P. roqueforti est une moisissure connue comme contaminant des ensilages et des aliments, mais aussi utilisée comme ferment pour la production de fromages à pâte persillée. Cette espèce est également connue pour son potentiel de production de métabolites secondaires à impact positif, comme les arômes, mais aussi à impact négatif, tels que des mycotoxines (dont la roquefortine C -ROQC- et l’acide mycophénolique -MPA-). Dans la première partie de ce travail, l’occurrence de la ROQC, du MPA et de l’aflatoxine M1 (seule mycotoxine réglementée dans les produits laitiers), dans une collection de 86 fromages à pâte persillée de différentes variétés et origines géographiques (15 pays), a été réalisée. Il apparaît que, si l’aflatoxine est inférieure au seuil de détection (LOD) de la méthode, les concentrations en ROQC et MPA sont quant à elles très variables et qu’une co-occurrence de ces mycotoxines existe dans 51% des échantillons testés. Dans la seconde partie, la toxicité de ces mycotoxines sur modèles cellulaires intestinal ou monocytaires a été évaluée. L’étude de cytotoxicité en mono-exposition a permis d’établir des valeurs de référence (CI50); d’autre part, un effet synergique des deux mycotoxines aux plus fortes concentrations testées a été révélé sur la lignée cellulaire intestinale (Caco-2). L’implication du mécanisme apoptotique après une exposition précoce (3 et 6 heures) à la ROQC et/ou au MPA a aussi été mise en évidence sur le modèle THP-1 ainsi qu’une absence d’implication de l’apoptose après 24 heures de mono ou de co-exposition sur modèle intestinal Caco-2. Enfin, dans la troisième partie, une étude de facteurs biotique ou abiotiques pouvant potentiellement moduler la production de ROQC et de MPA a été réalisée. L’évaluation du potentiel mycotoxigénique de 96 souches de P. roqueforti a mis en évidence que le milieu cheese-agar (mimant la composition physico-chimique du fromage) était beaucoup moins favorable que le milieu YES pour la production des deux mycotoxines et une grande variabilité dans la capacité de production des souches a été observée. L’étude de l’influence des facteurs abiotiques a montré qu’une température de 12°C ou une concentration en NaCl de 8% ou encore une atmosphère contenant 5% d’O2 diminuait significativement la production de ROQC et de MPA indépendamment de la croissance, alors que les autres facteurs (pH et présence de précurseurs de la ROQC) n’avaient pas d’effet dans les conditions testées. / P. roqueforti is a mould associated with silage and food contamination but also used as a ripening culture for the production of blue-veined cheese. Moreover, this species is also known for its potential to produce secondary metabolites. These metabolites can have a positive impact (e.g. aromas) or a negative impact (e.g. mycotoxins, including roquefortine C -ROQC- and mycophenolic acid -MPA-). In the framework of this work, we first studied the occurrence of MPA, ROQC and aflatoxin M1 (the only regulated mycotoxin in dairy products) within an 86 blue-veined cheese collection (representing 15 countries). Aflatoxin contents were always below the method detection limit. For MPA and ROQC, concentrations were highly variable and a co-occurrence of both mycotoxins was observed in 51% of the tested samples. Then, the toxicity of the 2 mycotoxins was established on various cell models (intestinal and monocytic). Mono-exposure studies allowed IC50 determination. Moreover, a synergistic effect was observed on Caco-2 cells when MPA and ROQC were used for co-exposure at the highest tested concentrations. While an apoptotic mechanism was observed at early THP-1 exposure stages (3 and 6h), no apoptose occurred for Caco-2 cells after 24 h of mono or co-exposure. Finally, a study of the biotic and abiotic factors potentially modulating P. roqueforti MPA and ROQC production was performed. Mycotoxigenic potential evaluation of 96 de P. roqueforti strains highlighted that a cheese-agar medium (mimicking cheese physico-chemical composition) was less favourable than the synthetic YES medium for mycotoxin production. A large variability in terms of mycotoxin production was also observed among the tested strains. Besides, while a significant effect of the temperature (12°C) and NaCl (8%) and O2 (5%) concentrations was observed on ROQC and MPA production, no significant effect of pH and ROQC amino-acid precursor addition could be detected on the production of both mycotoxins.

