Cleavage of β-lactone ring by serine protease. Mechanistic implications

Both enantiomers of 3-benzyl-2-oxetanone (1) were found to be slowly hydrolyzed substrates of α-chymotrypsin having k_cat values of 0.134±0.008 and 0.105±0.004 min^-1 for (R)-1 and (S)-1, respectively, revealing that α-CT is virtually unable to differentiate the enantiomers in the hydrolysis of 1. The initial step to form the acyl-enzyme intermediate by the attack of Ser-195 hydroxyl on the β-lactone ring at the 2-position in the hydrolysis reaction may not be enzymatically driven, but the relief of high ring strain energy of β-lactone may constitute a major driving force. The deacylation step is also attenuated, which is possibly due to the hydrogen bond that would be formed between the imidazole nitrogen of His-57 and the hydroxyl group generated during the acylation in the case of (R)-1, but in the α-CT catalyzed hydrolysis of (S)-1 the imidazole nitrogen may form a hydrogen bond with the ester carbonyl oxygen.