Old Yellow Enzymes as Oxime Reductases: New Variants by Substrate‐based Enzyme Engineering

The reduction of oximes was recently identified as a promiscuous activity of Old Yellow Enzymes (OYEs). This reaction involves a two‐step reduction of α‐oxime‐ß‐ketoesters to the corresponding amines, which spontaneously dimerise to yield pyrazine derivatives. This biotransformation is currently limited to substrates with small substituents like methyl/ethyl on the keto moiety. We used a structure‐based approach to engineer 12‐oxophytodienoate reductase 3 (OPR3) from Solanum lycopersicum as a prototypical OYE to accept oximes with bulkier substituents. To this end, three single and two double variants were prepared and tested on six oxime substrates. The engineered variants indeed showed activity on some of the bulkier substrates, which had not been converted at all by the wild‐type enzyme, including the diester compound diethyl‐2‐(hydroximino)malonate. While we were unable to identify variants capable of converting substrates with branched and aromatic substituents, the results demonstrate the validity of our engineering approach suggesting potential pathways for expanding the substrate scope of OYEs.