Computer-aided molecular modeling of the enantioselectivity of Pseudomonas cepacia lipase toward γ- and δ-lactones

Computer-aided molecular modeling was performed to investigate the experimentally determined enantioselectivities of Pseudomonas cepacia lipase (PCL) toward various saturated γ- and δ-lactones. Experimental data indicated that PCL preferentially hydrolyzes the (R)-enantiomers of both types of substrates. Interactions between the non-polar aliphatic alkyl chain of the (S)-enantiomers and the polar side chain of residue Y29 were identified to mediate enantioselectivity. Upon binding, the tyrosine was displaced, thus initiating a cascade of local geometry changes which led to the breakdown of the essential H-bond network at the active site H286. The lactone ring of the (S)-δ-enantiomers further added to this process, since it was forced into an unfavorable position by repulsion from Y29, directly affecting the position of H286. In contrast, the respective (R)-enantiomers fit without distorting side chains essential for catalysis in the binding pocket of PCL. In δ-lactones, the stereocenter was located close to the imidazole ring of H286, suggesting a more intense interaction with H286 as compared to γ-lactones. The length of the aliphatic chain adjacent to the stereocenter also affected the enantiopreference toward hydrolysis of δ-lactones, while for γ-lactones, the enantioselectivity did not significantly change with increasing alkyl chain length. In the cases of (S)-δ-octa- and (S)-δ-nonalactone, two alternative possible binding modes were examined, indicating that the respective substrate resolutions led to poor enantioselectivity as compared to the longer-chain δ-lactone substrates.