose (a) or pectin polysaccharides (c).Macromolecules may be formed by hypothetical covalent bond (d) between xyloglucan and pectin polysaccharides and between arabinoxylan and ferulic acid esters (A-F-F-A)(e)[START_REF] Cosgrove | Growth of the plant cell wall[END_REF].

Fig B- 1 .

 1 Fig B-1. The presumed inverting and retaining mechanism[START_REF] Mccarter | Mechanisms of enzymatic glycoside hydrolysis[END_REF]

Fig B- 2 .

 2 Fig B-2. Sugar binding subsite topologies in glycosyl hydrolases.

Five

  types of cellulases are commonly identified. Most cellulases are endo-1,4-β -Dglucanases or 1,4-β-D -glucan-4-glucanohydrolases (EC 3.2.1.4), which have an open active site (Fig B-2B, e.g. PDB: 1TML)

Fig B- 3 .

 3 Fig B-3. Structure of xylan and the sites of attack by xylanolytic enzymes[START_REF] Salis | Industrial Enzymes[END_REF]

Fig B- 4 .

 4 Fig B-4. Alignment of GH10 xylanases with characterized structures

  β-D-Xylosidases (β-D-xyloside xylohydrolases; EC 3.2.1.37) are exoglycosidases, hereafter referred to as xylosidases, that act on the nonreducing ends of short xylooligosaccharides (FigB-3). Xylosidases operate through either a retaining (e.g. family GH 39) or inverting (e.g. family GH43) mechanism and liberate D-xylose. It has been observed

Fig B- 5 .

 5 Fig B-5. Schematic representation of the proposed cellulosome architecture in A.cellulolyticus[START_REF] Bayer | The cellulosomes: multienzyme machines for degradation of plant cell wall polysaccharides[END_REF].

Fig B- 6 .

 6 Fig B-6. Functional model of glycan processing based on eight-gene B.thetaiotaomicron starch utilization system (Sus)(Martens et al., 2009).

Fig C- 1 :

 1 Fig C-1: Termites and their intestinal tract[START_REF] Hongoh | Toward the functional analysis of uncultivable, symbiotic microorganisms in the termite gut[END_REF].A the lower termites Coptotermes formosanus. B the interface (wood and humus)-feeding higher termite Termes comis. C the gut of C.formosanus. D the gut of T.comis. F forgut, M midgut. Bars=1mm.

1, 4 -

 4 linked D-xylopyranosyl residues that often bear substitutions at O-2 and/or O-3 positions.Moieties such as L-arabinofuranosyl (L-Araf), 4-O-methyl glucuronyl (D-MeGlcAp) and acetyl residues are frequent main chain substituents, and substituting L-Araf moieties can be esterified by ferulate at their O-5 position. The exact nature and pattern of xylan main chain decorations depends on the botanical source. Moreover, when xylan is extracted from a single plant species, the tissue source and the developmental status will determine the type of xylan that is observed(Scheller et al., 2010). Graminaceous plants are generally rich in glucuronoarabinoxylan (GAX), whereas the dominant xylan in typical dicotyledons is glucuronoxylan (GX), the difference between these two categories being the relative amounts of L-Araf and D-MeGlcAp present. Complete xylan degradation requires an extensive arsenal of enzymes that often work in synergy. The main chain is depolymerized by β-D-xylanases that hydrolyze internal β-1, 4 bonds, while decorations are removed by a wide variety of accessory enzymes, including α-L-arabinofuranosidases, α-D-glucuronidases, feruloyl and acetyl esterases. Finally, β-D-xylosidases breakdown xylo-oligosaccharides, removing D-Xyl

  Beechwood GX, D-Xyl and most other reagents were purchased from Sigma-Aldrich (Saint-Quentin Fallavier, France). Low viscosity wheat arabinoxylan (Ara:Xyl=38:62, LVWAX), acid-debranched wheat arabinoxylan (Ara:Xyl=22:78, ADWAX), enzyme-debranched wheat arabinoxylan (Ara:Xyl=30:70, EDWAX), rye arabinoxylan (Ara:Xyl=38:62, RAX), galactomannan (carob; low viscosity), xyloglucan (tamarind), arabinan (sugar beet), βglucan (barley; medium viscocity), xylobiose (X2), xylotriose (X3), xylotetraose (X4), xylopentaose (X5), xylohexaose (X6), p-nitrophenyl-β-D-xylopyranoside (pNPX), p-nitrophenyl-β-D-xylobiose (pNPX2), p-nitrophenyl-β-D-xylotriose (pNPX3), p-nitrophenyl-β-D-xylotetraose (

