![]() In this study, for the first time, we provide the structural and biochemical insights for a fungi‐specific class III OYE homolog and dissect the oxidoreductase mechanism. Moreover, the loss of extended C‐terminus does not alter the monomeric nature of ArOYE6. Intriguingly, ArOYE6 structure contains a novel C‐terminus (369‐445 residues) made of three α‐helices, which stabilizes the FMN binding pocket as its mutation/truncation lead to complete loss of FMN binding. Site‐directed mutations in the conserved catalytic (His196, His199, Tyr201) and FMN‐binding (Lys249, Arg348) residues render the enzyme inactive. The large hydrophobic cavity situated above FMN, specifically accommodates 12‐oxo‐phytodienoic acid and N‐ethylmaleimide substrates as observed in ArOYE6‐FMN‐substrate ternary complex models. We demonstrate that ArOYE6 exists as a monomer in solution, forms (β/α)8 barrel structure harboring non‐covalently bound FMN at cofactor binding site, and utilizes reduced nicotinamide adenine dinucleotide phosphate as its preferred reductant. Here, we present the 1.6 Å X‐ray structure of a class III member, old yellow enzyme 6 from necrotrophic fungus Ascochyta rabiei (ArOYE6), in FMN‐bound form (PDB ID‐7FEV) and provide mechanistic insights into their catalytic capability. ![]() class III, whose biochemical and structural intricacies remain unexplored to date. The yeast‐ and bacteria‐like OYEs have been structurally characterized earlier, however, filamentous fungal pathogens possess a novel OYE class i.e. Old Yellow Enzymes (OYEs) play critical role in antioxidation, detoxification and ergot alkaloid biosynthesis processes in various organisms.
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