Structural basis of stereoselectivity in Candida rugosa lipase-catalyzed hydrolysis of secondary alcohols

Lipases are widely applied catalysts for highly enantioselective resolution of chiral secondary alcohols. While stereopreference is determined predominantly by the substrate structure, stereoselectivity (enantioselectivity and diastereoselectivity) depends on atomic details of interactions between substrate and lipase. Experimentally obtained stereoselectivity and activity in the hydrolysis of butanoic acid esters of two secondary alcohols with two neighbouring stereocenters by Candida rugosa lipase have been investigated by computer-aided molecular modeling of tetrahedral substrate intermediates in complex with the lipase. Breakdown of this intermediate is considered to be the rate-limiting step. Sterical interactions of stereo isomers with the side chain of catalytic histidine led to different orientations of the imidazole. The distance d(HNε-Oalc) between HNε of the imidazole side chain of catalytic histidine and the alcohol oxygen of the substrate was identified to correlate with the experimentally determined reactivity order of the four stereo isomers. Modelled distances d(HNε-Oalc) were short (≤ 1.8 Å) for RR stereo isomers, which were also experimentally found to be hydrolyzed most rapidly; distances d(HNε-Oalc) were about 2 Å for SS and SR stereo isomers, which were converted at similar rates but at lower rate than RR stereo isomers; finally, distances d(HNε-Oalc) for SR stereo isomers were greater than 4 Å, in accordance with very slow conversion of SR stereo isomers