  bran and aliquots were removed at regular time intervals and heated at 95°C for 10 min to terminate the reaction. Each sample was centrifuged at 20,000 x g for 5 min and quantified by HPAEC-PAD on a Dionex PA1 column equipped with Carbo-Pac PA-1 guard and analytical columns (4*50mm and 4*250mm, respectively). Separation of oligosaccharides was achieved by isocratic elution with 100 mM NaOH at a flow rate of 1 ml/min from 0 to 5 min, a gradient of 0 to 120 mM sodium acetate in 100 mM NaOH from 5 min to 25 min, and isocratic elution with 500 mM sodium acetate in 100 mM NaOH from 25 min to 35 min. Then the column was re-equilibrated with 100 mM NaOH for another 10 min. Calibration was achieved using D-Xyl and XOS (X2, X3, X4, X5 and X6) at concentrations from 5 to 100 µM.

Figure 1 .

 1 Figure 1. Modular architecture of Xyn10C-like protein. (A) Schematic representation of the putative xylan-active gene cluster. (B) Amino acid alignment of Pm25 with other native GH10 xylanases designated by their PDB codes. PDB: 3CUJ, Cellulomonas fimi Xylanase/Cellulase Cex (CfXyn10A); 1US3, Cellvibrio japonicus xylanase 10C; 1CLX, Pseudomonas fluorescens xylanase A; 1UQY, Cellvibrio mixtus Xylanase B; 3W24, Thermoanaerobacterium saccharolyticum xylanase A; The β strands and α helices are shown above the aligned sequences. Numbering corresponds to the sequences of Pm25. Residues in red background are conserved and within blue frames are conservatively substituted. (C) Amino acid alignment of CBM4s in Pm25 with well characterized CBM4s in literature. CBM4-1 is the first CBM4 in Pm25; CBM4-2 is the second CBM4 in Pm25; PDB: 2Y6G, CBM4 from Rhodothermus marinus xylanase; 1GU3, CBM4 from Cellulomonoas fimi endoglucanase C; 1GUI, CBM4 from Thermotoga maritima laminarinase 16A; 3K4Z, CBM4

Figure 2 .

 2 Figure 2. Taxonomic distribution of GH10 xylanase. The nodes are colored by the taxonomic annotation listed on the right.

Figure 3 .

 3 Figure 3. Optimal physiochemical parameters. (A). Optimum pH. (B) Thermoactivity at pH 7.5. (C) Thermostability at pH 7.5. (D) Thermostablity at pH 9.0.

Figure 4 .

 4 Figure 4. Mutational and truncation constructs. (A) Domain organization of Pm25 (WT) and modified constructs. (B) SDS-PAGE of the purified WT and M1-M7. (C) SDS-PAGE of the purified CBM constructs M8-M10.

Figure 5 .

 5 Figure 5. Progressive degradation of 0.2mM of XOS, (A) X6, (B) X5, (C) X4 by Pm25. (D) The potential subsite mapping of Pm25 with X6, X5 and X4.

Figure 6 .

 6 Figure 6. Affinity gel electrophoresis of different CBM mutations in Pm25. The control gel is lack of ligand, while the other one is with 0.06% (w/v) of RAX. (A) Different retardation of CBM4-1(M8) and its mutants. 1, M8; 2, Y213A; 3, Q216A; 4,N218A; 5,Y257A; 6,W259A; 7,N261A. The Kd values for each constructs are on the right. (B) Different retardation of CBM4-2 (M9) and its mutants. 1, M9; 2, Y378A; 3, Q381A; 4, N383A; 5, Y422A; 6, W424A. The Kd values for each constructs are on the right.

Figure 7 .

 7 Figure 7. Hydrolysis of LVWAX by Pm25 and its mutants. The time-dependent degradation of LVWAX by (A) Pm25 (WT), (B) M5 and (C) M6.

Figure 8 .

 8 Figure 8. Hydrolytic patterns of Pm25 (WT) and mutants towards wheat bran.

Figure S1 .

 S1 Figure S1. Affinity gel electrophoresis of CBMs in Pm25 towards 0.5% (w/v) xyloglucan.

Figure S3 .

