Engineering of carbonyl reductase for asymmetric reduction of difficult-to-reduce ketone tetrahydrofuran-3-one

Tetrahydrofuran-3-one (3TF) was regarded as a difficult-to-reduce ketone for carbonyl reductases due to its high stereosymmetry, result of which, the biosynthesis of chiral 3-hydroxytetrahydrofuran (3HTF), a key precursor of pharmaceuticals for treatment of HIV or diabetes, has been limited. Present study mined a robust carbonyl reductase CmCR from Candida metapsilosis with high activity towards 3TF using isopropanol as co-substrate (500 mM, 22 g/L/h space-time yield, 67% ee). Based on the orientation of substrate's etheryl oxygen in the substrate binding pocket, CmCR was rationally designed and two mutants MuR and MuS were screened out to completely reduce 3TF into (R)-HTF and (S)-HTF respectively with the highest 3TF loading and productivity (200 mM, 4.4 g/L/h space-time yield, 99% ee for MuR while 500 mM, 11 g/L/h space-time yield, 99% ee for MuS). The structural mechanism for the enhanced stereoselectivity was revealed that the mutagenesis changed the electrostatic potential surrounding the substrate entrance and promoted 3TF to approach, bind and form prereaction state with MuR or MuS in one certain direction, by which 3TF was converted into 3HTF with excellent optical purity. The success in this study provides a viable approach for rational design of carbonyl reductases with high enantioselectivity towards target substrates of high symmetry.