The specificity of actinidin and its relationship to the structure of the enzyme

The kinetic parameters kcat, Km and kcat/Km, have been determined for the actinidin-catalysed hydrolyses of N-substituted amino acid esters and amides and compared to the corresponding values for papain (EC 3.4.22.2). Substrates with aromatic N-substituents have lower kcat/Km values for actinidin (EC 3.4.22.14); the difference is much smaller for substrates with aliphatic substituents. The lower kcat/Km values for actinidin generally correspond to higher Km values suggesting that the strength of substrate binding differs between the two enzymes. This difference is explained in terms of the differences in the substrate binding sites found in X-ray crystallographic studies.