 S3 Figure S3. Binding of inactive Pm25 and its truncated derivatives to wheat bran. BSA was used as negative control. Lane M, molecular mass markers. Lane 1, unbound fraction; lane 2, the third wash of the pellet; lane 3, bound fraction.

Figure S4 .

 S4 Figure S4. The time-dependent degradation of LVWAX by (A) M3 and (B) M4.

Fig 1 .

 1 Fig 1. Presence of xylan active enzymes in the putative PUL. Pm25, Pm22 and Pm12 are endo-acting on the main chain of xylan; Pm115 can remove the glucuronic acid decorations, and Pm21 can liberate xylose from xylobiose as well as arabinofuranose from side chain of xylan.

  The amino acid sequence of Pm12, Pm 22 and Pm 21 were rooted from its original sequences with the GenBank accession number CCO21040.1, CCO21038.1, and CCO21041.1, respectively. Pm 12 and Pm22 were cloned after removal of signal peptide predicted by SignalP4.1. Due to unsuccessful expression of Pm22, Pm 24, the truncation version of the GH11 module (residues 637-841) was also constructed. All three sequences were cloned into pDEST17 as described elsewhere (G.Arnal et al. manuscript in preparation, Vincentelli and Romier 2013). Pm115 (CCO21037.1) was cloned without the signal peptide into pET28 using the forward primers 5'-CAGCCATATGGCTAGCCAGTCCCGGGAGAGTAGA TTC-3', and reverse primer 5'-GGTGGTGGTGCTCGAGTCATTTGTCAATGGTTTTAC-3'. The recombinant proteins harboring His6 tags at their N-terminal extremities, were expressed in RosettaDE3 with autoinducing medium, except for Pm 115 which was expressed within TunerDE3 in LB (Luria-Bertani) medium induced by IPTG (isopropyl-β-D-l-thiogalactopyranoside, 0.2 mM final).

  of BGX was performed by incubating 1350 µl of 2% (w/v) BGX (citric acid 50 mM, dibasic sodium phosphate 100 mM, 1 mg/ml BSA) with 150 µl of enzyme mix in an Eppendorf™ Thermomixer™ R at 1400rpm, 37 °C. The enzyme amounts were set to make sure only around 10% of substrates is consumed in the first 15 min. Enzyme doses in the reaction were as follows, xylanase (Pm12, Pm24, Pm25, Pm25Y213AY378A and Pm25ΔCBMs), 2 nM; xylosidase (Pm21), 200 nM; α-Glucuronidase (Pm115), 10 nM. The enzyme combinations were made with "core enzymes" mixing with Pm25 and mutants thereof. There are four different core enzyme combinations, Pm12 and Pm24 (core2); core2 + Pm115 (Pm115@core3), core2+ Pm21 (Pm21@core3) and the four enzymes mentioned above Pm12, Pm24, Pm21+ Pm115 (core4). Reactions were conducted for 15 min, and after each time point(1, 2, 3, 4, 5, 10, 15 min), 190 µl of reaction was taken out and heated at 95 °C for 10 min to stop the reaction. The samples were ready to be analyzed after centrifuge at maximum speed for 10 min.The released X1 and XOS were quantified by HPAEC-PAD on a Dionex PA1 column equipped with Carbo-Pac PA-1 guard and analytical columns (4*50 mm and 4*250 mm, respectively). Separation of oligosaccharides was achieved by isocratic elution with 100 mM NaOH at a flow rate of 1 ml/min from 0 to 5 min, a gradient of 0 to 120 mM sodium acetate in 100 mM NaOH from 5 min to 25 min, and isocratic elution with 500 mM sodium acetate in 100 mM NaOH from 25 min to 35 min. Then the column is re-equilibrated with 100 mM NaOH for another 10 min. Calibration was achieved using X1 and XOS (X2, X3, X4, X5 and X6) at concentrations from 5 to 100 µM.For the analysis of xylan hydrolysis products by MALDI-TOF MS, 1 µl of hydrolysates was mixed with 1µl of matrix solution (10 mg/ml 2, 5-dihydroxybenzoic acid dissolved in acetonitrile-water [1:1, v/v], and 0.1 % [w/v] trifluoroacetic acid). One microliter of the mixture was mixed with matrix (DMA/DHB) and spotted onto the MALDI-TOF MS plate and allowed to dry before the analysis. Positive mass spectra were collected with a AUTOFLEX Speed instrument (linear mode, laser 1000Hz).

