David Ribeaucourt
email: ribeaucourt.david@gmail.com

Bastien Bissaro

Fanny Lambert

Mickael Lafond

Jean-Guy Berrin
email: jean-guy.berrin@inrae.fr

Biocatalytic oxidation of fatty alcohols into aldehydes for the flavors and fragrances industry

Keywords: Flavors and Fragrances, Fragrant aldehydes, Biocatalysis, Enzymes, Copper-Radical Oxidase, Fatty Alcohol, Fatty Aldehyde, AA5_2

From Egyptian mummies to the Chanel n°5 perfume, fatty aldehydes have long been used and keep impacting our senses in a wide range of foods, beverages and perfumes. Natural sources of fatty aldehydes are threatened by qualitative and quantitative variability while traditional chemical routes are insufficient to answer the society shift toward more sustainable and natural products. The production of fatty aldehydes using biotechnologies is therefore the most promising alternative for the flavors and fragrances industry. In this review, after drawing the portrait of the origin and characteristics of fragrant fatty aldehydes, we present the three main classes of enzymes that catalyze the reaction of fatty alcohols oxidation into aldehydes, namely alcohol dehydrogenases, flavin-dependent alcohol oxidases and copper radical alcohol oxidases. The constraints, challenges and opportunities to implement these oxidative enzymes in the flavors and fragrances industry are then discussed. By setting the scene on the biocatalytic production of fatty aldehydes, and providing a critical assessment of its potential, we expect this review to contribute to the development of biotechnology-based solutions in the flavors and fragrances industry.

Introduction

Flavors and fragrances (F&F) are found in a wide variety of consumer goods: foods and beverages, fine fragrances, pharmaceuticals, cosmetics, soap, household products, etc. Today, more than 3,000 molecules are used to manufacture products that affect our senses of smell and taste [START_REF] Zviely | Aroma Chemicals[END_REF]. While F&F usually make up only a minor quantitative fraction of the final product, they play a major role in its qualitative perception by consumers. In 2019, the value of the global F&F market reached 31.5 billion U.S. dollars and is predicted to grow by 3.5 % per year up to 2024 (IAL Consultants, 2020). Current industrial production of F&F relies to a large extent on chemical synthesis providing large quantities of material at cost-effective prices.

However, chemical synthesis has many drawbacks as (i) it is based on high energy consuming processes, (ii) it requires the use of metallic or toxic catalysts, (iii) it often relies on fossil material [START_REF] Johannsen | Process for the Preparation of Fatty Aldehydes from Renewable Resources[END_REF], and (iv) it generates large amounts of wastes and thus raises environmental concerns [START_REF] Braga | Generation of Flavors and Fragrances Through Biotransformation and De Novo Synthesis[END_REF][START_REF] Silveira | An Overview of Biotechnological Processes in the Food Industry[END_REF]. Chemical processes are usually poorly regio-and enantio-selective resulting in racemic mixtures that require further downstream processing (e.g. purification) to avoid unwanted off-flavors [START_REF] Berger | Biotechnology as a source of natural volatile flavours[END_REF][START_REF] Longo | Production of Food Aroma Compounds: Microbial and Enzymatic Methodologies[END_REF][START_REF] Serra | Biocatalytic preparation of natural flavours and fragrances[END_REF]. The growing societal environmental concerns go along with a steadily increasing demand for "natural" products [START_REF] Román | The importance of food naturalness for consumers: Results of a systematic review[END_REF], a feature usually not satisfied by established chemical routes. Many F&F are directly extracted from natural raw materials, using empirical processes such as distillation. This approach provides natural products with complex olfactive profiles but suffers from extremely low production yields. For instance, vanilla requires the meticulous manual pollination of each flower [START_REF] Frey | Natural Flavors and Fragrances: Chemistry, Analysis, and Production, in: Natural Flavors and Fragrances[END_REF][START_REF] Gallage | Vanillin-Bioconversion and Bioengineering of the Most Popular Plant Flavor and Its De Novo Biosynthesis in the Vanilla Orchid[END_REF]. Raw material supply is also vulnerable to environmental (i.e. climate variability, pathogens expansion, etc.), economic, social and political fluctuations [START_REF] Braga | Generation of Flavors and Fragrances Through Biotransformation and De Novo Synthesis[END_REF]. Consequently, there is a growing demand to provide alternatives for the production of natural F&F. Paradoxically, growing demand for food ingredients of natural origin has occasionally led to shortage of some plant resources such as vanilla or peppermint [START_REF] Dunkel | Nature's Chemical Signatures in Human Olfaction: A Foodborne Perspective for Future Biotechnology[END_REF][START_REF] Malik | Biotechnological Interventions for Production of Flavour and Fragrance Compounds[END_REF]. The tremendous increase of consumer demand over natural, and sustainable products, combined with the limitations of chemical and extractive routes call for a middle path, which likely lies in the rise of biotechnologies [START_REF] Berger | Biotechnology of flavours-the next generation[END_REF][START_REF] Check Hayden | Synthetic-biology firms shift focus[END_REF][START_REF] Pessôa | Prospective Research and Current Technologies for Bioflavor Production[END_REF][START_REF] Schwarz | Bio-based flavours and fragrances | Advanced Technologies for Industry[END_REF][START_REF] Silveira | An Overview of Biotechnological Processes in the Food Industry[END_REF][START_REF] Waltz | Engineers of scent[END_REF].

Among F&F compounds, aldehydes constitute a prominent class of molecules broadly used as food products additives [START_REF] Tsuzuki | Higher Straight-Chain Aliphatic Aldehydes: Importance as Odor-Active Volatiles in Human Foods and Issues for Future Research[END_REF]. Aldehydes are also key components of perfumes [START_REF] Surburg | Common Fragrance and Flavor Materials: Preparation, Properties and Uses[END_REF]. Most notably, straight chain (C6 to C13) saturated aldehydes, henceforth denoted as "fatty aldehydes", occupy a central role in the F&F field (see section 1), and will thus be the focus of the present review.

Biotechnological production of aldehydes can be tackled by two approaches: (i) de novo synthesis, which relies on the metabolization of simple substrates (e.g. glucose) by natural or engineered organisms to yield the final desired product (this approach will not be covered here but has been reviewed elsewhere [START_REF] Gounaris | Biotechnology for the production of essential oils, flavours and volatile isolates. A review[END_REF][START_REF] Kunjapur | Microbial Engineering for Aldehyde Synthesis[END_REF]), or by (ii) biotransformation/bioconversion, which consist in biocatalytic chemical modification of a precursor structurally related to the product through one or several step(s) catalyzed by enzyme(s) [START_REF] Braga | Generation of Flavors and Fragrances Through Biotransformation and De Novo Synthesis[END_REF][START_REF] Pessôa | Prospective Research and Current Technologies for Bioflavor Production[END_REF]. In the first section of this review, we briefly draw the portrait of aldehydes as key aroma and fragrance molecules. In the second section, we review the main enzymes of interest for oxidation of fatty alcohols, namely: alcohol dehydrogenases (ADHs -EC 1.1.1.1), flavin-dependent alcohol oxidases (FAD-AOXs -EC 1.1.3.13) and copper radical alcohol oxidases (CRO-AlcOx -EC 1.1.3.13). We then discuss the available resources to find new biocatalysts and the critical enzymatic and process criteria to consider for implementation of efficient bioprocesses. In the last section, we discuss some problematics regarding transfer from lab-scale to industrial F&F exploitation.

Aldehydes in the flavors and fragrances industry 1.1.

A brief history of fragrant aldehydes.

First recorded uses of fragrant substances date back to the Antiquity. In that time, odorants materials (e.g. tree barks, aromatic plants, resins…) were usually burnt to release and spread their scent. Ancient Egyptians (3150-30 BC) used to prepare cedarwood oil for the embalming process to protect mummies from insects and microbial attacks [START_REF] Balasubramanian | Scented Oils and Perfumes[END_REF][START_REF] Sawamura | Citrus Essential Oils: Flavor and Fragrance[END_REF]. During most of the Human history, essential oils have been the main source of fragrant materials and were often reserved to an elite and/or dedicated to specific usages (e.g. religious practices). Essential oils are still today of great importance for perfumers and flavorists. From late 19 th century and over the 20 th century, a modern era for F&F, marked by the democratization of their use opened up, notably thanks to the advent of both analytical and organic chemistry [START_REF] Brenna | Biotechnological Production of Flavors[END_REF][START_REF] Sell | The Chemistry of Fragrances: From Perfumer to Consumer[END_REF][START_REF] Taylor | Food Flavour Technology[END_REF].

Aldehydes are intimately linked to the early days of fragrance chemistry.

Cinnamaldehyde [START_REF] Chiozza | Sur la production artificielle de l'essence de cannelle[END_REF][START_REF] Richmond | Preparation of cinnamaldehyde[END_REF] and benzaldehyde were some of the first isolated and characterized chemical compounds. The latter being the first perfumery molecule isolated from nature and synthetically produced in 1832 [START_REF] Wöhler | Untersuchungen über das Radikal der Benzoesäure[END_REF]. Natural benzaldehyde from bitter almond, pits of apricots, peaches or plums is recovered in low yields by enzymatic hydrolysis of amygdalin (a cyanogenic glycoside), a process that must be conducted carefully to get rid of the poisonous cyanide [START_REF] Clark | Benzaldehyde. Perfumer & Flavorist[END_REF]. (Bio)catalytic production processes yielding simpler and safer product profiles are therefore of obvious interest. In 1874, Tiemann and Haarmann reported the first isolation and chemical synthesis of vanillin [START_REF] Tiemann | Ueber das Coniferin und seine Umwandlung in das aromatische Princip der Vanille[END_REF]. In 1921, the introduction of a combination at high dosage of the C10, C11 and C12 saturated fatty aldehydes and of 2-methylundecanal, allowed the design of the landmark famous perfume "Chanel N°5". This perfume truly popularized the use of synthetic ingredients in modern perfumery.

Characteristics and origin of commonly used fatty aldehydes

Despite their importance as reactive intermediates in various chemical routes [START_REF] Zhou | Bioaldehydes and beyond: Expanding the realm of bioderived chemicals using biogenic aldehydes as platforms[END_REF], the valorization of aldehydes per se is rare. Exceptions to this are some low molecular weight aldehydes (e.g. acetaldehyde, formaldehyde) and aldehydes used as perfumes, fragrances ingredients and flavoring agents in the F&F industry. In this domain, aldehydes are a staple, ubiquitous in food [START_REF] Parker | 1 -Introduction to aroma compounds in foods[END_REF][START_REF] Tsuzuki | Higher Straight-Chain Aliphatic Aldehydes: Importance as Odor-Active Volatiles in Human Foods and Issues for Future Research[END_REF], and necessary for the elaboration of most perfumes [START_REF] Kohlpaintner | Aldehydes, Aliphatic, in: Ullmann's Encyclopedia of Industrial Chemistry[END_REF], illustrated by a dedicated odor descriptive termi.e. "aldehydic" [START_REF] Surburg | Common Fragrance and Flavor Materials: Preparation, Properties and Uses[END_REF]. Aldehydes used in F&F are very odorant compounds with a low odor threshold and are therefore highly diluted in final products [START_REF] Pessôa | Prospective Research and Current Technologies for Bioflavor Production[END_REF][START_REF] Tsuzuki | Higher Straight-Chain Aliphatic Aldehydes: Importance as Odor-Active Volatiles in Human Foods and Issues for Future Research[END_REF].

Among the short-chain aliphatic aldehydes, acetaldehyde is a key compound in fruity flavoring [START_REF] Rowe | Overview of Flavor and Fragrance Materials[END_REF]. Fatty aldehydes ≥ C6 display a dual "citrus-like/fatty" character. Their industrial relevance is mainly restricted to C6, & C8-C13 aldehydes [START_REF] Kohlpaintner | Aldehydes, Aliphatic, in: Ullmann's Encyclopedia of Industrial Chemistry[END_REF][START_REF] Surburg | Common Fragrance and Flavor Materials: Preparation, Properties and Uses[END_REF]. Indeed, increase of the alkyl chain length entails the predominance of the "fatty", unpleasant, character, rendering the use of compounds > C13 inappropriate for most applications as F&F.

