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On the Specificity of Streptococcal Proteinase

From the ¹ From The Rockefeller University, New York, New York 10021

In order to investigate the specificity of streptococcal proteinase, the peptides formed during proteolysis of the reduced, carboxymethylated phenylalanyl chain of insulin have been isolated and identified. One of the bonds most rapidly hydrolyzed was the Phe-Tyr linkage in the -Phe-Phe-Tyr-sequence. With this result as a lead, a series of synthetic substrates was tested. The results of experiments with acyl dipeptides and with the insulin chain showed that the chief requirement for hydrolysis is the presence of a bulky side chain on the ammo acid adjacent, on the amino-terminal side, to the residue contributing the carbonyl to the susceptible peptide bond. This side chain appears to be important for the binding of substrate to the active center of the enzyme. The residues actually linked by the bond are not so critical, although the results with the acyl dipeptides indicate that there may be a secondary hydrophobic binding of the side chain of the amino acid donating the nitrogen to the susceptible peptide bond. Studies of V_max and K_m for the hydrolysis of several benzyloxycarbonyl-L-phenylalanyl peptides showed that, in these cases, deacylation of an acyl enzyme is not rate-limiting. The variations of V_max and K_m with pH were studied. A graph of log V_max with respect to pH showed one inflection at pH 6.4 whereas a graph of pK_m with respect to pH had two inflections, one at pH 6.4 and the other at pH 8.4. These results are compatible with the conclusion that an unprotonated imidazole ring and the protonated form of the single sulfhydryl group are essential for activity.

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