Fig 2 .

 2 Fig 2. The product (DP from 1 to 28) profile of by Pm25 and its mutants. The amounts of xylose and XOS in products were quantified by HPAEC-PAD after reactions of 30 min (A) and 14h (B). The variety of medium-sized products after treatment of Pm25, Pm25Y213AY378A and Pm25ΔCBMs were shown in (C), (D) and (E) respectively.

  ) using BGX as the substrate. Although Pm12, Pm24, Pm21 all display activity on WAX and BGX, BGX was chosen because it contains significant amounts of glucuronic acid substitutions (around 13% of glucuronic acid substitutions, Megazyme), necessary to investigate Pm115 activity. The inclusion of Pm21 (Pm21@core3 and core4) in the reaction mixture drastically increased in xylose release, consistent with the fact that we showed it is a xylosidase (Fig3A). Core4 yielded to a higher amount of xylose compared to Pm21@core3 (Fig3A), indicating that Pm115 may facilitate the action of Pm21 by removing substitutions (i.e. homosynergy between SCEs). Likewise, the amounts of oligosaccharides X2-6 released were lowest with core4, followed by Pm115@core3 (Fig 3B-F), suggesting that Pm115 doesn't complement Pm12 and Pm24 activities (i.e. no significant hetersynergy between SCE and MCE).

Fig 3 .

 3 Fig 3. The product (DP from 1 to 6) profile of different combinations of core enzymes. The amount (in µM) of xylose (A), xylobiose (B), xylotriose (C), xylotetraose (D), xylopentaose (E), and xylohexaose (F) was quantified by HPAEC-PAD.

Fig 4 .

 4 Fig 4. The product (DP from 5 to 28) profile of by different combinations of core enzymes.The populations of medium-sized products after treatment of core2 (A), Pm115@core3 (B), Pm21@core3 (C) and core4 (D) were accessed by MALDI-TOF. The number of methylated glucuronic acids in the products were counted after the treatment of core2(E) , Pm115@core3 (F), Pm21@core3 (G) and core4 (H).

Fig 5 .

 5 Fig 5. The product profile (DP from 1 to 6) of Pm25 and mutants in combination with core4. The amount of xylose (A), xylobiose (B) and xylotriose (C), xylotetraose (D), xylopentaose (E) and xylohexaose (F) were quantified by HPAEC-PAD. In order to study whether the Pm25-borne CBM4s positively or negatively contribute to synergy between PUL enzymes, the product profile of the wild type and mutant Pm25 with core4 (Fig 5) and core2 (Fig 6) were compared. As mentioned before, Pm21 plays a role in shaping the product profile (Fig 4C and D) and indeed the addition of either wild type or mutated Pm25 made no real difference to core4 (Fig 5) and Pm21@core3 (data not shown) although Pm25 mutants slightly accelerates and increases the release of X2-X5 (Fig 5B-E),

Fig 6 .

 6 Fig 6. The product (DP from 5 to 28) pattern of Pm25 and Pm25ΔCBMs in combination with core2. The populations of medium-sized products after treatment of (A) core2+Pm25, (B) core2+Pm25ΔCBMs were represented as intensity ratio. The numbers of methylated glucuronic acids in the products were counted after the treatment of core2+Pm25 (C) and core2+Pm25ΔCBMs (D).

2 .

 2 Fill capillaries by sticking the capillary horizontally into the reaction tube to aspirate the sample (see Note 20). The capillary containing the highest ligand concentration is placed in the front of the sample tray. 3. Place the sample tray in the instrument and perform the scan with the MO.Control software (see Note 21).