Hexanal (C6) releases a "green-note" odor, reminiscent of a "cut-grass" (Table 1). Fatty aldehydes with even number (C8, C10, C12) occur naturally in the oil glands of citrus peel (i.e. in the flavedo) yielding "green", "fresh" and "citrus-like" notes. In these oils, C8 and C10 aldehydes usually provide the "fresh/citrus" character while C12 gives the floral note [START_REF] Truong | Favored Conformations of Carbonyl Compounds: A Structural Study of n-Octanal[END_REF]. Octanal (C8) is also of importance in orange juice, being one of the main contributors of its flavor (Averbeck and Schieberle, 2009;[START_REF] Porat | The flavor of citrus fruit[END_REF], Overall, citrus aromas represent the most important tonality of the flavoring market (20 % of the demand) (IAL Consultants, 2020). Traditional extraction of citrus essential oil is performed by "cold press" extraction (also referred to as "expression"), by applying physical means to shatter the oil gland and recover the so-called "peel-oil", which is the most widely produced essential oil worldwide. Production of citrus peel oil is usually conducted concomitantly with production of citrus juice. Most of the global production of these juices relies on Brazilian and Florida citrus crops [START_REF] Berger | Flavours and Fragrances: Chemistry, Bioprocessing and Sustainability[END_REF][START_REF] Diaz | Citrus peel oil deterpenation with supercritical fluids: Optimal process and solvent cycle design[END_REF][START_REF] Porat | The flavor of citrus fruit[END_REF][START_REF] Sell | The Chemistry of Fragrances: From Perfumer to Consumer[END_REF] and the emergence of the citrus greening disease -Huánglóngbìng disease (lit. "Yellow dragon disease")raises concern about our dependency on this major source of natural molecules. The bacteria Candidatus Liberibacter spp. are the causal agent of this severe disease that affects fruit flavor and causes premature death of the infected trees [START_REF] Baldwin | Effect of Liberibacter Infection (Huanglongbing Disease) of Citrus on Orange Fruit Physiology and Fruit/Fruit Juice Quality: Chemical and Physical Analyses[END_REF][START_REF] Dala-Paula | Effect of Huanglongbing or Greening Disease on Orange Juice Quality, a Review[END_REF].

Overall, socio-environmental concerns together with the urge for a renewal of supply sources call for the development of alternative, greener routes for the production of fragrant aldehydes. Biotechnology and more specifically biocatalysis, offers promising perspectives to tackle this challenge. In the following sections, we present some of the major available oxidative enzymes for the production of fatty aldehydes.

Overview of bio-oxidative catalysts

The use of enzymes to produce F&F molecules has been for a long time restricted to a small set of reactions. However, over the past decades, considerable improvements have been made in the development of biotechnological solutions and the relevance of the enzymatic approach is now widely recognized [START_REF] Fasim | Large-scale production of enzymes for biotechnology uses[END_REF][START_REF] Musa | Recent advances in alcohol dehydrogenase-catalyzed asymmetric production of hydrophobic alcohols[END_REF]. Powerful tools have been developed to improve biocatalysts and their use including computer-assisted evolution of enzymes [START_REF] Arnold | Innovation by Evolution: Bringing New Chemistry to Life (Nobel Lecture)[END_REF], immobilization techniques, biphasic systems, thermostability engineering, large-scale recombinant enzyme production and isolation of highly enantio-and regio-selective enzymes (see section 3. and 4.) [START_REF] Cao | Enzyme Catalyst Engineering toward the Integration of Biocatalysis and Chemocatalysis[END_REF][START_REF] Wu | Biocatalysis: Enzymatic Synthesis for Industrial Applications[END_REF].

Using enzymes, aldehydes can be obtained through either an oxidative or a reductive route, starting from an alcohol or a carboxylic acid as precursor substrate, respectively.

Carboxylic acid reductases perform the reduction of carboxylic acid to aldehydes at the expense of ATP and NADPH as co-factors. Such expensive organic co-factors constrain their use in whole cell systems. Yet, aldehydes are often toxic for the cells and thus, are further reduced to alcohols or reoxidized to carboxylic acids by endogenous enzymes. The reductive route will not be further discussed here, and readers are directed to some recent reviews on this topic [START_REF] Derrington | Carboxylic acid reductases (CARs): An industrial perspective[END_REF][START_REF] Maurer | Investigation of fatty aldehyde and alcohol synthesis from fatty acids by αDoxor CAR-expressing Escherichia coli[END_REF][START_REF] Winkler | Carboxylic acid reductase enzymes (CARs)[END_REF].

Three main classes of enzymes are used for alcohol oxidation and will be discussed hereinafter: ADHs, FAD-AOXs and CRO-AlcOx. On the one hand, ADHs are broadly available, deeply studied and catalyze a reversible reaction using expensive organic nicotinamide cofactor as electron acceptor (commonly NAD(P) + ). On the other hand, despite being less studied and available, FAD-AOXs and CRO-AlcOx use only molecular O2 as inorganic final electron acceptor to catalyze the irreversible oxidation of the alcohol into the corresponding aldehyde. Such characteristics entail significant practical differences in the implementation of the biocatalytic systems, as discussed hereinafter. An overview of the main advantages and drawbacks of each biocatalytic system is presented in Figure 1. In Table S1, we

provide the reader with more information on the enzymes discussed hereafter, including notably their origin, biochemical properties, 3D structures, kinetic parameters and conversion efficiencies. S1 for more details on properties and kinetic parameters of the displayed enzymes Note that on the Y-axis (kcat/KM) is displayed in logarithmic scale.

Alcohol dehydrogenases (ADHs)

2.1.1. Source of ADHs ADHs are ubiquitous in living organisms [START_REF] Thompson | Molecular evolution and functional divergence of alcohol dehydrogenases in animals, fungi and plants[END_REF]. ADHs, which are mostly intracellular, are involved in various metabolic pathways. In anaerobic bacteria and yeast, ADHs reduce acetaldehyde to ethanol, whose toxicity inhibits the growth of other microorganisms [START_REF] Hernández-Tobías | Natural alcohol exposure: Is ethanol the main substrate for alcohol dehydrogenases in animals?[END_REF] while recycling NAD(P) + [START_REF] Reid | Molecular Characterization of Microbial Alcohol Dehydrogenases[END_REF]. In humans, ADHs promote the opposite reaction to detoxify deleterious ethanol [START_REF] Edenberg | The Genetics of Alcohol Metabolism: Role of Alcohol Dehydrogenase and Aldehyde Dehydrogenase Variants[END_REF].

Structure and catalytic mechanism of ADHs

In terms of 3D structure, ADHs share a conserved Rossmann fold for cofactor binding, but otherwise display a high structural diversity. The three main classes of ADHs are distinguished according to sequence length and structural/mechanistic features: short-chain dehydrogenases (SDRs) encompass small ADHs condensed in a single domain while mediumchain dehydrogenases (MDRs), and long-chain dehydrogenases (LDRs), both harbor a cofactor binding domain and a catalytic domain [START_REF] An | Structural insights into alcohol dehydrogenases catalyzing asymmetric reductions[END_REF]. SDRs are metal-free enzymes while MDRs usually make use of two Zn co-factors (one involved in catalysis and the second one playing a structural rolesee Figure 2A). LDRsa more heterogenous classcan require either Zn and/or Fe as co-factor, or sometime no metal ion at all [START_REF] Vidal | Review of NAD(P)H-dependent oxidoreductases: Properties, engineering and application[END_REF]. The Zn-containing ADHs are generally found as dimers (mostly in higher plants and mammals) or tetramers (in bacteria and yeasts) [START_REF] An | Structural insights into alcohol dehydrogenases catalyzing asymmetric reductions[END_REF]. ADHs operate through a hydride transfer from the substrate to the NAD(P) + co-factor with concomitant release of a proton (Figure 2B) [START_REF] Crichton | Chapter 12 -Zinc -Lewis Acid and Gene Regulator[END_REF]. The reaction they catalyze is reversible. These enzymes can both oxidize alcohol or reduce carbonyl compounds (note that some ADHs are also called ketone reductases -KRED), depending on the enzyme and substrate, the oxidation or reduction route can be preferred and the pH of the medium can influence the direction of the reaction [START_REF] D'auria | Alcohol Dehydrogenase, in: Handbook of Food Enzymology[END_REF].

Yet, from a thermodynamic standpoint, ADH-catalyzed reductive reaction is usually strongly favored over alcohol oxidation [START_REF] Hollmann | Oxidation of Alcohols, Aldehydes, and Acids[END_REF]. ADHs are the most intensively studied group of oxidoreductases. Among well-studied ADHs, the prominent horse liver ADH (HADH) was shown to accept 1-octanol as substrate (kcat/KM = 5.8×10 4 M -1 .s -1 ; [START_REF] Cea | Effect of chain length on the activity of free and immobilized alcohol dehydrogenase towards aliphatic alcohols[END_REF][START_REF] Tani | Thermostable NADP + -Dependent Medium-Chain Alcohol Dehydrogenase from Acinetobacter sp. Strain M-1: Purification and Characterization and Gene Expression in Escherichia coli[END_REF][START_REF] Whitaker | Handbook of Food Enzymology[END_REF]. Yeast ADHs (YADHs) from S. cerevisiae YADH1 and YADH2 were also found active on C2 to C10 alcohols [START_REF] De Smidt | The alcohol dehydrogenases of Saccharomyces cerevisiae : a comprehensive review[END_REF][START_REF] Dickinson | The activity of yeast ADH I and ADH II with long-chain alcohols and diols[END_REF][START_REF] Leskovac | The three zinc-containing alcohol dehydrogenases from baker's yeast, Saccharomyces cerevisiae[END_REF][START_REF] Ottone | Enhanced long-chain fatty alcohol oxidation by immobilization of alcohol dehydrogenase from S. cerevisiae[END_REF][START_REF] Pal | Activity of Yeast Alcohol Dehydrogenases on Benzyl Alcohols and Benzaldehydes. Characterization of ADH1 from Saccharomyces carlsbergensis and Transition State Analysis[END_REF][START_REF] Schöpp | Kinetics and reaction mechanism of yeast alcohol dehydrogenase with long-chain primary alcohols[END_REF]. YADHs are known to be unstable enzymes [START_REF] Trivedi | Optimization of adsorptive immobilization of alcohol dehydrogenases[END_REF]. -Of note, both HADH and YADH exist through multiple isoforms. The isoform will be specified hereinafter when possible. No specification means that the enzyme is either a mixture of isoforms or that no clear identification was provided in the original reference.

Comparison between HADH, human liver ADH and YADH on a wide range of substrates, was studied in the 1970s [START_REF] Pietruszko | Comparison of substrate specificity of alcohol dehydrogenases from human liver, horse liver, and yeast towards saturated and 2-enoic alcohols and aldehydes[END_REF]: HADH exhibited ca. 10-fold higher catalytic efficiency than both human liver ADH and YADH. Overall, a multitude of studies reported expression and characterization of new ADHs with observed activity on fatty alcohols (from C6 to C8) often among other substrates included in the initial specificity screening.

Unfortunately, most of these studies are restricted to spectrophotometric monitoring of NAD(P) + reduction/NAD(P)H oxidation, and thus lack direct analysis of product formation.

Among the characterized ADHs, potent catalysts for oxidation of fatty alcohols very likely exist, but are still untapped as studies for applicability of these enzymes in biocatalysis are predominantly focused on their use for asymmetric reduction processes and for their enantioselective oxidation of secondary alcohols [START_REF] Dong | Biocatalytic Oxidation Reactions: A Chemist's Perspective[END_REF][START_REF] Matsuda | Recent progress in biocatalysis for asymmetric oxidation and reduction[END_REF][START_REF] Turner | Enantioselective Oxidation of C-O and C-N Bonds Using Oxidases[END_REF]. In particular, the production of ketones using ADHs is of interest for other F&F applications such as the synthesis of carvone [START_REF] De Carvalho | Carvone: Why and how should one bother to produce this terpene[END_REF][START_REF] Werf | Stereoselective Carveol Dehydrogenase from Rhodococcus erythropolis DCL14: A Novel Nicotinoprotein Belonging to the Short Chain Dehydrogenase/Reductase Superfamily[END_REF].

ADHs from extremophiles

In this paragraph, we focus on some ADHs from extremophiles with interesting traits.

