4.
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.
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