Activity and enantioselectivity of serine proteases in transesterification reactions in organic media

The activity and enantioselectivity of -chymotrypsin and subtilisin Carlsberg in the transesterification of a series of N-acetyl-alanine and -phenylalanine esters in cyclohexane were investigated at constant water activity, both in the absence and presence of 18-crown-6. Isosteric variation of the leaving ability of the alcoholate group in the substrates by fluoro substitution provided information about the acylation step of the reaction. For less reactive esters, the rate of acylation is determined by expulsion of the leaving group, whereas for activated esters the rate is dictated by a physical step, most likely a relatively slow conformational change of the enzyme. This makes the enantioselectivity of the enzymes strongly dependent of the substrate activity and the composition of the reaction medium. The catalytic effect of 18-crown-6, observed for all reactions. can be attributed to an enhanced enzyme-substrate binding and an increase of the rate of acylation.