Indeed, while many ADHs from extremophiles have been described, some encompass activity on fatty alcohols [START_REF] Hirano | Purification and characterization of the alcohol dehydrogenase with a broad substrate specificity originated from 2-phenylethanolassimilating Brevibacterium sp. KU 1309[END_REF][START_REF] Holt | Cloning, sequencing and expression in Escherichia coli of the primary alcohol dehydrogenase gene from Thermoanaerobacter ethanolicus JW200[END_REF][START_REF] Inoue | Purification and Characterization of a Novel Alcohol Dehydrogenase from Leifsonia sp. Strain S749: a Promising Biocatalyst for an Asymmetric Hydrogen Transfer Bioreduction[END_REF][START_REF] Ma | Purification and characterization of NADP-specific alcohol dehydrogenase and glutamate dehydrogenase from the hyperthermophilic archaeon Thermococcus litoralis[END_REF][START_REF] Ma | An Unusual Oxygen-Sensitive, Iron-and Zinc-Containing Alcohol Dehydrogenase from the Hyperthermophilic Archaeon Pyrococcus furiosus[END_REF][START_REF] Yanai | Sulfolobus tokodaii ST0053 Produces a Novel Thermostable, NAD-Dependent Medium-Chain Alcohol Dehydrogenase[END_REF] and can exhibit attractive biochemical characteristics such as high thermostability and tolerance to solvent, two assets for the oxidation of poorly soluble substrates. A recombinant ADH from Bacillus stearothermophilus was used in an amination cascade reaction to convert fatty alcohols to aldehydes and was able to convert 50 mM C6 alcohol with a yield > 99%. The enzyme was also able to convert partially C8 (50 % yield) and C10 (2% yield) alcohols, and addition of 1,2-dimethoxyethane (10% (v/v)) as co-solvent improved the conversion of C10 up to 25 % [START_REF] Sattler | Redox Self-Sufficient Biocatalyst Network for the Amination of Primary Alcohols[END_REF]. Thermococcus hydrothermalis ADH can be driven preferentially toward reductive or oxidative reaction by adjusting the pH of the reaction with pH 10.5 for an optimal alcohol oxidation vs pH 7.5 for an optimal reductive reaction. Interestingly, this enzyme found as a dimer at pH 7.5 forms a tetramer at pH 10.5.

This ADH showed best activity on hexanol (>C6 not assayed) and was also able to oxidize some aromatic alcohols (i.e. benzyl alcohol) and monoterpenes (i.e. geraniol and nerol) [START_REF] Antoine | Cloning and over-expression in Escherichia coli of the gene encoding NADPH group III alcohol dehydrogenase from Thermococcus hydrothermalis[END_REF]. The ADH from Aeropyrum pernix K1 showed kinetic constants that increased with the alkyl chain length (tested up to C6 with a kcat/KM of 2.5 mM -1 .s -1 ) and strong thermostability with 24% residual activity after 30 min incubation at 98°C [START_REF] Hirakawa | Properties of an alcohol dehydrogenase from the hyperthermophilic archaeon Aeropyrum pernix K1[END_REF]. A thermostable enzyme from the psychrotolerant Flavobacterium frigidimaris KUC-1

showed increasing activity from C6 to C10, with high temperature resilience, being active from 0 to 85°C (optimum at 70°C) [START_REF] Kazuoka | A cold-active and thermostable alcohol dehydrogenase of a psychrotorelant from Antarctic seawater, Flavobacterium frigidimaris KUC-1[END_REF]. However, no heterologous production of the latter enzyme has been described. Alkanes/alkanols-degrading microorganisms can be a source of ADHs active on fatty alcohols as demonstrated by several studies [START_REF] Álvarez | Induction of NAD + dependent alcohol dehydrogenases with activity towards long chain aliphatic alcohols in mesophilic, thermophilic and extreme thermophilic microorganisms[END_REF][START_REF] Barth | Alcohol dehydrogenase (ADH) in yeasts II. NAD+-and NADP+dependent alcohol dehydrogenases in Saccharomycopsis lipolytica[END_REF][START_REF] Fox | Long-chain alcohol and aldehyde dehydrogenase activities in Acinetobacter calcoaceticus strain HO1-N[END_REF][START_REF] Nagashima | Long-chain n-alkanol dehydrogenase from Pseudomonas putida[END_REF][START_REF] Tassin | Separation and characterization of long-chain alcohol dehydrogenase isoenzymes from Pseudomonas aeruginosa[END_REF]Ueda and Tanaka, 1990;[START_REF] Weimer | Molecular structure and genetic regulation of SFA, a gene responsible for resistance to formaldehyde in Saccharomyces cerevisiae, and characterization of its protein product[END_REF][START_REF] Yamada | Subcellular localization of long-chain alcohol dehydrogenase and aldehyde dehydrogenase in n-alkane-grown Candida tropicalis[END_REF].

Noteworthily, two ADHs from long-chain alkane-degrading strain Geobacillus thermodenitrificans NG80-2 exhibited activity toward fatty alcohols including C6-C10 [START_REF] Liu | Two novel metalindependent long-chain alkyl alcohol dehydrogenases from Geobacillus thermodenitrificans NG80-2[END_REF].

Heterologous production of ADHs

Commercial ADHs [START_REF] Hummel | Large-scale applications of NAD(P)-dependent oxidoreductases: recent developments[END_REF] mainly originate from either animal tissues (horse liver), S. cerevisiae or other recombinant hosts, and can encompass multiple isoenzymes when purified from native host. Despite potential issues pertaining to the generation of aggregated proteins [START_REF] Utekal | Expression of soluble Saccharomyces cerevisiae alcohol dehydrogenase in Escherichia coli applicable to oxido-reduction bioconversions[END_REF], E. coli is predominantly used for the heterologous production of ADHs [START_REF] Höllrigl | TADH, the thermostable alcohol dehydrogenase from Thermus sp. ATN1: a versatile new biocatalyst for organic synthesis[END_REF][START_REF] Holt | Cloning, sequencing and expression in Escherichia coli of the primary alcohol dehydrogenase gene from Thermoanaerobacter ethanolicus JW200[END_REF][START_REF] Kim | Enhanced Production of Recombinant Alcohol Dehydrogenase Using the Genetically Engineered Escherichia coli Strain that Heterologously Expresses Carrot Heat Shock Protein 70[END_REF][START_REF] Liu | Two novel metalindependent long-chain alkyl alcohol dehydrogenases from Geobacillus thermodenitrificans NG80-2[END_REF] in the cytosolic compartment. For instance, the well-studied HADH (EE isoenzyme) has been recombinantly produced in E. coli [START_REF] Huang | Horse Liver Alcohol Dehydrogenase-Catalyzed Oxidative Lactamization of Amino Alcohols[END_REF][START_REF] Park | Isoenzymes of horse liver alcohol dehydrogenase active on ethanol and steroids. cDNA cloning, expression, and comparison of active sites[END_REF][START_REF] Quaglia | Horse Liver Alcohol Dehydrogenase: New Perspectives for an Old Enzyme[END_REF] but its expression at large scale is still challenging [START_REF] Hummel | Towards a Large-Scale Asymmetric Reduction Process with Isolated Enzymes: Expression of an (S)-Alcohol Dehydrogenase in E. coli and Studies on the Synthetic Potential of this Biocatalyst[END_REF][START_REF] Ottone | Enhanced long-chain fatty alcohol oxidation by immobilization of alcohol dehydrogenase from S. cerevisiae[END_REF]). Yet, successful recombinant expression of other ADHs in E. coli [START_REF] Wolberg | Biocatalytic Reduction of β,δ-Diketo Esters: A Highly Stereoselective Approach to All Four Stereoisomers of a Chlorinated β,δ-Dihydroxy Hexanoate[END_REF] and utilization at commercial scale have been reported for other applications [START_REF] Davis | Improved halohydrin dehalogenases and related polynucleotides[END_REF][START_REF] Patel | Biocatalytic synthesis of atorvastatin intermediates[END_REF], after having conducted protein engineering (see section 4.1).

Flavin-dependent alcohol oxidases (FAD-AOXs)

FAD-AOXs are found in both the large superfamily of glucose-methanol-choline (GMC)

oxidoreductases and the superfamily of Vanillyl Alcohol Oxidase/p-cresol methylhydroxylase (VAO/PCMH). Some FAD-AOXs are also classified into the Carbohydrate-Active enZymes database (CAZy, www.cazy.org ; [START_REF] Levasseur | Expansion of the enzymatic repertoire of the CAZy database to integrate auxiliary redox enzymes[END_REF][START_REF] Lombard | The carbohydrate-active enzymes database (CAZy) in 2013[END_REF] in the Auxiliary Activity 3 (AA3) and AA4 families. The AA3 CAZy family, initially built for carbohydrate oxidoreductases (e.g., cellobiose dehydrogenase, glucose 1-oxidase), contains phylogenetically-related enzymes active on either aryl alcohols (AA3_2 sub-family) or on small primary alcohols (e.g., methanol, ethanol; AA3_3 sub-family) [START_REF] Sützl | Multiplicity of enzymatic functions in the CAZy AA3 family[END_REF]. The AA4 family contains VAO that belong to subgroup 4 of the VAO/PCMH superfamily [START_REF] Ewing | The VAO/PCMH flavoprotein family[END_REF]. AA4 will not be further discussed in this review because they are specifically active on para-substituted phenolic compounds and not active on aliphatic substrates. Yet, these enzymes have been harnessed for other major F&F applications such as the production of vanillin [START_REF] Lambert | System for the production of aromatic molecules in Streptomyces[END_REF][START_REF] Lambert | Production of ferulic acid and coniferyl alcohol by conversion of eugenol using a recombinant strain of Saccharomyces cerevisiae[END_REF].

2.2.1. Source of FAD-AOXs FAD-AOXs are widespread [START_REF] Ewing | Oxidation using alcohol oxidases[END_REF], and some of the best-known FAD-AOXs originate from peroxisomes of methylotrophic yeasts, such as Pichia pastoris (Komagataella phaffii), Candida boidinii or Hansenula sp. where they play a key role in methanol metabolism [START_REF] Ozimek | Alcohol oxidase: A complex peroxisomal, oligomeric flavoprotein[END_REF][START_REF] Zavec | Beyond alcohol oxidase: The methylotrophic yeast Komagataella phaffii utilizes methanol also with its native alcohol dehydrogenase Adh2[END_REF]. Alternatively, a number of "long-chain alcohol oxidases" (LCAOs) also called "fatty-alcohol oxidases" (FAO)

(EC:1.1.3.20) have been described [START_REF] Ewing | Oxidation using alcohol oxidases[END_REF]. Owing to heterogenous nomenclature, these enzymes will hereinafter be referred to as "FAD-FAOX". FAD-FAOXs have been mostly isolated from "alkane-growing" yeasts [START_REF] Fukuda | Enzymes for Aerobic Degradation of Alkanes in Yeasts[END_REF], fungi [START_REF] Alvarado-Caudillo | Presence and physiologic regulation of alcohol oxidase activity in an indigenous fungus isolated from petroleumcontaminated soils[END_REF][START_REF] Kumar | Functional characterization of alcohol oxidases from Aspergillus terreus MTCC 6324[END_REF][START_REF] Pickl | Amination of ω-Functionalized Aliphatic Primary Alcohols by a Biocatalytic Oxidation-Transamination Cascade[END_REF][START_REF] Savitha | Alcohol oxidase of Aspergillus flavipes grown on hexadecanol[END_REF][START_REF] Silva-Jiménez | Intracellular distribution of fatty alcohol oxidase activity in Mucor circinelloides YR-1 isolated from petroleum contaminated soils[END_REF] and plants [START_REF] Banthorpe | Purification and properties of alcohol oxidase from Tanacetum vulgare[END_REF][START_REF] Cheng | Functional identification of AtFao3, a membrane bound long chain alcohol oxidase in Arabidopsis thaliana[END_REF][START_REF] Moreau | Oxidation of fatty alcohol in the cotyledons of jojoba seedlings[END_REF][START_REF] Zhao | Cloning and Characterization of Long-Chain Fatty Alcohol Oxidase LjFAO1 in Lotus japonicus[END_REF]. Early studies described interesting activities on fatty alcohols, albeit individual enzymes were not isolated/characterized. First evidences of FAD-FAOX enzymes (from yeasts) were described in the 80-90's [START_REF] Blasig | Degradation of long-chain n-alkanes by the yeast Candida maltosa[END_REF][START_REF] Il'chenko | Oxidase of higher alcohols in the yeast Torulopsis candida grown on hexadecane[END_REF][START_REF] Il'chenko | Isolation and characterization of alcohol oxidase from higher alcohols of the yeast Torulopsis candida grown on hexadecane[END_REF][START_REF] Krauzova | Formation of lauric acid and NADH in the oxidation of dodecanol by membrane fractions of the yeast Candida maltosa cultured on hexadecane[END_REF][START_REF] Krauzova | Possible pathways of the oxidation of higher alcohols by membrane fractions of yeasts cultured on hexadecane and hexadecanol[END_REF], followed by studies on the microsomal fractions of Aspergillus flavipes, Candida bombicola, C. maltose, C. apicola, C. tropicalis and Yarrowia lipolytica [START_REF] Hommel | Evidence for two fatty alcohol oxidases in the biosurfactantproducing yeast Candida (Torulopsis) bombicola[END_REF][START_REF] Hommel | The inducible microsomal fatty alcohol oxidase of Candida (Torulopsis) apicola[END_REF][START_REF] Kemp | Activity and substrate specificity of the fatty alcohol oxidase of Candida tropicalis in organic solvents[END_REF][START_REF] Kemp | Light sensitivity of the n-alkane-induced fatty alcohol oxidase from Candida tropicalis and Yarrowia lipolytica[END_REF][START_REF] Kemp | Inducible long chain alcohol oxidase from alkane-grown Candida tropicalis[END_REF][START_REF] Mauersberger | Substrate specificity and stereoselectivity of fatty alcohol oxidase from the yeast Candida maltosa[END_REF].

