Peptide Bond Synthesis Catalyzed by Subtilisin, Papain, and Pepsin

Abstract

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→Cbz-(AA)_n Leu-X, in which AA=various amino acid residues, n=1-3 and X=-NH₂, -OEt, -OBu^t, -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=-OBu^t 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_??_>-OBu^t>-NH₂, -OEt. Leu-NH₂, however, was quite efficient when its molar concentration was raised to twenty times that of the carboxyl component.