  displays a typical quenching sigmoidal curve not compatible with proper MST experiment. In such case it is recommended to perform a SDS-denaturation Test or SD-test to investigate if the quenching comes from a binding event or non-specific interactions with the ligand and the fluorophore, nonspecific adsorption to tube walls, protein aggregation, or just because the fluorophore is in close vicinity to the binding site (see Note 25). To perform a SD-Test, the samples 1-3 and 14-16 from the 16 reaction series are centrifuged 10 min at maximum speed in order to avoid sample adsorption to the tube walls. 10 µl of SD mix is added to each sample and well mixed by pipetting up and down three times. The mixture is incubated for 5 minutes at 95 °C and centrifuge briefly in order to denature the protein. The fluorescence intensity of each sample is measured after filling capillaries (see Note 26). In our case, protein denaturation leads to restore uniform fluorescence intensity in all capillaries, meaning that the variation of fluorescence was induced by a binding event. Select the tab "Initial Fluorescence Analysis Set" in the MO.Affinity Analysis software, select the raw data and then move to "Dose Response Fit". Choose Kd Model, fix the concentration of CBM4, a sigmoid curve is derived and well fitted to the data.2. Choose ∆Raw fluorescence [counts] as y-axis, and then the dose response curve appearsshowing that the fluorescence intensity decreases with increasing concentrations of the ligand (Fig.2b). The sigmoidal curve needs to have (1) a clear baseline and a clear saturation (at least 3 points respectively),(2) an amplitude/noise > 3(see Note 24).

Fig. 1

 1 Fig. 1 CBM3a. (A) Initial fluorescence of CBM3a towards different concentrations of cellulose nanocristals. The data showed a homogenous initial fluorescence, which indicated that MST experiment could be performed. (B) Measurement of CBM3a binding to cellulose nanocristals. Kd apparent = 0.24 ± 0.05 g/L. (C) Surface representation of CBM3a (pdb 1NBC). Arrow indicates residues involved in cellulose binding. * Indicate lysine residues potentially labeled with NHS-dye. None of these residues are in closed vicinity to the flat surface of the binding site.

Fig. 2

 2 Fig. 2 CBM4. (A) Initial fluorescence of CBM4 towards different concentrations of xylohexaose. Quenching of fluorescence indicated that MST experiment could not be performed. (B) Measurement of CBM4 binding to xylohexaose. Kd = 2.5 ± 0.3 10 3 µM. (C) Multiple protein sequence alignment of CBM4 with family 4 CBMs of known crystallographic structure. In grey, mains residues involved in ligand recognition. In black, CBM4 lysine residue potentially labeled with NHS-dye. Close proximity between Lys56 and Tyr57 in the binding site may explain the quenching of fluorescence during titration.

  . According to the difference in the mainchain, hemicelluloses can be

	roughly divided into xylan, xyloglucan (XG), glucomannan, mannan and arabinogalactan et
	al. Different plants contain different main hemicelluloses with acetylated (galacto)
	glucomannan (as well as arabinoglucuronoxylan), glucuronoxylan, and
	glucuronoarabinoxylan are major hemicellulose in softwood, hardwood, and grassy species,

respectively (c.f. Table

A-1

)

(Scheller and Ulvskov, 2010)

. In addition to its structural role, hemicelluloses in PCWs can fulfill other roles, such as signaling molecules and seed storage carbohydrates

(Scheller and Ulvskov, 2010)

.

Table A -

 A

		Dicot walls	Grass walls	Conifer walls
	Polysaccharide	Primary Secondary Primary Secondary Primary Secondary
	Xyloglucan	20-25	Minor	2-5	Minor	10	-b
	Glucuronoxylan	-	20-30	-	-	-	-
	Glucuronoarabinoxylan	5	-	20-40	40-50	2	5-15
	(Glucu)mannan	3-5	2-5	2	0-5	-	-
	Galactoglucomannan	-	0-3	-	-	+ b	10-30
	β-(1→3, 1→4)-glucan	Absent	Absent	2-15	Minor	Absent	Absent

1. Occurrence of hemicelluloses in primary and secondary cell walls of plants

(Scheller and Ulvskov, 2010)

Amout of polysaccharide in wall (% w/w) a a Actual values vary between different species and tissue types. b -, absent or minor; +, present but data not available.

Table A -

 A 2. Comparison of composition of plant cell wall in different groups of kingdomPlantae[START_REF] Sarkar | Plant cell walls throughout evolution: Towards a molecular understanding of their design principles[END_REF]

	Grass	low XG, high amount of mixed-linkage glucans, xylan	low HG, RG	high lignin with guaiacyl and syringyl units
	Gymnosperm	high XG, xylan glucomannan and structural proteins	high HG, RG I and II	high lignin with guaiacyl units
	Leptosporangiate	low XG, high xylan, mannan, uronic acids, 3-O-methylrhamnose	low HG, RG	high lignin with guaiacyl units
	Eusporangiate and bryophyte	low XG, high mannan, uronic acids, 3-O-methylrhamnose	low HG, RG, phenolic compounds instead of lignin	-
		hemicellulose	primary cell wall	secondary cell wall
	Non-grass angiosperm	high XG, xylan and some mannan and structural proteins	high HG, RG I and II	high lignin with guaiacyl and syringyl units