Structure and catalytic mechanism of FAD-AOXs

GMC-type FAD-AOXs are multimeric enzymes usually found as homooctamers [START_REF] Ozimek | Alcohol oxidase: A complex peroxisomal, oligomeric flavoprotein[END_REF][START_REF] Silva-Jiménez | Intracellular distribution of fatty alcohol oxidase activity in Mucor circinelloides YR-1 isolated from petroleum contaminated soils[END_REF] or sometimes as homodimers [START_REF] Dickinson | Purification and some properties of alcohol oxidase from alkane-grown Candida tropicalis[END_REF][START_REF] Heath | An Engineered Alcohol Oxidase for the Oxidation of Primary Alcohols[END_REF] in yeasts while fungal enzymes, which are less investigated, can be found as heterodimers [START_REF] Kumar | Dissociation and Reconstitution Studies of a Broad Substrate Specific Multimeric Alcohol Oxidase Protein Produced by Aspergillus terreus[END_REF]. Overall, all FAD-AOXs share a conserved FAD binding domain displaying both the sequence motif GxGxxG/A and a

Rossmann fold [START_REF] Dijkman | Flavoprotein oxidases: classification and applications[END_REF]. The flavin co-factor is usually non-covalently bound to the enzymes of the GMC superfamily in contrast to the members of the VAO/PCMH superfamily [START_REF] Ewing | Chapter Four -Vanillyl alcohol oxidase[END_REF][START_REF] Ewing | The VAO/PCMH flavoprotein family[END_REF][START_REF] Romero | Same Substrate, Many Reactions: Oxygen Activation in Flavoenzymes[END_REF]. The C-terminal substrate binding domain is more variable, allowing accommodation of different types of substrates [START_REF] Sützl | The GMC superfamily of oxidoreductases revisited: analysis and evolution of fungal GMC oxidoreductases[END_REF]. Although several crystallographic structures of FAD-AOXs from methylotrophic yeasts have been resolved (see Figure 3A), equivalent information for FAD-FAOXs is still lacking [START_REF] Cheng | Candida yeast long chain fatty alcohol oxidase is a c-type haemoprotein and plays an important role in long chain fatty acid metabolism[END_REF]. Intriguingly FAD-FAOXs of Candida sp. and Arabidospsis thaliana were reported to possess a heme binding motif "CXXC" and a possible covalently bound heme [START_REF] Cheng | Candida yeast long chain fatty alcohol oxidase is a c-type haemoprotein and plays an important role in long chain fatty acid metabolism[END_REF][START_REF] Cheng | Functional identification of AtFao3, a membrane bound long chain alcohol oxidase in Arabidopsis thaliana[END_REF][START_REF] Vanhanen | A Consensus Sequence for Long-chain Fatty-acid Alcohol Oxidases from Candida Identifies a Family of Genes Involved in Lipid ω-Oxidation in Yeast with Homologues in Plants and Bacteria[END_REF], the function of which remains to be elucidated. The general catalytic cycle of FAD-AOXs can be divided in two half reactions,

where FAD plays the role of an electron shuttle. In the reductive half reaction, two electrons are transferred from the substrate to the oxidized flavin via a hydride transfer mechanism [START_REF] Wongnate | The substrate oxidation mechanism of pyranose 2-oxidase and other related enzymes in the glucose-methanol-choline superfamily[END_REF]. Then, FADH2 is regenerated to FAD during the oxidative half reaction via the two electrons reduction of O2 into H2O2 (Figure 3B). As stated before, the oxidation of the alcohol is strictly irreversible.

Substrate specificity of FAD-AOXs

The substrate scope of FAD-AOXs is broad with enzymes active on aliphatic alcohols, aromatic alcohols, carbohydrates, steroids, etc. [START_REF] Ewing | Chapter Four -Vanillyl alcohol oxidase[END_REF]. Only enzymes that fit in the focus of this review will be described here. We nevertheless underscore that aryl alcohol oxidases from the AA3_2 subfamily exhibit some activity on unsaturated fatty alcohols [START_REF] Urlacher | Pecularities and applications of aryl-alcohol oxidases from fungi[END_REF]. Among the FAD-AOXs active on saturated aliphatic alcohols, some of the most prominent members are from methylotrophic yeasts, which are efficient at oxidizing methanol and other short-chain alcohols, while their activity gradually decreases with extension of the alkyl-chain [START_REF] Ozimek | Alcohol oxidase: A complex peroxisomal, oligomeric flavoprotein[END_REF]. In aqueous medium, FAD-AOX from P. pastoris is found as an homooctamer where the subunits interact via their substrate binding domain [START_REF] Koch | Crystal Structure of Alcohol Oxidase from Pichia pastoris[END_REF]. As a result, the active site appears buried into the octameric structure. These FAD-AOXs are thus not competent catalysts for fatty-alcohols oxidation in aqueous conditions, although their substrate scope can be extended by protein engineering (see section 4.1) or by carrying out reactions in biphasic system (see section 4.3).

In contrast, FAD-FAOXs are more efficient on fatty alcohols. Indeed, yeast FAD-FAOX from

Candida tropicalis exhibited activity on C12 and longer alcohols (but not on C8 and C10) [START_REF] Cheng | Candida yeast long chain fatty alcohol oxidase is a c-type haemoprotein and plays an important role in long chain fatty acid metabolism[END_REF]. A FAD-FAOXs from another strain of C. tropicalis (ATCC 20336)

showed activity over C6-C16 primary alcohols, with highest specific activity on C8, followed by C12 alcohols and low KM on both substrates (Table S1) [START_REF] Eirich | Cloning and characterization of three fatty alcohol oxidase genes from Candida tropicalis strain ATCC 20336[END_REF][START_REF] Eirich | Fatty alcohol oxidase genes and proteins from Candida tropicalis and methods relating thereto[END_REF]. A plant FAD-FAOX from jojoba (Simmondsia chinensis) displayed activity on a range of fatty alcohols, notably C8, and exhibited best performances on C12, with one of the lowest KM hitherto reported for fatty-alcohols active oxidases (see table S1) [START_REF] Rajangam | Molecular Characterization of the Fatty Alcohol Oxidation Pathway for Wax-Ester Mobilization in Germinated Jojoba Seeds[END_REF].

Among fungal FAD-FAOXs, an enzyme purified from the microsomal fraction Aspergillus terreus (MTCC 6324) was biochemically characterized, showing best activity on C7 and a broad activity on aromatic and secondary alcohols [START_REF] Kumar | Dissociation and Reconstitution Studies of a Broad Substrate Specific Multimeric Alcohol Oxidase Protein Produced by Aspergillus terreus[END_REF][START_REF] Gadda | Hydride Transfer Made Easy in the Reaction of Alcohol Oxidation Catalyzed by Flavin-dependent Oxidases[END_REF], 2006).

We also underscore that a FAD-FAOX from A. fumigatus was successfully applied in a cascade reaction where C6-C8 fatty alcohols (10 mM) were fully converted to aldehydes [START_REF] Pickl | Amination of ω-Functionalized Aliphatic Primary Alcohols by a Biocatalytic Oxidation-Transamination Cascade[END_REF].

Heterologous expression of FAD-FAOXs

Heterologous expression of FAD-FAOXs became possible only two decades ago when genes encoding an enzyme from C. tropicalis (NCYC 470) and two isoenzymes from C. cloacae were cloned [START_REF] Slabas | Improvements in or relating to fatty acid metabolism[END_REF][START_REF] Vanhanen | A Consensus Sequence for Long-chain Fatty-acid Alcohol Oxidases from Candida Identifies a Family of Genes Involved in Lipid ω-Oxidation in Yeast with Homologues in Plants and Bacteria[END_REF]. Since then, several fungal, yeast and plant FAD-FAOXs have subsequently been successfully produced in E. coli [START_REF] Cheng | Functional identification of AtFao3, a membrane bound long chain alcohol oxidase in Arabidopsis thaliana[END_REF][START_REF] Eirich | Cloning and characterization of three fatty alcohol oxidase genes from Candida tropicalis strain ATCC 20336[END_REF][START_REF] Rajangam | Molecular Characterization of the Fatty Alcohol Oxidation Pathway for Wax-Ester Mobilization in Germinated Jojoba Seeds[END_REF][START_REF] Takahashi | Identification of the fatty alcohol oxidase FAO1 from Starmerella bombicola and improved novel glycolipids production in an FAO1 knockout mutant[END_REF]. FAD-FAOXs are multimeric, membrane-bound enzymes showing biophysical complexity [START_REF] Kumar | Purification and properties of a novel broad substrate specific alcohol oxidase from Aspergillus terreus MTCC 6324[END_REF]) which can complicate their heterologous expression and hamper their implementation at large scale. Solubilization of membrane-bound FAD-FAOXs can lead to significant decrease in activity [START_REF] Kakoti | Microsome-bound alcohol oxidase catalyzed production of carbonyl compounds from alcohol substrates[END_REF] although it was reported that preservation of activity is possible depending on the enzyme [START_REF] Cheng | Functional identification of AtFao3, a membrane bound long chain alcohol oxidase in Arabidopsis thaliana[END_REF]. Furthermore, a recombinant FAD-FAOX from A. fumigatus (LCAO_af), was successfully employed in a cascade with a ɷ-transaminase for conversion of fatty alcohols to amines [START_REF] Pickl | Amination of ω-Functionalized Aliphatic Primary Alcohols by a Biocatalytic Oxidation-Transamination Cascade[END_REF].

In this study, lyophilized recombinant E. coli cells were used to avoid the solubilization and purification of the enzyme and to provide an expression system compatible with the scaled-up production of such FAD-FAOXs. So far, only E. coli has been used as recombinant production host and there is no data as to whether the utilization of eukaryotic hosts (e.g. P. pastoris) may improve protein expression.

Copper Radical Alcohol Oxidases (CRO-AlcOx)

Copper radical oxidases (CROs) are classified into the CAZy database as members of the Auxiliary Activity 5 family (AA5). They have been further segregated into two subfamilies, namely, AA5_1 containing glyoxal oxidases (GLOX -EC 1.2.3.15) that catalyze the oxidation of aldehydes (most certainly through their hydrated gem-diol form) to carboxylic acids, and AA5_2 that gathers enzymes acting on the alcohol moieties of diverse substrate scaffolds to generate aldehydes (see below the "substrate specificity" paragraph for more details). GLOXs were well covered elsewhere [START_REF] Daou | Glyoxal oxidases: their nature and properties[END_REF][START_REF] Kersten | Copper radical oxidases and related extracellular oxidoreductases of wood-decay Agaricomycetes[END_REF], and will therefore not be further discussed here.

Source of CRO-AlcOx

To date, all characterized CROs originate from filamentous fungi. The only exceptions are a CRO (distantly related to AA5_1) from the filamentous bacterium Streptomyces lividans, the substrate preference of which remains to clarified [START_REF] Chaplin | Active-site maturation and activity of the copper-radical oxidase GlxA are governed by a tryptophan residue[END_REF][START_REF] Chaplin | GlxA is a new structural member of the radical copper oxidase family and is required for glycan deposition at hyphal tips and morphogenesis of Streptomyces lividans[END_REF][START_REF] Petrus | The DyP-type peroxidase DtpA is a Tat-substrate required for GlxA maturation and morphogenesis in Streptomyces[END_REF] and galactose oxidase-like enzymes from A. thaliana (Šola et al., 2019; Šola et al., 2021).

Within the AA5_2 subfamily, CRO-AlcOx originate from plant pathogens ascomycetes (e.g.

Colletotrichum sp. or Magnaporthe sp.). Recent investigations on the biological context of these enzymes revealed an essential role of Colletotrichum orbiculare and Magnaporthe oryzae CRO-AlcOx in the penetration of the plant through oxidation of plant cuticular long chain alcohols [START_REF] Bissaro | Unravelling the role of alcohol copper radical oxidases in fungal plant pathogens[END_REF].

