pH-dependence and structure–activity relationships in the papain-catalysed hydrolysis of anilides

The pH-dependence of the Michaelis–Menten parameters for the papain-catalysed hydrolysis of N-acetyl-l-phenylalanylglycine p-nitroanilide was determined. The equilibrium binding constant, Ks, is independent of pH between 3.7 and 9.3, whereas the acylation constant, k+2, shows bell-shaped pH-dependence with apparent pKa values of 4.2 and 8.2. The effect of substituents in the leaving group on the acylation constant of the papain-catalysed hydrolysis of hippuryl anilides and N-acetyl-l-phenylalanylglycine anilides gives rise in both series to a Hammett ρ value of -1.04. This indicates that the enzyme provides electrophilic, probably general-acid, catalysis, as well as the nucleophilic or general-base catalysis previously found. A mechanism involving a tetrahedral intermediate whose formation is general-base-catalysed and whose breakdown is general-acid-catalysed seems most likely. The similarity of the Hammett ρ values appears to exclude facilitated proton transfer as a means through which the specificity of papain is expressed.