The ability of the NiSOD binding loop to chelate zinc(ii): the role of the terminal amino group in the enzymatic functions†
Equilibrium and detailed spectroscopic characterization of zinc(II) complexes with NiSOD binding loop and their related model fragments are reported in the whole investigated pH-range. The zinc(II) complexes of L1 (HCDLPCGVY-NH2), L2 (Ac-HCDLPCGVY-NH2) and L3 (HCDLACGVY-NH2) and the nickel(II) and zinc(II) complexes of L4 (HCDLPCG-NH2) were studied by pH-potentiometric and several spectroscopic methods. The results indicated that the macrochelate coordinated zinc(II) complexes are dominant in a whole pH-range and the side chain donors of the peptides are involved in the metal binding. Therefore, the deprotonation and coordination of the peptide backbone occur only in a strongly alkaline solution. The acetylation of the peptide amino terminus (L2) significantly enhances the zinc(II) binding ability compared to the corresponding nickel(II) complexes. L2 complexes of zinc(II) are 2 or 3 orders of magnitude more stable than the corresponding nickel(II) complexes. This effect clearly shows the crucial role of the terminal amino group in the nickel binding for the NiSOD enzyme.