Crossing the Border: From Keto‐ to Imine Reduction in Short‐Chain Dehydrogenases/Reductases

The family of NAD(P)H‐dependent short‐chain dehydrogenases/reductases (SDRs) comprises numerous biocatalysts capable of C=O or C=C reduction. The highly homologous noroxomaritidine reductase (NR) from Narcissus sp. aff. pseudonarcissus and Zt_SDR from Zephyranthes treatiae, however, are SDRs with an extended imine substrate scope. Comparison with a similar SDR from Asparagus officinalis (Ao_SDR) exhibiting keto‐reducing activity, yet negligible imine‐reducing capability, and mining the Short‐Chain Dehydrogenase/Reductase Engineering Database indicated that NR and Zt_SDR possess a unique active‐site composition among SDRs. Adapting the active site of Ao_SDR accordingly improved its imine‐reducing capability. By applying the same strategy, an unrelated SDR from Methylobacterium sp. 77 (M77_SDR) with distinct keto‐reducing activity was engineered into a promiscuous enzyme with imine‐reducing activity, thereby confirming that the ability to reduce imines can be rationally introduced into members of the “classical” SDR enzyme family. Thus, members of the SDR family could be a promising starting point for protein approaches to generate new imine‐reducing enzymes.