Asymmetric Proton Transfer Catalysis by Stereocomplementary Old Yellow Enzymes for C═C Bond Isomerization Reaction

Native and promiscuous catalytic activities of flavin-dependent 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,β-butenolide 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.