Table A -

 A 3. Major agricultural wastes and their worldwide availability, bioethanol production potential and compositions[START_REF] Sarkar | Bioethanol production from agricultural wastes: An overview[END_REF]

				Agrowaste
			Rice straw	Wheat straw	Corn straw	Bagasse
	Global	Availability (million tons)	731.3	354.34	128.02	180.73
	productio n	Bioethanol potential (Gl) a	205	104	58.6	51.3
	Polymer / dry	Cellulose (%) Hemicellulose (%)	32-47 19-27	35-45 20-30	42.6 21.3	65
	matter	Lignin (%)	5-24	8-15	8.2	18.4
		Glucose (%)	41-43.4	38.8 ± 0.5	39	38.1
	monomer	Xylose (%)	14.8-20.2	22.2 ± 0.3	14.8	23.3
	/ dry	Mannose (%)	1.8	1.7 ± 0.2	0.3	-
	matter	Galactose (%)	0.4	2.7 ± 0.1	0.8	1.1
		Arabinose (%)	2.7-4.5	4.7 ± 0.1	3.2	2.5
	a potential annual bioethanol global production, Gl, giga liter.

  was done using InterPro protein sequence analysis (https://www.ebi.ac.uk/interpro/) and the accession S0DFK9. Multiple protein sequence alignment of CBM4s was done using Clustal Omega at EBI (https://www.ebi.ac.uk/Tools/msa/clustalo/) and the alignment of the secondary structure elements of Pm25 with other structurally-characterized GH10 family members was achieved using both Clustal Omega and ESPript 3 at

http://espript.ibcp.fr/ESPript/ESPript/

(Robert et al., 2014)

.

Table 1 . Kinetic parameters of Pm25.

 1

		Activity	KMapp	kcat	kcat/KM
	Substrate	(U/mg)	(mg/ml)	(/s)	(/min/mM)
	Beechwood GX	7.4 ± 0.1	3.1 ± 0.2 10.3 ± 0.2
	LVWAX	25.8 ± 0.8 2.0 ± 0.2 36.2 ± 1.1
	ADWAX	21.7 ± 0.5 1.0 ± 0.1 30.4 ± 0.7
	EDWAX	21.4 ± 0.4 1.7 ± 0.1 29.9 ± 0.5
	RAX	30.3 ± 0.6 1.4 ± 0.1 42.4 ± 0.8
	X6				292.8 ± 36.8
	X5				35.5 ± 2.4
	X4				3.75 ± 0.1
	pNPX4				2381.7 ± 381.7

KMapp, apparent KM.

Table 2 . Binding of the recombinant proteins to soluble substrates as determined in affinity gel electrophoresis.

 2 The unit for Kd is in mg/ml; + , retardation observable; -, no retardation.

		M1	M7	M8	M9	M10
	Ligand			Kd (10 -2 )
	RAX	2.8 ± 0.1	18.8 ± 3.5	31.9 ± 0.8	18.1 ± 2.2	5.1 ± 0.4
	LVWAX	3.7 ± 0.3	27.1 ± 1.9	22.7 ± 1.1	15.9 ± 1.9	3.7 ± 0.2
	ADWAX	2.3 ± 0.2	15.6 ± 0.2	10.9 ± 0.7	8.0 ± 0.1	6.4 ± 0
	EDWAX	2.0 ± 0.1	16.7 ± 0.4	12.6 ± 0.8	9.2 ± 0.

Table S2 . The concentration series of RAX used for different constructs.

