A series of peptides with a positively charged cobalt(III)-complex group attached to the carboxylate terminal was synthesized. The behavior of these metallopeptides as acceptor nucleophiles in acyl transfer reactions catalyzed by the three serine proteases bovine pancreatic α-chymotrypsin, porcine pancreatic trypsin, and proteinase K from Tritirachium album was examined. The efficiency of the substrates was assessed by kinetic measurement of the partition between aminolysis and hydrolysis. The results are discussed with special reference to coulombic interactions between the metal-ligated substrates and charged residues on the enzyme surfaces. The idea of using the metallopeptides in practical enzymatic peptide synthesis is put forward.