Significant improvement of the enantioselectivity of a halohydrin dehalogenase for asymmetric epoxide ring opening reactions by protein engineering

Halohydrin dehalogenases (HHDHs) have attracted much attention due to their ability to synthesize enantiomerically enriched epoxides and β-haloalcohols. However, most of the HHDHs exhibit low enantioselectivity. Here, a HHDH from the alphaproteobacteria isolate 46_93_T64 (AbHHDH), which shows only poor enantioselectivity in the catalytic resolution of rac-PGE (E = 9.9), has been subjected to protein engineering to enhance its enantioselectivity. Eight mutants (R89K, R89Y, V137I, P178A, N179Q, N179L, F187L, F187A) showed better enantioselectivity than the wild type. The best single mutant N179L (E = 93.0) showed a remarkable 9.4-fold increase in the enantioselectivity. Then, the single mutations were combined to produce the double, triple, quadruple, and quintuple mutants. Among the combinational mutants, the best variant (R89Y/N179L) showed an increased E value of up to 48. The E values of the variants N179L and R89Y/N179L for other epoxides 2–7 were 12.2 to > 200, which showed great improvement compared to 1.2 to 10.5 for the wild type. Using the variant N179L, enantiopure (R)-PGE with > 99% ee could be readily prepared, affording a high yield and a high concentration.