ACTION OF PEPSIN ON CATIONIC SYNTHETIC SUBSTRATES

The kinetic parameters for the peptic hydrolysis of a series of cationic peptide derivatives have been determined at several pH values. The substrates used were of the type AX-YB, where A = Z-His, Z-Gly- His, Z-His-Gly, Bz-Lys, Gly-Gly, or Gly-Gly-Gly; X = Phe, Gly, Tyr, or Trp; Y = Phe, Gly, Tyr, or Trp; and B = OMe, OEt, or NH2. Significant differences were found in the values of Km (assumed to approximate the dissociation constant of the productive enzyme-substrate complex) for comparable substrates in which A = Gly-Gly of Z-His. Other structural modifications caused relatively small changes in Km; in numerous cases, significant changes in kcat were found. Of special interest was the large difference in the kcat values for the hydrolysis of Z-Gly-His-Phe-Phe-OEt and of Z-His-Gly-Phe-Phe, Gly, Tyr, or Rrp; Y = Phe, Gly, Tyr, or Rrp; and of the first substrate, but not of the second one, resembled that for Z-His-Phe-Phe-OEt. For a series of comparable substrates AX-YB, when Y = Phe, the presence in the X position of Phe (or p-nitro-L-phenylalanyl) led to much higher values of kcat than with Tyr or Trp; replacement of these residues by Gly caused significantly slower hydrolysis. When X = Phe, the presence of Phe, Tyr, or Trp in the Y position gave substrates of approximately equal susceptibility; replacement of these residues by Gly or Leu caused equally slow cleavage. The data in this communication are discussed in relation to the hypothesis that both the substrate and the catalytic region of the enzyme undergo conformational changes as a consequence of multiple cooperative interactions. © 1969, American Chemical Society. All rights reserved.