A comparative study of Zn(II) and Co(II) binding to glycyl-L-tyrosine, a pseudosubstrate for carboxypeptidase A

A comprehensive investigation of the interaction of Zn(II) and Co(II) with the dipeptide glycyl-L-tyrosine has been carried out. The carboxyl, amino, and tyrosyl pKa values, as well as the distribution of solution complexes, have been determined by analytical potentiometry. The amide pKa value was determined by relating the proton magnetic resonance (PMR) titration behavior of the tyrosyl alpha-hydrogen resonance to an H2-acidity function for concentrated solutions of aqueous base. Both metals behave in a qualitatively similar manner, yielding equivalent species as a function of pH. Both metals formed bis-peptide complexes, involving amino and peptide carbonyl coordination near pH = 8, with Zn(II) demonstrating a substantially higher affinity for the ligand. No evidence could be found for direct, metal-promoted phenolic dissociation, although the tyrosyl pKa value was sensitive to metal binding at other loci on the dipeptide molecule. At high pH, both systems ionized two additional protons. In the Co(II) system, these correspond to amide protons. However, it is not entirely clear whether the protons in the Zn(II) system originate from the peptide linkage or metal-bound water molecules.