Rapid asymmetric reduction of ethyl 4-chloro-3-oxobutanoate using a thermostabilized mutant of ketoreductase ChKRED20.

Ethyl (S)-4-chloro-3-hydroxybutanoate ((S)-CHBE) is an important chiral intermediate for the synthesis of "blockbuster" drug statins. The carbonyl reductase ChKRED20 from Chryseobacterium sp. CA49 was found to catalyze the bio-reductive production of (S)-CHBE with excellent stereoselectivity (>99.5 % ee). Perceiving a capacity for improvement, we sought to increase the thermostability of ChKRED20 to allow a higher reaction temperature. After one round of error-prone PCR (epPCR) library screening followed by the combination of beneficial mutations, a triple-mutant MC135 was successfully achieved with substantially enhanced thermostablity. The activity of MC135 at 50 °C was similar to the wild type. However, at its temperature optima of 65 °C, the mutant displayed 63 % increase of activity compared to the wild type and remained >95 % activity after being incubated for 15 days, while the wild type had a half-life of 11.9 min at 65 °C. At a substrate/catalyst ratio of 100 (w/w), the mutant catalyzed the complete conversion of 300 g/l substrate within 1 h to yield enantiopure (S)-CHBE with an isolated yield of 95 %, corresponding to a space-time yield of 1824 mM/h.