Key Residues for Controlling Enantioselectivity of Halohydrin Dehalogenase from Arthrobacter sp. Strain AD2, Revealed by Structure-Guided Directed Evolution

Halohydrin dehalogenase fromAgrobacterium radiobacter AD1 (HheC) is a valuable tool in the preparation ofR enantiomers of epoxides and β-substituted alcohols. In contrast, the halohydrin dehalogenase fromArthrobacter sp. AD2 (HheA) shows a lowS enantioselectivity toward most aromatic substrates. Here, three amino acids (V136, L141, and N178) located in the two neighboring active-site loops of HheA were proposed to be the key residues for controlling enantioselectivity. They were subjected to saturation mutagenesis aimed at evolving anS -selective enzyme. This led to the selection of two outstanding mutants (the V136Y/L141G and N178A mutants). The double mutant displayed an inverted enantioselectivity (fromS enantioselectivity [ES ] = 1.7 toR enantioselectivity [ER ] = 13) toward 2-chloro-1-phenylethanol without compromising enzyme activity. Strikingly, the N178A mutant showed a large enantioselectivity improvement (ES > 200) and a 5- to 6-fold-enhanced specific activity toward (S )-2-chloro-1-phenylethanol. Further analysis revealed that those mutations produced some interference for the binding of nonfavored enantiomers which could account for the observed enantioselectivities. Our work demonstrated that those three active-site residues are indeed crucial in modulating the enantioselectivity of HheA and that a semirational design strategy has great potential for rapid creation of novel industrial biocatalysts.