Probing the Cu2+ and Zn2+ binding affinity of histidine-rich glycoprotein

The zinc(II) and copper(II) binding ability of two oligopeptide fragments, Ac-HHPHG-NH2 and Ac-HHPHGHHPHG-NH2, derived from the repeat-region of the His–Pro-rich domain of histidine-rich glycoprotein (HRG) and the structure of the formed complexes have been investigated by potentiometry, NMR-, UV–visible-, CD-, SRCD- and EPR spectroscopy. Exclusive coordination of the side-chain imidazoles of the peptides has been observed with both metal ions in the acidic and neutral pH range. While the three His units of the pentapeptide resulted in a modest stability of the ML complexes, the decapeptide with its increased number of His residues offered a high-affinity metal binding site for both metal ions with the participation of at least four nitrogen donors. Due to the high number of potential donor groups, the formation of binding isomers of the protonated and parent complexes is very likely. Both peptides show a synchrotron radiation (SR) CD-pattern resembling to that of the polyproline II structure, similarly to that of the His–Pro-rich domain of the HRG protein. The longer sequence was shown to bind a second metal ion in the slightly acidic pH-range. The determined stability data suggest a remarkable extra stabilization emerging in the decapeptide for the coordination of the second metal ions, as compared to the ML complexes of the pentapeptide. Whether the observed cooperativity has similarities to the cooperative metal binding feature of HRG or the two phenomena have different sources is a question yet to be clarified.