Origin of the Enantioselectivity of Lipases Explained by a Stereo-Sensing Mechanism Operative at the Transition State

The stereoelectronic considerations and the molecular modeling using an X-ray structure of Rhizomucor miehei lipase suggested that the lipase-catalyzed reactions proceed under the stereoelectronic control. This suggestion was supported by the semiempirical MO (MNDO-PM3) calculations carried out on the imidazole-catalyzed transesterification as a model reaction. The stereoelectronic effect operates at the transition state (TS) more effectively than at the tetrahedral intermediate (THI). The stabilization energy due to the stereoelectronic effect operating at the TS was estimated to be ca. 5 kcal mol^-1. The enantioselectivities for 1-phenylethanol, l-phenyl-2-propanol, and 1-cyclohexylethanol were estimated in terms of the lipase-induced strain caused at the TS. A TS model generally applicable to chiral secondary alcohols is proposed. The kinetic study supported the TS model. The result that the enantioselectivity in the lipase-catalyzed transesterifications arises from the difference in V_max between the two enantiomers rather than from the difference in K_m indicates that the ability of Upases to discriminate between the enantiomers at the TS is high, while the ability to recognize the chirality in the binding step is poor. Furthermore, the difference in V_max between the enantiomers was found to result not from the enhanced reactivity of the (R)-enantiomers but from the reduced reactivity of the (S)-enantiomers.