The Comparative Specificity of Acid Proteinases

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Abstract
Examination of the kinetic parameters for the hydrolysis, by acid proteinases, of a single peptide bond (between p-nitro-L-phenylalanyl and L-phenylalanyl) in a series of oligopeptides has shown that secondary interactions are important factors in determining the catalytic efficiency. Comparison of the action of highly purified pepsinlike enzymes (Rhizopus proteinase, Mucor proteinase, rennin) with that of swine pepsin A indicates significant differences among them, either in the binding of the substrate (as estimated by K(m)), or in the catalytic efficiency (as measured by k(cat)), or both. It may be concluded from these data that, in their action on oligopeptide substrates, the specificity of proteinases operating by a similar catalytic mechanism cannot be explained solely in terms of the amino acid residues flanking the sensitive peptide bond; in addition, the specificity includes significant contributions from secondary interactions arising from complementary relations between parts of the substrate and of the enzyme at a distance from the catalytic site. Data are also presented for the effect of urea (about 1 M) on the kinetic parameters of several acid proteinases; under the conditions of these studies, the binding of the substrate is affected to a much lesser degree than is the catalytic efficiency.