Breddam_1987_Carlsberg.Res.Commun_52_65

The unblocked cysteinyl residue (Cys341), located at the binding site of carboxypeptidase Y, does not react with the bulky cyanylating reagent 2-nitro-5-thiocyanatobenzoic acid unless the enzyme is denatured. However, it reacts specifically with small concentrations of14CNBr forming the corresponding alkyl thiocyanate. This results in reductions in the kcat/Km values for the hydrolysis of ester and peptide substrates to 5-25% of the values obtained with unmodified enzyme, due both to decreases in the kcat values and increases in the Km values. The rate of deacylation of the indoleacryloyl acyl enzyme intermediate, formed by reaction of the cyanylated enzyme with indoleacryloyl imidazole, is reduced to 20% of the value obtained with the unmodified enzyme. The results suggest that Cys341 participates in catalysis although it is not essential.