Discovery and Redesign of a Family VIII Carboxylesterase with High (S)-Selectivity toward Chiral sec-Alcohols

Highly enantioselective lipase has been widely utilized in the preparation of versatile enantiopure chiral sec -alcohols through kinetic or dynamic kinetic resolution. Lipase is intrinsically ( R )-selective, and it is difficult to obtain ( S )-selective lipase. Recent crystal structures of a family VIII carboxylesterase have revealed that the spatial array of its catalytic triad is the mirror image of that of lipase but with a catalytic triad that is distinct from lipase. We, therefore, hypothesized that the family VIII carboxylesterase may exhibit ( S )-enantioselectivity toward sec -alcohols similar to ( S )-selective serine protease, whose catalytic triad is also spatially arrayed as its mirror image. In this study, a homologous enzyme (carboxylesterase from Proteobacteria bacterium SG_bin9, PBE) of a known family VIII carboxylesterase (pdb code: 4IVK ) was prepared, which showed not only moderate ( S )-selectivity toward sec -alcohols such as 3-butyn-2-ol and 1-phenylethyl alcohol but also ( R )-selectivity toward particular sec -alcohols among the substrates explored. Furthermore, the ( S )-selectivity of PBE has been significantly improved by rational redesign based on molecular modeling. Molecular modeling identified a binding pocket composed of Ser381, Ala383, and Arg408 for the methyl substituent of ( R )-1-phenylethyl acetate and suggested that larger residues may increase the enantioselectivity by interfering with the binding of the slow-reacting enantiomer. As predicted, substituting Ser381with larger residues (Phe, Tyr, and Trp) significantly improved the ( S )-selectivity of PBE toward all sec -alcohols explored, even the substrates toward which the wild-type PBE exhibits ( R )-selectivity. For instance, the enantioselectivity toward 3-butyn-2-ol and 1-phenylethyl alcohol was improved from E = 5.5 and 36.1 to E = 2001 and 882, respectively, by single mutagenesis (S381F).