The kinetics of transesterification by a-chymotrypsin of a racemic mixture of phenylalanine propyl ester with 1,4-butanediol

The kinetics of an enzymic transesterification reaction were studied by considering a model reaction: the transesterification of a racemic mixt. of phenylalanine Pr ester with 1,4-butanediol by chymotrypsin (EC 3.4.21.1). The model developed, describing the evolution of the yields of both the hydrolysis and the transesterification product with time, is based on a 2-step mechanism which involves the formation of an acyl-enzyme intermediate with release of the alc., followed by the decompn. of the intermediate by a nucleophile. The substrate, L-phenylalanine Pr ester, forms an acyl-enzyme intermediate which decomps. into 3 products, L-phenylalanine as a result of the attack by water, L-phenylalanine Pr ester as a result of the attack by propanol and L-phenylalanine 4-hydroxybutyl ester, the desired transfer product, as a result of the attack by 1,4-butanediol. The formation of an acyl-enzyme intermediate from the L-phenylalanine 4-hydroxybutyl ester explains the presence of a max. in the yield of this product. The D-phenylalanine Pr ester acts as a competitive inhibitor. The affinity consts. of L-phenylalanine 4-hydroxybutyl ester and L-phenylalanine Pr ester are found to be nearly identical, 8.6 and 9.311.(g.min)-1 resp. The rate consts. of product formation are found to be 0.00126, 0.247, and 0.29911.(g.min)-1 for L-phenylalanine, L-phenylalanine Pr ester, and L-phenylalanine 4-hydroxybutyl ester, resp. [on SciFinder (R)]