ABSTRACT
The uncertainties in the outcome of protease-catalyzed reactions presently limit the general applicability of the enzymatic approach to peptide synthetic chemistry. In most instances the reaction pathways which prove to be dead ends can be bypassed by alternative strategies. Under these circumstances, one should adopt a versatile, synthetic design which then allows for subsequent improvisations. The fragments thus obtained are then assembled in the presence of another protease that does not endanger the preformed peptide bonds to yield the final completed peptide. A major limitation of the method is determined by the lack of a complete set of proteases whose stringent specificity would enable the synthetic chemist to prepare any conceivable peptide linkage without interfering with pre-existing bonds. Boc-Tyr-Gly-N2H2Ph, a precursor of the abovementioned dipeptide, could be synthesized via chymotrypsin-catalysis, while an alternative approach to the synthesis of this Boc-dipeptidyl-phenylhydrazide via papain-controlled coupling of Boc-Tyr-OH and H-Gly-N2H2Ph failed because Boc-Tyr-N2H2Ph was formed instead.