Structure and catalytic mechanism of CRO-AlcOx

Currently, all the structurally characterized CROs belong to the AA5_2 subfamily as no structure of GLOX (AA5_1) is available. CROs are monomeric enzymes with a conserved architecture, displaying a characteristic conserved seven bladed β-propeller fold ("Kelch motif" -Pfam 01344) (Figure 4A). The predominance of β-sheets in their structure has been suggested

to contribute to their strong stability [START_REF] Ito | Novel thioether bond revealed by a 1.7 A crystal structure of galactose oxidase[END_REF]. The catalytic module exhibits a remarkably solvent-exposed active site [START_REF] Firbank | Crystal structure of the precursor of galactose oxidase: an unusual self-processing enzyme[END_REF]. The rather open and solventexposed topology of the CRO active site, which is in striking contrast with FAD-AOX and ADH (Figure 5), may explain why CRO can act on bulky substrates such as fatty alcohols (Ribeaucourt et al., 2021a) or plant waxes [START_REF] Bissaro | Unravelling the role of alcohol copper radical oxidases in fungal plant pathogens[END_REF]. CRO-AlcOx are generally composed of a single catalytic module unlike most other CROs and aside from the CRO-AlcOx from Magnaporthe oryzae, that harbors a cell wall integrity and stress response (WSC) module [START_REF] Oide | Carbohydrate-binding property of a cell wall integrity and stress response component (WSC) domain of an alcohol oxidase from the rice blast pathogen Pyricularia oryzae[END_REF]. The role of this WSC module is still unclear and may not be directly involved in the catalytic mechanism, as its deletion did not affect the activity, nor the specificity of the enzyme [START_REF] Oide | Carbohydrate-binding property of a cell wall integrity and stress response component (WSC) domain of an alcohol oxidase from the rice blast pathogen Pyricularia oryzae[END_REF]. CROs are monocopper metalloenzymes that catalyze the twoelectron aerobic oxidation of alcohols into aldehydes (Figure 4B). No organic co-factor is involved in the catalytic mechanism, which distinguish them from previously discussed ADHs and FAD-AOXs. To catalyze the two electrons transfer, a crosslinked 3′-(S-cysteinyl)-tyrosine free radical (Cys-Tyr • ) acts in combination with the Cu 2+ ion, hence the term "copper radical".

CROs rely on a characteristic "ping-pong" mechanism composed of a reductive half-reactioni.e. substrate oxidation and reduction of the active siteand the complementary reverse oxidative half-reactioni.e. reoxidation of the active site and concomitant reduction of O2 to H2O2 (Figure 4B). CROs can also be found in an inactive "semi-reduced" form which could increase their stability or be involved in activity regulation in vivo [START_REF] Bissaro | Unravelling the role of alcohol copper radical oxidases in fungal plant pathogens[END_REF].

Substrate specificity of CRO-AlcOx

CROs have been for long restricted to very few archetypal members and their scope of application accordingly limited. Indeed, since their discovery in 1959, the founding member of CROs, namely the galactose oxidase (EC 1.1.3.9) from Fusarium graminearum (FgrGalOx) has been the main object of most studies. Numerous applications have been described for the FgrGalOx and the boundaries of its substrate specificity have been pushed far beyond its natural carbohydrate preference [START_REF] Birmingham | A Single Enzyme Oxidative "Cascade" via a Dual-Functional Galactose Oxidase[END_REF][START_REF] Deacon | Enhanced Fructose Oxidase Activity in a Galactose Oxidase Variant[END_REF][START_REF] Escalettes | Directed Evolution of Galactose Oxidase: Generation of Enantioselective Secondary Alcohol Oxidases[END_REF][START_REF] Herter | Galactose Oxidase Variants for the Oxidation of Amino Alcohols in Enzyme Cascade Synthesis[END_REF][START_REF] Mattey | Natural heterogeneous catalysis with immobilised oxidase biocatalysts[END_REF][START_REF] Mckenna | Enzyme cascade reactions: synthesis of furandicarboxylic acid (FDCA) and carboxylic acids using oxidases in tandem[END_REF][START_REF] Sun | Modification of Galactose Oxidase to Introduce Glucose 6-Oxidase Activity[END_REF][START_REF] Vilím | Catalytic Promiscuity of Galactose Oxidase: A Mild Synthesis of Nitriles from Alcohols, Air, and Ammonia[END_REF][START_REF] Wilkinson | Structural and kinetic studies of a series of mutants of galactose oxidase identified by directed evolution[END_REF]. It is only recently, with the advent of natural diversity exploration, notably rendered possible by the availability of abundant genomic data, that the true potential of CROs scope of action was unveiled [START_REF] Andberg | A Novel Colletotrichum graminicola Raffinose Oxidase in the AA5 Family[END_REF][START_REF] Mathieu | Discovery of a fungal copper radical oxidase with high catalytic efficiency towards 5-hydroxymethylfurfural and benzyl alcohols for bioprocessing[END_REF][START_REF] Mollerup | A family AA5_2 carbohydrate oxidase from Penicillium rubens displays functional overlap across the AA5 family[END_REF][START_REF] Paukner | Expression, purification, and characterization of galactose oxidase of Fusarium sambucinum in E. coli[END_REF][START_REF] Paukner | Galactose Oxidase from Fusarium oxysporum -Expression in E. coli and P. pastoris and Biochemical Characterization[END_REF][START_REF] Yin | Structure-function characterization reveals new catalytic diversity in the galactose oxidase and glyoxal oxidase family[END_REF]. Remarkably, a subgroup of AA5_2 CROs, so-called CRO-AlcOx, appeared to be of particular relevance for fragrant aldehydes production [START_REF] Forget | Determination of biocatalytic parameters of a copper radical oxidase using real-time reaction progress monitoring[END_REF][START_REF] Oide | Carbohydrate-binding property of a cell wall integrity and stress response component (WSC) domain of an alcohol oxidase from the rice blast pathogen Pyricularia oryzae[END_REF]Ribeaucourt et al., 2021bRibeaucourt et al., , 2021a;;[START_REF] Yin | Structure-function characterization reveals new catalytic diversity in the galactose oxidase and glyoxal oxidase family[END_REF]. These enzymes, mainly isolated from Colletotrichum species, all exhibit a similar substrate specificity profile and comparable efficiencies. They can oxidize a large panel of primary alcohols, aryl alcohols and activated aliphatic alcohols being excellent substrates.

Importantly, CRO-AlcOx can also convert non-activated alcohols with increasing affinity towards longer alkyl chains (Ribeaucourt et al., 2021a;[START_REF] Yin | Structure-function characterization reveals new catalytic diversity in the galactose oxidase and glyoxal oxidase family[END_REF]. For instance, 1-hexanol and 1-octanol (3 mM) could be fully converted by the AlcOx from Colletotrichum graminicola (CgrAlcOx) and C. destructivum (CdeAlcOx) within 15 min and under mild conditions (23°C, in aqueous media) (Ribeaucourt et al., 2021b(Ribeaucourt et al., , 2021a)). Despite stable/increasing catalytic efficiencies up to C10, conversion of C10 alcohol was only partial compared to C6-C8 (unpublished results; see Table S1), which might be the result of increasing insolubility of the substrate or stronger inhibition of CgrAlcOx by the hydrated form of the aldehyde (Scheme 1) [START_REF] Forget | Determination of biocatalytic parameters of a copper radical oxidase using real-time reaction progress monitoring[END_REF]Ribeaucourt et al., 2021a). Preparative-scale conversion was also demonstrated using a crude preparation of CgrAlcOx (supernatant of a recombinant P. pastoris strain) yielding 0.72 g of octanal from 2 g of 1-octanol (Ribeaucourt et al., 2021a).

Heterologous expression of CRO-AlcOx

AA5_2 are secreted fungal enzymes and necessitate post-translational modification such as the formation of a thioether bond for the Cys-Tyr • co-factor. E. coli and P. pastoris have been the main expression systems used, although Aspergillus nidulans [START_REF] Baron | Structure and mechanism of galactose oxidase. The free radical site[END_REF][START_REF] Mcpherson | Galactose oxidase: molecular analysis and mutagenesis studies[END_REF], A. oryzae and Fusarium venenatum [START_REF] Xu | Expression and characterization of a recombinant Fusarium spp. galactose oxidase[END_REF] have also been reported as possible hosts for the archetypal FgrGalOx. Detailed comparison of E. coli and P.

pastoris expression systems have been described elsewhere [START_REF] Spadiut | A comparative summary of expression systems for the recombinant production of galactose oxidase[END_REF] for the FgrGalOx. Poor expression yields were obtained in E. coli unless a custom-variant strain was used. To date, satisfactory yields of CRO-AlcOx have exclusively been obtained using P.

pastoris as host. Scaled-up production in P. pastoris has been developed in bioreactor for both CgrAlcOx and FgrGalOx [START_REF] Anasontzis | Effects of temperature and glycerol and methanol-feeding profiles on the production of recombinant galactose oxidase in Pichia pastoris[END_REF]Ribeaucourt et al., 2021a). Post translational activation of the enzyme is usually done by incubation with copper under aerobic conditions.

This process can be done directly in situ by adding copper salt into the expression medium [START_REF] Spadiut | A comparative summary of expression systems for the recombinant production of galactose oxidase[END_REF][START_REF] Yin | Structure-function characterization reveals new catalytic diversity in the galactose oxidase and glyoxal oxidase family[END_REF]. CRO-AlcOx, like other CROs, are usually recovered after purification in their "semi-reduced" inactive state and must therefore be oxidized to generate the fully active form prior to utilization [START_REF] Forget | Determination of biocatalytic parameters of a copper radical oxidase using real-time reaction progress monitoring[END_REF][START_REF] Whittaker | The radical chemistry of galactose oxidase[END_REF][START_REF] Whittaker | The active site of galactose oxidase[END_REF]. This activation is usually achieved by incubation with a peroxidase (EC 1.11.1.7) (see section 3.2.3). For all enzymes, surface was colored according to the ratio of non-polar/polar (NPP) residues using Protein-Sol web tool (https://proteinsol.manchester.ac.uk/; [START_REF] Hebditch | Protein-Sol: a web tool for predicting protein solubility from sequence[END_REF]. The NPP ratio scale, indicative of increasing hydrophobicity level from 0.6 to 2.3, displayed at the bottom left hand corner of panel C applies to all panels. Note that the structures displayed are not shown at the same scale.

Toward efficient natural biocatalysts

Nature remains the main source of enzymatic diversity providing a wide range of ingenious "ready-to-use" catalysts. In this paragraph, we tackle the question of the discovery and exploitation of efficient natural fatty alcohols-active biocatalysts.

Exploration of biodiversity

In the quest for suitable catalysts, the exploration of biodiversity can prove to be a powerful approach. A great potential, still largely untapped, exists in the microbial kingdom, as exemplified by the discovery and application of FAD-FAOXs from alkane-growing yeasts and fungi [START_REF] Kemp | Activity and substrate specificity of the fatty alcohol oxidase of Candida tropicalis in organic solvents[END_REF][START_REF] Kemp | Inducible long chain alcohol oxidase from alkane-grown Candida tropicalis[END_REF][START_REF] Pickl | Amination of ω-Functionalized Aliphatic Primary Alcohols by a Biocatalytic Oxidation-Transamination Cascade[END_REF] (see section 2.2). Diversity among CROs was also identified while investigating phylogenetical differences in fungal sequences of the GalOx family [START_REF] Andberg | A Novel Colletotrichum graminicola Raffinose Oxidase in the AA5 Family[END_REF][START_REF] Mathieu | Discovery of a fungal copper radical oxidase with high catalytic efficiency towards 5-hydroxymethylfurfural and benzyl alcohols for bioprocessing[END_REF][START_REF] Yin | Structure-function characterization reveals new catalytic diversity in the galactose oxidase and glyoxal oxidase family[END_REF]. The tremendous amount of genomic information available today [START_REF] Sharon | Genomes from Metagenomics[END_REF] combined with, powerful bioinformatics/modeling tools and platforms for high throughput (HTP) recombinant expression [START_REF] Haon | Recombinant protein production facility for fungal biomass-degrading enzymes using the yeast Pichia pastoris[END_REF][START_REF] Vincentelli | High-Throughput Protein Production and Purification: Methods and Protocols[END_REF] allow to generate wide libraries of natural enzyme candidates. Once access to enzyme diversity is achieved, the next most challenging task is most likely the design of a suitable, HTP-compatible screening method to detect desired substrate specificities and/or properties. Most generic HTP methods are often indirect and fail to provide fine details as to the sought-after reaction products. Getting a higher level of information is often at the expense of the throughput frequency. For instance, for substrate screening targeting fatty alcohol oxidases, the well-established ABTS (or equivalent Amplex Red)/horseradish peroxidase (HRP) coupled assay can be used. This assay has the advantage of enabling real-time kinetic monitoring of the enzymatic reaction. However, extensive controls are required to avoid false positive/negative reactions owing to the indirect nature of the assay and to the reactivity of the radicals generated. Other indirect assays based on the detection of H2O2 exist such as the xylenol orange-based assay [START_REF] Ewing | A Xylenol Orange-Based Screening Assay for the Substrate Specificity of Flavin-Dependent para-Phenol Oxidases[END_REF], this method avoids the use of a secondary enzyme, but does not allow direct real-time monitoring of the reaction. A direct assay detecting the production of aldehydes would be more suitable, and applicable to all classes of enzymes. Commercial colorimetric assays are available and an HTP method using amino benzamidoxime has been developed recently and validated on CARs enzyme [START_REF] Horvat | Amino Benzamidoxime (ABAO)-Based Assay to Identify Efficient Aldehyde-Producing Pichia pastoris Clones[END_REF][START_REF] Ressmann | Substrate-Independent High-Throughput Assay for the Quantification of Aldehydes[END_REF]. A HTP-derivatization strategy coupled to liquid-and supercritical fluid-chromatography was also implemented for screening of GalOx variants, which enabled product analysis. However, this method requires a more complex procedure (derivatization) and equipment [START_REF] Huffman | Design of an in vitro biocatalytic cascade for the manufacture of islatravir[END_REF]. Redirecting "nature's technology" for our needs require thoughtful reaction set-up. Parameters that can significantly affect the efficiency of the biocatalysts for fatty aldehydes production (co-factor regeneration, need for accessory enzymes, overoxidation) are discussed hereafter.

