Sequential Enzymatic Conversion of α‐Angelica Lactone to γ‐Valerolactone through Hydride‐Independent C=C Bond Isomerization

A case of hydride‐independent reaction catalyzed by flavin‐dependent ene‐reductases from the Old Yellow Enzyme (OYE) family was identified. α‐Angelica lactone was isomerized to the conjugated β‐isomer in a nicotinamide‐free and hydride‐independent process. The catalytic cycle of C=C bond isomerization appears to be flavin‐independent and to rely solely on a deprotonation–reprotonation sequence through acid–base catalysis. Key residues in the enzyme active site were mutated and provided insight on important mechanistic features. The isomerization of α‐angelica lactone by OYE2 in aqueous buffer furnished 6.3 mm β‐isomer in 15 min at 30 °C. In presence of nicotinamide adenine dinucleotide (NADH), the latter could be further reduced to γ‐valerolactone. This enzymatic tool was successfully applied on semi‐preparative scale and constitutes a sustainable process for the valorization of platform chemicals from renewable resources.