 S2

	RAX‰
	(w/v)	#1	#2	#3	#4	#5	#6	#7	#8	#9	#10	#11	#12
			0.			0.10
	M8	0.9	6	0.3	0.15	5	0.06	0.03
			1.						0.10
	M8_Y213A	2.1	5	1.2	0.9	0.6	0.3	0.15	5	0.06	0.03
			1.						0.10
	M8_Q216A	1.5	2	0.9	0.6	0.45	0.3	0.15	5	0.06	0.03
			0.			0.10
	M8_N218A	0.9	6	0.3	0.15	5	0.06	0.03
			1.						0.10
	M8_Y257A	2.1	5	1.2	0.9	0.6	0.3	0.15	5	0.06	0.03
	M8_W259		1.					0.10
	A	1.5	2	0.9	0.6	0.3	0.15	5	0.06	0.03
			0.			0.10
	M8_N261A	0.9	6	0.3	0.15	5	0.06	0.03
			1.					0.12	0.10
	M9	1.5	2	0.9	0.6	0.3	0.15	6	5	0.06	0.03
			2.
	M9_Y378A	3	1	1.5	1.2	0.9	0.6	0.45	0.3	0.15	0.105	0.06
			1.					0.10
	M9_Q381A	1.5	2	0.9	0.6	0.3	0.15	5	0.06
			1.					0.12	0.10
	M9_N383A	1.5	2	0.9	0.6	0.3	0.15	6	5	0.09	0.06
			1.							0.10
	M9_Y422A	2.1	5	1.2	0.9	0.6	0.45	0.3	0.15	5	0.06
	M9_W424		1.					0.10
	A	1.5	2	0.9	0.6	0.3	0.15	5	0.06
		0.4	0.
	M11	5

Table S3 . Relative activity (%) of Pm25 and mutants thereof towards polysaccharides.

 S3

	Substrate	Pm25	M 1	M 2
	RAX	100 ± 0.89	0.19 ± 0	0.24 ± 0
	LVWAX	100 ± 6.57	1.08 ± 0.03	0.76 ± 0.02
	Beechwood GX	100 ± 4.67	4.50 ± 0.15	3.08 ± 0.13

Table 1 . Biochemical properties and kinetic parameters of Pm12, Pm24 and Pm25.

 1

	Enzyme		Pm12	Pm24	Pm25 a
	Final yield of purified protein (mg/L)	126	12.8	34.9
	Theoretical pI		5.16	5.43	4.34
	Mol mass (kDa)		43.23	26.57	83.97
	Optimal pH b		6.5-7.5 (6.5)	5.0-9.0 (6.0)	4.5-9.0 (7.5)
	Optimal T°C b		45-60 (50)	60-65 (40)	60-65 (50)
	Km app (g/L)	BGX	1.2 ± 0.2	1.5 ± 0.2	3.1 ± 0.2
		WAX	3.0 ± 0.6	3.2 ± 0.4	2.0 ± 0.2
		BGX	296.2 ± 11.8	179.7 ± 7.6	10.3 ± 0.2
	Kcat(/s)
		WAX	608.1 ± 43.4	799.0 ± 29.0	36.2 ± 1.1

a The biochemical properties of Pm25 have been reported elsewhere

[article 1]

  1 , Cédric Y. Montanier 1 and Claire Dumon 1 *

	1 LISBP, Université de Toulouse, CNRS, INRA, INSA, Toulouse, France
	*To whom correspondence should be addressed: LISBP-INSA Toulouse, 135 Avenue de,
	Rangueil, 31077 Toulouse, France. Phone: + 33 (0)5 61 55 94 93; Fax: +33 (0)5 61 55 94 00;

  2 mL PCR tubes. Transfer 20 µl of the highest ligand concentration into vial 1 (see Note 16), then proceed to cascade dilution as follow: add 10 µl of dilution buffer into vial 2 to vial 16. Transfer 10 µl from vial 1 to vial 2, mix by pipetting up and down three times (see Note 17). Then transfer 10 µl from vial 2 to 3. Repeat this step successively for all vials. Remove 10 µl from vial 16 after mixing to end up with 10 µL final.1. Prepare 2 X stock solution of labeled protein (see Note 18). CBM3a and CBM4 were diluted to 8.6 10 -4 g/L and 0.114 μM respectively with dilution buffer and Pluronic ® F-127 (0.1% final). Add 10 µl of protein stock to each vial of the dilution series (see Note 19).

Acknowledgement:

We acknowledge the Fédération de Recherche Agrobiosciences, Interactions et Biodiversité (FR 3450), CNRS, Université de Toulouse, UPS, Castanet-Tolosan, France, and the IDEX "UNITI" Université de Toulouse (GO-AHEAD project) for the Nanotemper Monolith NT.115 facilities.

intern ever! I am so lucky to have you involve in my PhD.

I gratefully acknowlege the support of the Chinese Scholarship Council that funded my 42months of thesis.