Accessory enzymes

Alcohol dehydrogenases and oxidases often require the use of accessory enzymes to harness the best of their ability. An overview of the most important accessory enzymes is given hereinafter.

Co-factor recycling systems

One of the main hurdles for the industrial application of ADH is the need for expensive organic co-factors (e.g. NAD(P) + ) where stochiometric addition is not an affordable option.

Furthermore, the oxidation of alcohols is thermodynamically disfavored compared to the reduction of aldehydes by ADH [START_REF] Hummel | Large-scale applications of NAD(P)-dependent oxidoreductases: recent developments[END_REF]. Both problems can be solved by in situ co-factor regeneration. This can be achieved by chemical approaches, although coupled enzyme systems are most commonly applied. Yet, the enzymatic regeneration of the oxidized form of the cofactors (NAD(P) + )needed for ADH-catalyzed oxidation reaction (Fig.

2)is less investigated than the regeneration of the reduced cofactors (NAD(P)H) (useful when running ADH in "reductase mode"). This lack is often a critical obstacle in the implementation of ADH-catalyzed oxidation in industry. Among the enzymes described for this purpose, glutamate dehydrogenase, lactate dehydrogenase, and NAD(P)H oxidases (NOX) are the most prominent ones. In particular, NOX have attracted a lot of attention in the past years. They can directly oxidize the reduced cofactor using oxygen as co-substrate and releasing H2O or H2O2.

Recently, the use of an ADH/NOX coupled-system was demonstrated at pilot-scale for the oxidation of lactols [START_REF] Bartsch | Optimization of Alcohol Dehydrogenase for Industrial Scale Oxidation of Lactols[END_REF], with the use of an engineered NOX enzyme [START_REF] Petschacher | Cofactor Specificity Engineering of Streptococcus mutans NADH Oxidase 2 For NAD(P) + Regeneration in Biocatalytic Oxidations[END_REF]. Alternatively, regeneration can be achieved by a substrate-coupled approach, where the ADH itself reoxidizes the co-factor by reducing a secondary substrate (see Figure 2). To this end, a cheap secondary substrate must be chosen and added in excess (e.g.

acetone, which also acts as substrate-solubilizing agent) [START_REF] Höllrigl | TADH, the thermostable alcohol dehydrogenase from Thermus sp. ATN1: a versatile new biocatalyst for organic synthesis[END_REF]. Care must be taken to avoid enzyme inhibition or any other drawbacks for downstream processes by these additional compounds. Case to case study is essential to select the most suitable approach. For instance, Vrsalović and colleagues studied the conversion of 1-hexanol by either isolated YADH1 or with permeabilized baker's yeast cells [START_REF] Vrsalović Presečki | Coenzyme Regeneration in Hexanol Oxidation Catalyzed by Alcohol Dehydrogenase[END_REF][START_REF] Vrsalović Presečki | Mathematical modelling of the dehydrogenase catalyzed hexanol oxidation with coenzyme regeneration by NADH oxidase[END_REF]. Substrate-coupled regeneration was probed, but the "sacrificial" oxidant (i.e. acetaldehyde) strongly inhibited the isolated YADH and utilization of a coupled enzyme system was more suitable. More exhaustive coverage of recycling systems can be found in recent reviews [START_REF] Liu | Recent advances in enzymatic oxidation of alcohols[END_REF][START_REF] Puetz | Biocatalytic Oxidation of Alcohols[END_REF][START_REF] Rehn | Application of NAD(P)H oxidase for cofactor regeneration in dehydrogenase catalyzed oxidations[END_REF][START_REF] Wu | Methods for the regeneration of nicotinamide coenzymes[END_REF].

Catalase

Both, FAD-AOXs and CRO-AlcOx, as part of their catalytic cycle, release H2O2 as byproduct, which, when accumulated, is generally deleterious for the enzyme and must be therefore eliminated. Catalases (EC 1.11.1.6), which belong to a catalytically very efficient class of enzymes, are already used at industrial scale in textile bleaching processes [START_REF] Lončar | Catalases as biocatalysts in technical applications: current state and perspectives[END_REF], and are widely applied to circumvent AOXs inactivation. Catalases conveniently recycles H2O2 into H2O and ½ O2, supplying back molecular oxygen which can be rate-limiting in the reaction due to its limited solubility in aqueous media (0.25 mM at room temperature).

Yet, some enzymes seem to possess an intrinsic resistance to H2O2 as described for a microsome-bound preparation of a FAD-FAOX from A. terreus, able to convert ≥85% of C7-C12 alcohols (10 mM) without any addition of catalase [START_REF] Kakoti | Microsome-bound alcohol oxidase catalyzed production of carbonyl compounds from alcohol substrates[END_REF].

Peroxidase

CROs are usually recovered in their inactive "semi-reduced" form and require activation prior to utilization. Peroxidases or chemical oxidants (e.g. ferricyanide, ammonium hexachloroiridate, Mn(III) compounds etc…) can be used to this end [START_REF] Forget | Determination of biocatalytic parameters of a copper radical oxidase using real-time reaction progress monitoring[END_REF][START_REF] Hamilton | Trivalent copper, superoxide, and galactose oxidase[END_REF][START_REF] Johnson | Biocatalytic oxidation of alcohols using galactose oxidase and a manganese(III) activator for the synthesis of islatravir[END_REF][START_REF] Pedersen | Process Requirements of Galactose Oxidase Catalyzed Oxidation of Alcohols[END_REF]. However, these chemical compounds are often less efficient than peroxidases and are required in large amounts, Thus, to date no equivalent alternative to peroxidase has been found and more investigations are still needed. The HRP is the most common enzyme used as activating agent, mainly for its commercial availability and stability in reaction mixtures. Of note, recent work demonstrated that natural fungal peroxidases are better activators of CRO-AlcOx than HRP [START_REF] Bissaro | Unravelling the role of alcohol copper radical oxidases in fungal plant pathogens[END_REF]. Strikingly, although the activation of CRO by peroxidases has been known for long, the underlying mechanism remains obscure. Current state of the art points toward an oxidative activation mechanism independent of any peroxidase catalytic activity but most likely mediated by a protein-protein interaction [START_REF] Bissaro | Unravelling the role of alcohol copper radical oxidases in fungal plant pathogens[END_REF][START_REF] Forget | Determination of biocatalytic parameters of a copper radical oxidase using real-time reaction progress monitoring[END_REF]. Indeed, no peroxidase substrate seems involved into the activation, which suggest that chemical oxidants may act in a similar way. We underscore that the required amount of peroxidase greatly varies depending on the substrates: only minimal amounts are needed for activated alcohols while stochiometric amounts (relative to the AlcOx) or more are required for oxidation of non-activated alcohols (Ribeaucourt et al., 2021a). The necessity to add large amount of peroxidase to foster the reaction is unambiguously the most important barrier for the implementation of CROscatalyzed oxidation of fatty alcohols at industrial scale. Peroxidases are expensive enzymes as their recombinant production remains challenging. Deeper understanding of this mechanism is needed to provide access to alternative activation methods. Very recently, an electrochemical activation method was investigated to substitute the peroxidase [START_REF] Zhang | Electrochemical Activation of Galactose Oxidase: Mechanistic Studies and Synthetic Applications[END_REF]. This approach proved to be efficient for the conversion of various aromatic alcohols and aliphatic polyols catalyzed by several FgrGalOx mutants.

Overoxidation

Overoxidation of aldehydes into carboxylic acids is frequently observed and affectin a substrate-, pH and enzyme-dependent mannerevery class of enzymes aforementioned, namely ADHs [START_REF] Hinson | An examination of octanol and octanal metabolism to octanoic acid by horse liver alcohol dehydrogenase[END_REF][START_REF] Hollmann | Enzymemediated oxidations for the chemist[END_REF][START_REF] Velonia | Dismutation of aldehydes catalyzed by alcohol dehydrogenases[END_REF], FAD-AOXs [START_REF] Ferreira | Kinetic and chemical characterization of aldehyde oxidation by fungal aryl-alcohol oxidase[END_REF][START_REF] Martin | Production of Hydroxy Acids through Selective Double Oxidation of Diols by a Flavoprotein Alcohol Oxidase[END_REF][START_REF] Romero | Alcohol oxidation by flavoenzymes[END_REF] and CROs [START_REF] Birmingham | A Single Enzyme Oxidative "Cascade" via a Dual-Functional Galactose Oxidase[END_REF][START_REF] Kelleher | Re-examination of the products of the action of galactose oxidase. Evidence for the conversion of raffinose to 6''-carboxyraffinose[END_REF][START_REF] Parikka | Oxidation of methyl α-D-galactopyranoside by galactose oxidase: products formed and optimization of reaction conditions for production of aldehyde[END_REF]. The aldehydes cannot be oxidized directly by these enzymes, as their H atom is not abstractable per se and need an adduct brought either by the enzyme [START_REF] Olson | Mechanism of Aldehyde Oxidation Catalyzed by Horse Liver Alcohol Dehydrogenase[END_REF], or by the environment. In most cases, the observed oxidation of aldehydes is in fact directly dependent on the propension of the aldehyde to undergo hydration to form the corresponding geminal-diol (gem-diol) (Scheme 1) [START_REF] Dong | Biocatalytic Oxidation Reactions: A Chemist's Perspective[END_REF] and the subsequent ability of the enzyme to accept the latter as a substrate. Destabilization of the dipole at the carbonyl group favor nucleophilic addition of H2O, consequently shifting the equilibrium toward the hydrated species. Electron-withdrawing groups will thence favor gem-diol, while electron-donating groups or resonance effect should stabilize the carbonyl group [START_REF] Bell | The Reversible Hydration of Carbonyl Compounds[END_REF][START_REF] Hutchings | Correlation analyses of the aqueous-phase acidities of alcohols and gem-diols, and of carbonyl hydration equilibria using electronic and structural parameters[END_REF]. Aldehydes bearing an alkyl group, like fatty aldehydes are known to be prone to hydration [START_REF] Buschmann | The Reversible Hydration of Carbonyl Compounds in Aqueous Solution Part II: The Kinetics of the Keto/Gem-diol Transition[END_REF][START_REF] Buschmann | The Reversible Hydration of Carbonyl Compounds in Aqueous Solution. Part I, The Keto/Gem-diol Equilibrium[END_REF][START_REF] Mcclelland | Structure-reactivity effects in the hydration of benzaldehydes[END_REF].

Aldehyde overoxidation and disproportionation (i.e., when two molecules of aldehyde yields one molecule of acid and one molecule of alcohol, also known as "Cannizzaro reaction") by ADHs [START_REF] Velonia | Dismutation of aldehydes catalyzed by alcohol dehydrogenases[END_REF] has been investigated for some applications in pharmaceuticals or food additives [START_REF] Contente | Uncommon overoxidative catalytic activity in a new halo-tolerant alcohol dehydrogenase[END_REF][START_REF] Könst | Enantioselective Oxidation of Aldehydes Catalyzed by Alcohol Dehydrogenase[END_REF][START_REF] Tassano | Biocatalytic Parallel Interconnected Dynamic Asymmetric Disproportionation of α-Substituted Aldehydes: Atom-Efficient Access to Enantiopure (S)-Profens and Profenols[END_REF]. Production of carboxylic acid has been reported early for the prominent HADH, exhibiting a different behavior depending on the substrate. HADH oxidation of ethanol resulted in a mixture of aldehyde and carboxylic acid, while for 1-octanol, octanoic acid was the only detectable end-product (Hinson andNeal, 1975, 1972;[START_REF] Oppenheimer | Horse Liver Alcohol Dehydrogenase-Catalyzed Aldehyde Oxidation[END_REF].

