Enantioselective Regulation of Short‐Chain Dehydrogenases via Key Sites in its Loop and Adjacent Regions for the Enantiodivergent Reduction of Difficult‐To‐Reduce Ketones

Short-chain dehydrogenases (SDRs) are powerful catalysts for the asymmetric reduction of prochiral ketones in pharmaceutical products. Herein, through gene mining and evolutionary analysis, we obtained two major types of SDRs (SDR-1 and SDR-2) for the enantioselective complementary reduction of N-Boc-piperidone, which gives the corresponding product (R)- or (S)-N-Boc-piperidol, an intermediate of the interleukin inhibitor and lymphoma treatment drug (imbruvica), respectively. By integrating multiple sequence alignment, site-directed mutagenesis and computational modeling, we proposed a "loop regulation" mechanism for the enantioselective control of SDRs, through which residues in the loop region could potentially fine-tune their enantioselectivity. Further, site-directed mutagenesis assays showed that two key residues (L201 and F205 for SDR-1, F92 and H93 for SDR-2) in the loop and its adjacent region played critical roles in fine-tuning the enantioselectivity of SDRs. Understanding this mechanism of SDR stereo preference in catalyzing asymmetric reduction, we further switched the enantioselectivity of the homologous enzymes. The obtained enzymes catalyzed the enantiodivergent synthesis of chiral heterocyclic alcohols with different ring sizes and substituents (25-99% conversion and 25-99% ee (R/S)), including piperidols, 4-hydroxy azepanes, 3-hydroxy azepanes and pyrrolidinols. These findings could potentially guide future attempts at protein engineering of stereoselective SDRs. image