Transition-state models are useful for versatile biocatalysts: Kinetics and thermodynamics of enantioselective acylations of secondary alcohols catalyzed by lipase and subtilisin

Lipases and subtilisins are versatile enzymes capable of showing high enantioselectivity and broad substrate specificity simultaneously. The transition-state models previously proposed to rationalize this important feature were intensively examined from kinetic and thermodynamic viewpoints. Kinetic measurements reaffirmed that chiral discrimination originates from the transition state and that the enantioselectivity results from the reduced activity of the enzymes for the slower-reacting enantiomer, but not from the enhanced activity for the faster-reacting enantiomer relative to a reference alcohol, cyclopentanol. The larger substituent of the slower-reacting enantiomers interacts repulsively with the protein in the transition state, and even the larger substituent of the faster-reacting enantiomers interacts unfavorably to some degree with the protein. A number of thermodynamic parameters, ΔΔH^‡ and ΔΔS^‡, for the subtilisin-catalyzed acylations of secondary alcohols were determined. A linear compensation effect was found between the ΔΔH^‡ and ΔΔS^‡ values. As the ΔΔH^‡ value becomes negatively large, the ΔΔS^‡ value also becomes negatively large. This observation is explained in terms of the transition-state model. Because the widely accepted concepts such as the lock-and-key mechanism and the induced-fit mechanism cannot account for the peculiar behavior of these enzymes toward unnatural substrates, a new category, the non-lock-and-key mechanism, has been proposed.