ACTIVITY AND ENANTIOSELECTIVITY OF SERINE PROTEASES IN TRANSESTERIFICATION REACTIONS IN ORGANIC MEDIA

The activity and enantioselectivity of alpha-chymotrypsin and subtilis in Carlsberg in the transesterification of a series of N-acetyl-alanin e and -phenylalanine esters in cyclohexane were investigated at consta nt water activity, both in the absence and presence of 18-crown-6, Iso steric variation of the leaving ability of the alcoholate group in the substrates by fluoro substitution provided information about the acyl ation step of the reaction, For less reactive esters, the rate of acyl ation is determined by expulsion of the leaving group, whereas for act ivated esters the rate is dictated by a physical step, most likely a r elatively slow conformational change of the enzyme. This makes the ena ntioselectivity of the enzymes strongly dependent of the substrate act ivity and the composition of the reaction medium, The catalytic effect of 18-crown-6, observed for all reactions can be attributed to an enh anced enzyme-substrate binding and an increase of the rate of acylatio n.