Semirational engineering of an aldo-keto reductase KmAKR for overcoming trade-offs between catalytic activity and thermostability.

Enzyme engineering usually generates trade-offs between activity, stability, and selectivity. Herein, we report semirational engineering of an aldo-keto reductase (AKR) KmAKR for simultaneously enhancing its thermostability and catalytic activity. Previously, we constructed KmAKRM9 (W297H/Y296W/K29H/Y28A/T63M/A30P/T302S/N109K/S196C), which showed outstanding activity towards t-butyl 6-chloro-(3R,5S)-dihydroxyhexanoate ((3R,5S)-CDHH), and t-butyl 6-cyano-(3R,5R)-dihydroxyhexanoate, the key chiral building blocks of rosuvastatin and atorvastatin. Under the guidance of computer-aided design including consensus residues analysis and molecular dynamics (MD) simulations, K164, S182, S232, and Q266 were dug out for their thermostability conferring roles, generating the "best" mutant KmAKRM13 (W297H/Y296W/K29H/Y28A/T63M/A30P/T302S/N109K/S196C/K164E/S232A/S182H/Q266D). The Tm and T5015 values of KmAKRM13 were 10.4 and 6.1°C higher than that of KmAKRM9 , respectively. Moreover, it displayed a significantly elevated organic solvent tolerance over KmAKRM9 . Structural analysis indicated that stabilization of the α-helixes mainly contributed to thermostability enhancement. Under the optimized conditions, KmAKRM13 completely asymmetrically reduced 400 g/l t-butyl 6-chloro-(5S)-hydroxy-3-oxohexanoate ((5S)-CHOH) in 8.0 h at a high substrate to catalyst ratio (S/C) of 106.7 g/g, giving diastereomerically pure (3R,5S)-CDHH (>99.5% d.e.P ) with a space-time yield (STY) of 449.2 g/l·d.

Tailoring an aldo-keto reductase KmAKR for robust thermostability and catalytic efficiency by stepwise evolution and structure-guided consensus engineering.

t-Butyl 6-cyano-(3R,5R)-dihydroxyhexanoate ((3R,5R)-2) is an advanced chiral diol intermediate of the cholesterol-lowering drug atorvastatin. KmAKRM5 (W297H/Y296W/K29H/Y28A/T63M) constructed in our previous work, displayed good biocatalytic performance on (3R,5R)-2. In the present work, stepwise evolution was applied to further enhance the thermostability and activity of KmAKRM5. For thermostability enhancement, N109 and S196 located far from the active site were picked out by structure-guided consensus engineering, and mutated by site-directed mutagenesis (SDM). For catalytic efficiency improvement, the residues A30 and T302 adjacent to the substrate-binding pocket were subjected to site-saturation mutagenesis (SSM). As a result, the "best" mutant KmAKRM9 (W297H/Y296W/K29H/Y28A/T63M/A30P/T302S/N109K/S196C) was developed, of which T5015 and Tm were 5.0 °C and 8.2 °C higher than those of KmAKRM5. Moreover, compared to KmAKRM5, KmAKRM9 displayed a 1.9-fold (846 vs 2436 min) and 6.7-fold (126 vs 972 min) longer half-lives at 40 and 50 °C, respectively. Structural analysis suggested that beneficial mutations introduced additional hydrophobic interactions and hydrogen bonds, contributing rigidification of the flexible loops and the increase of internal forces, hence increasing the thermostability and activity. 5 g DCW (dry cell weight) L-1KmAKRM9 completely reduced 350 g L-1t-butyl 6-cyano-(5R)-hydroxy-3-oxo-hexanoate ((5R)-1), within 3.7 h at 40 °C, yielding optically pure (3R,5R)-2 (d.e.p > 99.5%) with a space-time yield (STY) of 1.82 kg L-1 d-1. Hence, KmAKRM9 is a robust biocatalyst for the synthesis of (3R,5R)-2.