 Gal-Coëffet, M.-F. Le, Williamson, G., Archer, D.B., and Williamson, M.P. (1997). Solution structure of the granular starch binding domain of Aspergillus niger glucoamylase bound to β-cyclodextrin. Structure 5, 647-661. Southall, S.M., Simpson, P.J., Gilbert, H.J., Williamson, G., and Williamson, M.P. (1999).

The starch-binding domain from glucoamylase disrupts the structure of starch. FEBS Lett. 447, 58-60. Stam, M.R., Danchin, E.G.J., Rancurel, C., Coutinho, P.M., and Henrissat, B. (2006).

Dividing the large glycoside hydrolase family 13 into subfamilies: Towards improved functional annotations of α-amylase-related proteins. Protein Eng. Des. Sel. 19, 555-562. Stephen, A.M. (1983). Other Plant Polysaccharides. In The Polysaccharides, VOL. 2, pp. 97-193. Sticklen, M.B. (2008). Plant genetic engineering for biofuel production: towards affordable cellulosic ethanol. Nat. Rev. Genet. 9, 433-443. Straus, D., and Gilbert, W. (1985). Genetic engineering in the Precambrian: structure of the chicken triosephosphate isomerase gene. Mol. Cell. Biol. 5, 3497-3506. Suh, J.-H., and Choi, Y.-J. (1996a). Synergism among endo-xylanase, beta-xylosidase, and acetyl xylan esterase from Bacillus stearothermophilus. J. Microbiol. Biotechnol. 6, 173-178.

ARTICLE I SUPPLEMENTARY MATERIAL: Materials and Methods:

Enzyme specificity of Pm25 and mutants thereof. To compare the specific activity of Pm25 and mutants thereof, RAX (0.75%, w/v), LVWAX (0.4%, w/v) and beechwood GX (1%, w/v) were employed. The final concentration of Pm25 used to degrade beechwood GX and other soluble substrates is 35 and 7 mg/L, respectively. Owing to the weaker activity of the mutants (M1 and 2), for activity assays the final enzyme concentration was increased 100-fold compared to assays using the wild type Pm25.

Comparison of the product profile of Pm25 and its mutants

In order to study the intramolecular synergy between the CBM4s and GH10 in Pm25, the product profile of the wild type and mutant Pm25 against BGX were compared. In order to characterize the effects of the above enzymes on medium-sized XOS, mass spectroscopy analysis was performed. Oligosaccharides from DP 5 to 28 were monitored and grouped by size range (Fig 2C, D andE). The intact enzyme released 1.5-fold less DP 5-8 than the CBM-truncated construct, while the double mutant (lacking both CBMs) produced an intermediate result (Fig 2C,D and E).

Taking into account the similarity of these results, we suggest that interplay between the catalytic domain and the CBMs in Pm25 is not an advantage in xylan depolymerization, and may even somehow hinder the hydrolysis at least in the beginning of the reaction (i.e. antisynergy). Similar behavior were found for another CBM-insertional enzyme, SusG (Koropatkin and Smith, 2010).

What can be done in the future to further address this topic?

The function of the CBMs in Pm25 is probably relative to insoluble substrate/complex biomass. It would be interesting to compare the wild type and mutant Pm25 in degrading complex biomass with/without other PUL enzymes. In this respect, new techniques are required to compare the medium-sized products as the HPAEC/MALDI-TOF are not practical.

In the PUL, there is an unknown protein (Figure 1A, Article I) (Dodd et al., 2011), whose gene locus is always between SusD protein and Pm25. It was predicted to be SusE-like protein. In our PUL, we produced the SusE-like protein and showed that it can bind to beechwood xylan. It would be interesting to further characterize this protein and better understand its role in the PUL.

Until now, no crystal structure of Xyn10C protein is published. Its 3D structure would be very informative to understand its domain organization, pH tolerance, and in designing chimeric proteins. Although several attempt to obtain the crystal structure was unsuccessfull (E. Ioannou's thesis). And it would be interesting to compare the crytal structure of wild type Pm25 and the CBM-truncated mutant thereof.

The deletion of the PUL where there is Xyn10C affected the bacterial growth on simple xylan (Rogowski et al., 2015). To further understand the function of Xyn10C, the xyn10Cknock out Bacteroidale strain could be constructed and see how it affect the growth on xylan with/without the rescue experiments, the different mutant of Pm25 could be used to restore the function and could may be help to understand if the CBM have a critical role in the xylan degradation.