Overoxidation in FAD-AOXs-catalyzed reactions must also be carefully considered.

Indeed, in a recent study, a mutant of the Phanerochaete chrysosporium FAD-AOX (PcAOX) yielded >99 % of carboxylic acid when oxidizing C6 alcohol [START_REF] Martin | Production of Hydroxy Acids through Selective Double Oxidation of Diols by a Flavoprotein Alcohol Oxidase[END_REF]. In contrast, a choline oxidase variant (AcCO6) engineered for fatty alcohol oxidation (see section 4.1) [START_REF] Heath | An Engineered Alcohol Oxidase for the Oxidation of Primary Alcohols[END_REF] revealed only traces of acid for C6 and between 1.5 -8 % for C7-C12 (Table S1) despite expectable strong activity on gem-diols [START_REF] Gadda | Hydride Transfer Made Easy in the Reaction of Alcohol Oxidation Catalyzed by Flavin-dependent Oxidases[END_REF]. Very few studies on LCAO included products analyses and, therefore, overoxidation has rarely been evaluated. The aforementioned study on LCAO_Af revealed no overoxidation for C4 to C8 but led to 5-26% carboxylic acid for C9 to C11 [START_REF] Pickl | Amination of ω-Functionalized Aliphatic Primary Alcohols by a Biocatalytic Oxidation-Transamination Cascade[END_REF].

Overoxidation was also monitored when using CRO-AlcOx and in this case, different mechanisms were observed. While a gem-diol dependent, CRO-catalyzed overoxidation was monitored for benzyl substrates, the overoxidation of fatty aldehydes was rather triggered by exposure to HRP.

Scheme 1: Aldehyde hydration and overoxidation routes. Formation of gem-diol upon aldehyde hydration in acidic or alkaline conditions and subsequent enzymatic oxidation of the gem-diol to carboxylic acid.

Overoxidation can be circumvented or limited by different level of control on reaction conditions. The substrate structure will firstly give indication on the propension of the aldehyde to undergo hydration and subsequent probability of overoxidation. In terms of hydration rate, gem-diols seem readily generated in a matter of seconds or less upon exposure to H2O at room temperature [START_REF] Hilal | Hydration Equilibrium Constants of Aldehydes, Ketones and Quinazolines[END_REF]. Setting the pH of the reaction around neutrality, when possible, could slow down the rate of gem-diol formation [START_REF] Doussin | Structure-activity relationship for the estimation of OHoxidation rate constants of carbonyl compounds in the aqueous phase[END_REF]. Gem-diols are often disfavored substrates compared to the initial alcohols, especially for enzymes with preferences upon primary alcohols, as gem-diols are structurally similar to secondary alcohols.

Thus, tight control of alcohol concentration and reaction time may be one of the main levers to avoid the formation of unwanted carboxylic acid (Ribeaucourt et al., 2021a). The implementation of a biphasic system can also be an alternative to remove the aldehyde from the aqueous phase and prevent its hydration (see section 4.2).

Aside from the enzymatic-catalyzed overoxidation, intrinsic reactivity of aldehydes can result in autooxidation [START_REF] Marteau | Oxidative degradation of fragrant aldehydes. Autoxidation by molecular oxygen[END_REF][START_REF] Sankar | The benzaldehyde oxidation paradox explained by the interception of peroxy radical by benzyl alcohol[END_REF] or side reactions (e.g.

formation of hemi-acetals) which require careful controls of reaction conditions and product validation.

Biocatalytic system engineering

Protein engineering

Protein engineering is a complimentary approach to the exploration of biodiversity (section 3.1), enabling to enhance or reprogram a natural enzymatic starting scaffold for a targeted process [START_REF] Woodley | Accelerating the implementation of biocatalysis in industry[END_REF]. Native enzymes are designed for in vivo finely tuned reactions. Early engineering approaches mostly relied on rational engineering based on structural and mechanistic information [START_REF] Wilkinson | A large increase in enzyme-substrate affinity by protein engineering[END_REF]. Directed evolution of enzymes though the introduction of random diversity was then widely adopted using error-prone PCR and DNA shuffling [START_REF] Chen | Tuning the activity of an enzyme for unusual environments: sequential random mutagenesis of subtilisin E for catalysis in dimethylformamide[END_REF][START_REF] Cobb | Directed Evolution: Past, Present and Future[END_REF][START_REF] Stemmer | Rapid evolution of a protein in vitro by DNA shuffling[END_REF]. The large libraries of generated mutants require HTP screening methods raising similar problematics than for biodiversity screening. The field is now rapidly evolving toward semi-rational approaches, harnessing structure/mechanistic information and computational modeling to focus the directed evolution effort on the most relevant residues [START_REF] Vogel | Enzyme Development Technologies[END_REF]. The resulting "smart" libraries are smaller allowing to apply more specific detection methods, closer to the final sought-after properties [START_REF] Lutz | Beyond directed evolution-semi-rational protein engineering and design[END_REF][START_REF] Sheldon | Broadening the Scope of Biocatalysis in Sustainable Organic Synthesis[END_REF].

In the context of this review, enzyme engineering is an asset to (i) improve substrate and organic solvent tolerance, (ii) improve thermal stability, (iii) switch enzyme specificity to nonnatural substrates, (iv) improve expression, (v) adapt enzyme to specific conditions (pH, temperature, presence of inhibitors) etc. For instance, a choline oxidase from Arthrobacter cholorphenolicus was recently engineered using structure-guided evolution [START_REF] Heath | An Engineered Alcohol Oxidase for the Oxidation of Primary Alcohols[END_REF].

With six mutated residues, the AcCO6 variant gained activity toward 1-hexanol and a broad array of saturated, unsaturated, aliphatic, cyclic and benzylic alcohols [START_REF] Heath | An Engineered Alcohol Oxidase for the Oxidation of Primary Alcohols[END_REF].

Notably, C6-C10 unsaturated alcohols were identified as good substrates and were converted (with little overoxidation) in 24 h at 30°C in air-saturated buffer, without any added catalase.

This enzyme variant also displayed improved thermostability and solvent tolerance as demonstrated by the implementation of biphasic biotransformation with heptane.

The enlargement of the substrate binding pocket has been successfully applied on several enzymes of interest to increase their activities towards bulky primary alcohols. With only two single-point mutations on YADH1, kinetic constants were improved by one order of magnitude on octanol, (kcat/KM = 5.5×10 4 M -s -1 ) for the T48S-W39A variant), reaching a catalytic efficiency comparable to HADH (EE isoenzyme) [START_REF] Green | Inversion of the substrate specificity of yeast alcohol dehydrogenase[END_REF]. Similarly, enlargement of the substrate pocket of PcAOX enabled oxidation of 1-hexanol and diols, when the wildtype enzyme showed only poor activity on these substrates [START_REF] Linke | An alcohol oxidase of Phanerochaete chrysosporium with a distinct glycerol oxidase activity[END_REF][START_REF] Martin | Production of Hydroxy Acids through Selective Double Oxidation of Diols by a Flavoprotein Alcohol Oxidase[END_REF][START_REF] Nguyen | Structure-Based Engineering of Phanerochaete chrysosporium Alcohol Oxidase for Enhanced Oxidative Power toward Glycerol[END_REF].

The CRO FgrGalOx enzyme has been the subject of many engineering studies, bending its properties far from its initial characteristics, yielding for instance, enantioselective oxidation of secondary alcohols or glucose-6-oxidase activity [START_REF] Escalettes | Directed Evolution of Galactose Oxidase: Generation of Enantioselective Secondary Alcohol Oxidases[END_REF][START_REF] Sun | Modification of Galactose Oxidase to Introduce Glucose 6-Oxidase Activity[END_REF] (see section 2.3.2). The FgrGalOx was also recently engineered to catalyze the first step of a cascade to produce a drug for HIV treatment [START_REF] Huffman | Design of an in vitro biocatalytic cascade for the manufacture of islatravir[END_REF]. Twelve rounds of mutations starting from a previously generated variant [START_REF] Rannes | Glycoprotein labeling using engineered variants of galactose oxidase obtained by directed evolution[END_REF] were run leading to 34 mutated residues in the evolved enzyme enabling an 11-fold improvement for 2ethynylglycerol oxidation and reversing the enzyme enantio-specificity. This study, among the numerous studies performed on the FgrGalOx, let envision a great potential for CRO-AlcOx enzymes, which naturally possess attractive features for fatty alcohol conversion and can benefit from the knowledge gathered on structurally/phylogenetically related CROs.

Whole-cell catalysis

When considering industrial applications, whole-cell and isolated enzymes approaches must be put into balance [START_REF] Hoyos | 3.28 -Biocatalyzed Production of Fine Chemicals[END_REF]. The whole-cell approach is attractive for some enzymes like ADHs that require in situ supply and regeneration of a cofactor [START_REF] Romano | Preparative Biotransformations: Oxidation of Alcohols[END_REF][START_REF] Sheldon | Broadening the Scope of Biocatalysis in Sustainable Organic Synthesis[END_REF]. Implementation of multi-enzymatic systems into a chassis through synthetic biology is a powerful approach to reprogram metabolic pathways, as exemplified by recent work on the production of sclareol in pathway-engineered E. coli [START_REF] Schalk | Method for producing sclareol[END_REF][START_REF] Schalk | Toward a Biosynthetic Route to Sclareol and Amber Odorants[END_REF]). Yet, this approach has inherent drawbacks such as low enzyme activity levels, cell membrane permeability-related issues, growth inhibition by substrate or product toxicity etc. [START_REF] Garzón-Posse | Whole Cells as Biocatalysts in Organic Transformations[END_REF]. Indeed, aldehydes are particularly hard to produce through this approach as they are undesirable intermediates for most cells and are often readily converted to carboxylic acids or alcohols by endogenous enzymes [START_REF] Liu | Recent advances in enzymatic oxidation of alcohols[END_REF].

Two studies using permeabilized S. cerevisiae cells producing YADH outlined such difficulties: the conversion of C6 alcohol yielded only hexanoic acid due to overoxidation attributed to the endogenous aldehyde dehydrogenase [START_REF] Vrsalović Presečki | Coenzyme Regeneration in Hexanol Oxidation Catalyzed by Alcohol Dehydrogenase[END_REF][START_REF] Vrsalović Presečki | Mathematical modelling of the dehydrogenase catalyzed hexanol oxidation with coenzyme regeneration by NADH oxidase[END_REF]. Whole-cell aldehyde production for F&F has been recently reviewed elsewhere [START_REF] Ben Akacha | Microbial and enzymatic technologies used for the production of natural aroma compounds: Synthesis, recovery modeling, and bioprocesses[END_REF][START_REF] Kunjapur | Microbial Engineering for Aldehyde Synthesis[END_REF] and will not be further discussed here.

Biphasic systems and enzyme immobilization

Biphasic systems are attractive to trap the produced aldehydes in the organic phase and avoid their hydration and subsequent overoxidation, especially for hydrophobic fatty aldehydes [START_REF] Gandolfi | An easy and efficient method for the production of carboxylic acids and aldehydes by microbial oxidation of primary alcohols[END_REF][START_REF] Heath | An Engineered Alcohol Oxidase for the Oxidation of Primary Alcohols[END_REF]. Biphasic systems are also a convenient way to (i) overcome low water solubility of the substrates, (ii) limit product inhibition and (iii) facilitate product recovery trough simple distillation (Domínguez de [START_REF] Domínguez De María | On the (Un)greenness of Biocatalysis: Some Challenging Figures and Some Promising Options[END_REF]. The use of biphasic systems to avoid overoxidation can yet be limited, when hydration occurs directly within the active site as reported for a FAD-AOX [START_REF] Gadda | Kinetic mechanism of choline oxidase from Arthrobacter globiformis[END_REF][START_REF] Salvi | Structure of choline oxidase in complex with the reaction product glycine betaine[END_REF].

