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International audience; Biocatalytic pathways for the synthesis of (-)-menthol, the most sold flavor worldwide, are highly sought-after. To access the key intermediate (R)-citronellal used in current major industrial production routes, we established a one-pot bienzymatic cascade from inexpensive geraniol, overcoming the problematic biocatalytic reduction of the mixture of (E/Z)-isomers in citral by harnessing a copper radical oxidase (CgrAlcOx) and an old yellow enzyme (OYE). The cascade using OYE2 delivered 95.1% conversion to (R)-citronellal with 95.9% cc, a 62 mg scaleup affording high yield and similar optical purity. An alternative OYE, GIuER, gave (S)-citronellal from geraniol with 95.3% conversion and 99.2% ee.

International audience; 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.

Copper radical alcohol oxidases (CRO-AlcOx), which have been recently discovered among fungal phytopathogens, are attractive for the production of fragrant fatty aldehydes. With the initial objective to investigate the secretion of CRO-AlcOx by natural fungal strains, we undertook time course analyses of the secretomes of three Colletotrichum species (C. graminicola, C. tabacum, and C. destructivum) using proteomics. The addition of a copper-manganese-ethanol mixture in the absence of any plant-biomass mimicking compounds to Colletotrichum cultures unexpectedly induced the secretion of up to 400 proteins, 29 to 52% of which were carbohydrate-active enzymes (CAZymes), including a wide diversity of copper-containing oxidoreductases from the auxiliary activities (AA) class (AA1, AA3, AA5, AA7, AA9, AA11, AA12, AA13, and AA16). Under these specific conditions, while a CRO-glyoxal oxidase from the AA5_1 subfamily was among the most abundantly secreted proteins, the targeted AA5_2 CRO-AlcOx were secreted at lower levels, suggesting heterologous expression as a more promising strategy for CRO-AlcOx production and utilization. C. tabacum and C. destructivum CRO-AlcOx were thus expressed in Pichia pastoris, and their preference toward both aromatic and aliphatic primary alcohols was assessed. The CRO-AlcOx from C. destructivum was further investigated in applied settings, revealing a full conversion of C(6) and C(8) alcohols into their corresponding fragrant aldehydes. IMPORTANCE In the context of the industrial shift toward greener processes, the biocatalytic production of aldehydes is of utmost interest owing to their importance for their use as flavor and fragrance ingredients. Copper radical alcohol oxidases (CRO-AlcOx) have the potential to become platform enzymes for the oxidation of alcohols to aldehydes. However, the secretion of CRO-AlcOx by natural fungal strains has never been explored, while the use of crude fungal secretomes is an appealing approach for industrial applications to alleviate various costs pertaining to biocatalyst production. While investigating this primary objective, the secretomics studies revealed unexpected results showing that under the oxidative stress conditions we probed, Colletotrichum species can secrete a broad diversity of copper-containing enzymes (laccases, sugar oxidoreductases, and lytic polysaccharide monooxygenases [LPMOs]) usually assigned to “plant cell wall degradation,” despite the absence of any plant-biomass mimicking compound. However, in these conditions, only small amounts of CRO-AlcOx were secreted, pointing out recombinant expression as the most promising path for their biocatalytic application.

The oxidation of alcohols is a cornerstone reaction in chemistry, notably in the flavor and fragrance industry where long-chain aliphatic aldehydes are major odorant compounds. In a context where greener alternatives are sought after, biocatalysis holds many promises. Here, we investigated the ability of the alcohol oxidase from Colletotrichum graminicola (CgrAlcOx)—an organic cofactor-free enzyme belonging to the copper-radical oxidase (CRO) class—to convert industrially relevant long-chain aliphatic alcohols. CgrAlcOx is a competent catalyst for the conversion of octan-1-ol when supported by the accessory enzymes peroxidase and catalase. Detailed examination of the products revealed the occurrence of an overoxidation step leading to the production of carboxylic acid for some aliphatic aldehydes and benzaldehyde derivatives. The partition between aldehyde and acid products varied upon substrate properties (chain length and propensity to form geminal-diols) and enzyme specificity and could be tuned by controlling the reaction conditions. In silico analyses suggested an inhibitory binding mode of long-chain aliphatic geminal-diols and a substrate-induced fit mechanism for a benzyl alcohol derivative. By demonstrating their natural ability to perform long-chain aliphatic alcohol oxidation, the present study establishes the potential of fungal CRO-AlcOx as promising candidates for the green production of flavor and fragrance compounds. 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), by the Natural Sciences and Engineering Research Council of Canada through the "Strategic Projects - Natural Resources and Energy - Project (STPGP) entitled "FUNTASTIC - Fungal copper-radical oxidases as new biocatalysts for the valorization of biomass carbohydrates and alcohols" and by the PID2019-106370RB-I00 grant from the Spanish Ministry of Innovation and Sciences. We are grateful to MANE & Fils and the “Association Nationale Recherche Technologie” (ANRT) for funding the PhD fellowship of D.R. entitled “Discovery and structure-function study of new fungal copper radicals oxidases used as biocatalysts for the valorisation of alcohols and plant biomass.” This Convention Industrielle de Formation par la RecherchE (CIFRE) grant no. 2017/1169 runs from 1 April 2018 to 1 April 2021