Natural products are secondary metabolites produced by plants and microorganisms that often possess medicinal properties and are implicated in organismal defense. Drawbacks to utilizing natural products in the pharmaceutical industry are difficulties with isolation from biological sources and low yields that can lack stereospecificity from synthetic sources. It is paramount to solve these issues and to develop novel natural products to combat the growing antimicrobial resistance crisis, which was responsible for ~5 million deaths in 2019 alone. One approach is utilizing enzymes to synthesize existing natural products to improve the yields and stereospecificity issue. This dissertation is focused on the biochemical characterization of three enzymes-ZvFMO, OxaD, and CreE-that are implicated in the detoxification of natural products used for organismal defense or participate in the biosynthesis of novel natural products. Each of these enzymes belong to the flavin-dependent monooxygenase (FMO) family, which catalyze the oxygenation of a substrate, generating an oxidized product. ZvFMO, from the insect food crop pest, Zonocerus variegatus, was determined to catalyze a highly uncoupled oxygenation reaction of the nitrogen or sulfur atom of various substrates. OxaD, from Penicillium oxalicum F30, catalyzes novel sequential oxidation reactions of the indole nitrogen of roquefortine C. CreE, from Streptomyces cremeus, also catalyzes sequential nitrogen oxidation reactions to convert L-aspartate to nitrosuccinate en route to biosynthesis of cremeomycin. For each enzyme, the steady-state kinetics have been determined using an oxygen consumption assay and the rapid-reaction kinetics were measured using anaerobic time-resolved spectroscopy. All three enzymes feature a fast flavin reduction step and a slow flavin dehydration step. The oxygenation chemistry of each enzyme was found to proceed through a highly reactive oxygenating species, the C4a-hydroperoxyflavin. Site-directed mutagenesis efforts led to the identification of key active site residues involved in flavin motion and substrate binding, revealing important information about the active site architecture for enzyme engineering applications and drug discovery efforts. / Doctor of Philosophy / Natural products are compounds that are produced by many plants, fungi, and bacteria that have potent medicinal properties and can be used to defend the organism against pests. Unfortunately, using these compounds widely in the pharmaceutical industry is difficult because it is hard to isolate the compound of interest from the organism that produces it and attempts to produce it chemically can result in low yields. Additionally, the overuse of the current natural products, which are most of the antibiotics on the market today, has led to an extreme increase in the resistance of bacteria, fungi, and parasites to the natural product-based drug. Therefore, it is essential that a method is developed to produce novel natural products at high yields to combat the antimicrobial resistance crisis. One method is by using enzymes to generate the natural products of interest. Enzymes are biological catalysts that speed up reactions by ensuring that less energy is required to transition from a reactant to a product and are highly efficient. This dissertation focuses on the characterization of three enzymes that could aid in our understanding of natural product chemistry. All three enzymes insert an oxygen atom on a nitrogen of their respective reactant. The first enzyme ZvFMO, is from an insect and its reactivity causes the insect to become resistant to the natural product-based plant defense mechanism, demonstrating that ZvFMO is a great candidate for inhibitor design. OxaD is the second enzyme and is involved in producing natural products that have antimicrobial and anticancer properties. The last enzyme, CreE, is involved in generating the natural product, cremeomycin, which possesses potent antimicrobial and anticancer properties as well. The reactions of OxaD and CreE positions these enzymes as candidates to produce novel natural products and other efforts to expand their reactivity. The rates of each reaction step have been determined in this work. Key amino acids that contribute to the reaction chemistry and the uptake of the reactant have been identified, laying a solid foundation for drug discovery efforts.