Another limitation of these systems lies on the intrinsic enzyme tolerance to the waterimmiscible solvent, their related resistance to interfacial tension, the rate of phase transfer, and the level of O2 available in the case of AOXs. However, in some cases, utilization of an organic layer can improve or modify the activity of a given enzyme. The microsomal fraction of C.

tropicalis was for instance much more efficient in oxidizing C10 and C12 alcohols in an hydrophobic organic solvent containing 1-10% water as compared to aqueous media [START_REF] Kemp | Activity and substrate specificity of the fatty alcohol oxidase of Candida tropicalis in organic solvents[END_REF]. The FAD-AOX from P. pastorisactive on methanol and other short-chain alcohols has been used for conversion of 1-hexanol and bulkier substrates thanks to the implementation of biphasic systems [START_REF] Karra-Chaabouni | Biooxidation of n-hexanol by alcohol oxidase and catalase in biphasic and micellar systems without solvent[END_REF][START_REF] Murray | Bio-oxidation of aliphatic and aromatic high molecular weight alcohols by Pichia pastoris alcohol oxidase[END_REF]. Noteworthy, the substrate itself can sometimes be employed as the organic layer [START_REF] De Almeida | Efficient Aerobic Oxidation of trans -2-Hexen-1-ol using the Aryl Alcohol Oxidase from Pleurotus eryngii[END_REF]. Using naturally resilient or engineered enzymes, reactions can be run in water-saturated organic medium.

Another alternative to improve enzyme industrial applicability is immobilization.

Immobilization techniques are intended to facilitate downstream processes, improve turnover number by enhancing catalyst stability and reusability. They also can be applied to improve resistance of enzymes to organic solvents. Some examples are given elsewhere [START_REF] Basso | Industrial applications of immobilized enzymes-A review[END_REF][START_REF] Pessôa | Prospective Research and Current Technologies for Bioflavor Production[END_REF]. Immobilization of ADH enzymes has been extensively studied [START_REF] Bolivar | Improvement of the stability of alcohol dehydrogenase by covalent immobilization on glyoxyl-agarose[END_REF][START_REF] Vasić | Immobilization of alcohol dehydrogenase from Saccharomyces cerevisiae onto carboxymethyl dextran-coated magnetic nanoparticles: a novel route for biocatalyst improvement via epoxy activation[END_REF] but with the use of fatty alcohols as substrates is less common [START_REF] Cea | Effect of chain length on the activity of free and immobilized alcohol dehydrogenase towards aliphatic alcohols[END_REF]. YADH immobilized on glyoxyl agarose crosslinked with a polymer improved its stability and its activity on fatty alcohols [START_REF] Ottone | Enhanced long-chain fatty alcohol oxidation by immobilization of alcohol dehydrogenase from S. cerevisiae[END_REF]. A study conducted in 2010 combined immobilization of HADH and YADH and the use of organic solvents, including the use of the substrate itself as solvent [START_REF] Snijder-Lambers | Optimization of alcohol dehydrogenase activity and NAD(H) regeneration in organic solvents[END_REF]. In organic medium, YADH showed better activity on bulkier substrates compared to aqueous environment. Immobilization of YADH on amberlite XAD-8, resulted in 93-99 % loss of activity despite optimization attempts, while immobilized HADH remained fully active [START_REF] Snijder-Lambers | Optimization of alcohol dehydrogenase activity and NAD(H) regeneration in organic solvents[END_REF]. The aforementioned FAD-AOX AcCO6 has been immobilized on an affinity resin via the His-tag used for purification [START_REF] Thompson | A generic platform for the immobilisation of engineered biocatalysts[END_REF] and on an epoxy resin [START_REF] Mattey | Natural heterogeneous catalysis with immobilised oxidase biocatalysts[END_REF], improving its reusability. Only one study reported the immobilization of a FAD-FAOX (isolated from microsomal fraction of A. terreus) into a polyurethane foam matrix but resulted in only 20% conversion of C7 alcohol [START_REF] Kakoti | Microsome-bound alcohol oxidase catalyzed production of carbonyl compounds from alcohol substrates[END_REF]. Immobilization studies were performed on the M1 and M3-5 variants of the CRO FgrGalOx and improved their thermal stability, reusability and organic solvent tolerance [START_REF] Mattey | Natural heterogeneous catalysis with immobilised oxidase biocatalysts[END_REF]. To our knowledge, no attempts have yet been made for CRO-AlcOx. It is likely that immobilization of CRO-AlcOx and FAD-FAOX could greatly enhance their use in bi-phasic system or hydrophobic medium, avoiding solubility and overoxidation issues.

The main problematics and possible solutions (discussed in sections 3. and 4.) to consider for the applications of ADHs; FAD-AOXs or CRO-AlcOx for the oxidation of fatty alcohols are summarized in Figure 6.

Outlook

The current state of the art reveals that a great number of enzymes candidates already exists for the challenging oxidation of non-activated fatty alcohols. Some clear trends stand out (see Figure 1):

(i) ADHs constitute a tremendous reservoir of enzymes but scarce information exists on their biocatalytic performances for fatty alcohols turnover. The need for a nicotinamide co-factor adds complexity to this biocatalytic system. (iii) FAD-FAOX might be today the best suited enzymes for oxidation of fatty alcohols.

They display high catalytic efficiencies toward fatty alcohols (Figure 1B) and only require the "industrial-compatible" catalase as accessory enzyme. However, only scarce information is available regarding their large-scale recombinant production and their performances in biocatalytic systems.

To foster the translation of fatty aldehydes production at industrial scale, we draw the attention towards some crucial criteria that must be taken into account early in the research and development process: (i) Culture media used for growth of microorganisms should be simple and composed of low-cost ingredients (e.g. malt). Complex bioreactor media supplemented with vitamins or metals are costly when shifting to large scale processes for F&F and any additional compounds (chemicals, metals, accessory enzymes, etc.) will increase reaction costs and must be balanced with substantial gain in yields. (ii) Upscaled reactions in large tank reactors (e.g. 30-80 m 3 ) require important investments and solutions that are easily set up at lab scale (e.g., work in pressurized reactor, addition of large volume or organic solvent, etc.) can be difficult to transfer to industry or must be useful for multiple processes to justify new investments. (iii) The duration of the process has also a direct effect in the cost. Long-time immobilization of the material will impair profitability and could increase the probability of a contamination. Consequently, every step must be optimized: culture of the microorganism, production and recovery of the biocatalyst, conversion reaction and product recovery. (iv) The extraction procedure is also crucial and full conversion of the substrate is thus highly desirable to simplify downstream processes.

In the future, the biotechnological production of fatty aldehydes will increase to secure the production using greener systems. To set up a sustainable approach, natural sources of fatty alcohols must be considered. Although cheap chemical sources are available [START_REF] Surburg | Common Fragrance and Flavor Materials: Preparation, Properties and Uses[END_REF], natural sources of fatty alcohols are very limited today. Some new reservoirs could arise from forestry by-products and from the pulp and paper industry [START_REF] Corberán | Green oxidation of fatty alcohols: Challenges and opportunities[END_REF] and alternative sources could be established by implementing cascade reactions from natural esters (using for instance well-known lipases; [START_REF] Johannsen | Process for the Preparation of Fatty Aldehydes from Renewable Resources[END_REF] or carboxylic acid as starting raw material. In any case, the extraction of the substrate from raw materials must be done in compliance with local regulation to preserve their natural origin.

The hunt for ad-hoc biocatalysts to produce fatty aldehydes calls for highly integrated approaches harnessing fundamental knowledge on enzymes and their biological context, taking into account industrial constraints. We hope that the background information provided in this review will encourage new investigations to foster the sustainable development of biotechnological solutions for the F&F industry.

Figure 1 .

 1 Figure 1. Comparative overview of (A) the main advantages and drawbacks for each enzyme class discussed in this review and of (B) catalytic efficiencies of some CRO-AlcOx (blue bar), FAD-AOXs (yellow bars) and ADHs (pink bars) toward a range of fatty alcohols (C6-C12). For each enzyme, the kcat/KM values are shown with a different symbol depending on the substrate chain length (C6 to C12, see legend key in the figure). For illustrative purposes, we show a colored bar in the background that corresponds to the highest kcat/KM value measured for each enzyme. An asterisk (*) indicates when an X-ray crystallographic structure of the enzyme is available. See TableS1for more details on properties

Figure 2 .

 2 Figure 2. Example of an ADH structure and overview of ADHs catalytic cycle. (A) Cartoon representation of one HADH monomer in complex with NADH (PDB 2OHX (Al-Karadaghi et al., 1994)). NADH is displayed as yellow sticks. Zinc (Zn) ions are displayed as blue spheres; the Zn ion close to NADH plays a role in catalysis, the second Zn ion is structural. Note that HADH structure is used for illustrative purposes and is not meant to represent the entire, diverse class of ADHs. (B) Catalytic mechanism of ADH dehydrogenation of primary alcohols (pink arrows) (and reverse reduction reaction of aldehydes, shown in blue arrows) coupled to in situ NAD(P)H regeneration. In the NAD(P) + /NAD(P)H chemical structure only the nicotinamide group is shown (R corresponds to the adenine (monophosphate) dinucleotide chain)

Figure 3 .

 3 Figure 3. Example of a FAD-AOX structure and overview of FAD-AOXs catalytic cycle. (A) Cartoon representation of a FAD-AOX monomer from P. pastoris in complex with FAD (shown as yellow stick) (PDB 5HSA). This enzyme is shown as an example and is not not meant to represent the entire, diverse class of FAD-AOXs. (B) Generic catalytic mechanism of AOX-catalyzed oxidation of primary alcohols and H2O2 removal by catalase.

Figure 4 .

 4 Figure 4. Example of a CRO-AlcOx structure and overview of CRO-AlcOx catalytic cycle. (A) Cartoon representation of CgrAlcOx (PDB 5C92; (Yin et al., 2015)). Copper is displayed as orange sphere. Active sites residues (including copper coordinating residues) are shown as yellow sticks. (B) Generic catalytic mechanism of CRO-catalyzed oxidation of primary alcohols, H2O2 consumption by catalase and CRO activation by a peroxidase.

Figure 5 .

 5 Figure 5. Surface views of crystal structures of (A) A. pernix ADH active site (pdb 1H2B) (chain A) in complex with octanoic acid (inhibitor; shown as green stick); (B) S. cerevisiae YADH1 active site (pdb 4W6Z) (chain A) in complex with trifluoroethanol (shown as green stick); (C) C. graminicola CRO-AlcOx (pdb 5C92 -copper ion shown as orange sphere) in complex with 1-octanol (shown as green stick; docking model adapted from Ribeaucourt et al., 2021a); (D) P. pastoris FAD-AOX (pdb 5HSA) homooctamer. The red arrow indicates the position of the buried substrate binding cavity.For all enzymes, surface was colored according to the ratio of non-polar/polar (NPP) residues using Protein-Sol web tool (https://proteinsol.manchester.ac.uk/;[START_REF] Hebditch | Protein-Sol: a web tool for predicting protein solubility from sequence[END_REF]. The NPP ratio scale, indicative of increasing hydrophobicity level from 0.6 to 2.3, displayed at the bottom left hand corner of panel C applies to all panels. Note that the structures displayed are not shown at the same scale.

Figure 6 .

 6 Figure 6. Overview of some challenges and possible solutions for implementation of enzymatic oxidation of fatty alcohols. While bringing answers to certain challenges, some solutions might cause other unwanted drawbacks that are represented by red dashed lines.

  are promising catalysts with well-established large-scale recombinant production but require a peroxidase activation step for oxidation of unactivated alcohols. Research on CRO-AlcOx is still in its infancy since their discovery in 2015 by Yin et al. and better understanding of these microbial enzyme systems could bring alternative activation methods. The recent success in activating someFgrGalOx mutants by electrochemical means[START_REF] Zhang | Electrochemical Activation of Galactose Oxidase: Mechanistic Studies and Synthetic Applications[END_REF], open a new avenue to overcome the need of peroxidase and could be assessed on CRO-AlcOx likewise.

Table 1 : Overview of main fatty aldehydes used as flavors and fragrances. Names Number of C atoms Chemical structure Scent Main natural sources

 1

	Hexanal, Caproaldehyde	6	Green, grassy fruity	Occurs in many plants and fruits
	Octanal, Caprylaldehyde	8	Green, citrus-like fruity, floral, lemon, green grassy	Sweet orange peel oil
	Nonanal, Pelargonaldehyde	9	Soapy, citrus-like, rose-like, floral	Orange and rose oils
				Large occurrence in
	Decanal Caprinaldehyde	10	Green, citrus-like, fatty, peelly	various essential oils: citrus peel
				oils, neroli oil…
	Dodecanal Lauric aldehyde	12	Waxy, fatty, soapy, reminiscent of violets	Citrus and Pinus oils

Funding

This study was supported by the French National Agency for Research ("Agence Nationale de la Recherche") through the "Projet de Recherche Collaboratif International" ANR-NSERC (FUNTASTIC project, ANR-17-CE07-0047). PhD fellowship of D.R. was funded by V. MANE Fils and the "Association Nationale Recherche Technologie" (ANRT) through the Convention Industrielle de Formation par la RecherchE (CIFRE) grant no. 2017/1169.