Native and promiscuous catalytic activities of flavindependent Old Yellow Enzymes (OYEs) reported to date are initiated by the reduced flavin upon electron transfer. As a rare exception, the isomerization of a nonactivated CC bond was shown to be hydride-independent with two nonstereoselective yeast OYEs. Here, we report the asymmetric isomerization of a prochiral model substrate, γ-methyl β,γ-butenolide, to the corresponding (R)-and (S)-enantiomers of the γ-methyl α,βbutenolide in up to >99% ee by two stereocomplementary OYEs of algal and fungal origin, respectively, which operate by asymmetric proton transfer. Mechanistic studies based on two newly solved crystal structures, along with soaking experiments and site-directed mutagenesis, support the crucial role of partially nonconserved tyrosine residues for the activity and stereoselectivity of both (R)and (S)-isomerases. This study offers a unique view on the potential of flavoproteins in nonredox catalysis and provides hints for scouting olefin isomerases in likely stereodivergent classes of OYEs.

Alcohol dehydrogenases catalyze the regioselective lactonization of dialdehydes via a bio-Tishchenko-like reaction. The nicotinamide-dependent self-sufficient reduction–oxidation sequence proceeds through a formal intramolecular hydride shift.