Peptide bond synthesis catalyzed by subtilisin, papain, and pepsin

The peptide bond syntheses catalyzed by subtilisin BPN', papain [EC 3.4.22.2], and pepsin [EC 3.4.23.1] were studied comparatively at the optimum pH of the enzymes with the coupling system Cbz-(AA)n-OH + Leu-X leads to Cbz-(AA)n-Leu-X, in which AA = various amino acid residues, n = 1-3 and X = -NH2, -OEt, -OBut, -ODPM (diphenyl methyl ester) or -NH. The coupling with these enzymes differed depending upon the nature of (AA)n and X. For subtilisin-catalyzed coupling, the molecular size of the carboxyl component was most important. Thus, Cbz-Gly-Pro-Leu-OH was useful for synthesis in the presence of an equimolar concentration of Leu-NH. Either Ala-NH2 or Leu-NH2 was also useful as an amine component when present at a concentration several times that of the carboxyl component. Papain catalyzed the coupling between Cbz-AA-OH (AA = glycine, L-alanine, L-valine, L-glutamic acid, or L-phenylalanine) and Leu-X (X = -OBut and -ODPM). The coupling of Cbz-Gly-Phe-OH and Leu-X catalyzed by pepsin was markedly affected, depending upon the nature of X in the following order: -NH greater than -OBut greater than -NH2, -OEt. Leu-NH2, however, was quite efficient when its molar concentration was raised to twenty times that of the carboxyl component.