The Kinetics of Transesterification by α-Chymotrypsin of a Racemic Mixture of Phenylalanine Propyl Ester with 1,4-Butanediol

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Abstract
The kinetics of an enzymatic transesterification reaction have been studied by considering a model reaction: the transesterification of a racemic mixture of phenylalanine propyl ester with 1,4-butanediol by α-chymotrypsin (E.C. 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 two step mechanism which involves the formation of an acyl-enzyme intermediate with release of the alcohol, followed by the decomposition of the intermediate by a nucleophile. The substrate L-phenylalanine propyl ester forms an acyl-enzyme intermediate which decomposes into three products, L-phenylalanine as a result of the attack by water, L-phenylalanine propyl 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-hyroxybutyl ester explains the presence of a maximum in the yield of this product. The D-phenylalanine propyl ester acts as a competitive inhibitor. The affinity constants of L-phenylalanine 4-hydroxybutyl ester and L-phenylalanine propyl ester are found to be nearly identical, 8.6 and 9.311 · (g · min)-1 respectively. The rate constants of product formation are found to be 0.00126, 0.247 and 0.29911 · (g · min) -1 for L-phenylalanine, L-phenylalanine propyl ester and L-phenylalanine 4-hydroxybutyl ester, respectively.