Transition metal complexes of peptides with cysteine and histidine binding sites: a thermodynamic and structural study

The thiolate group of cysteine and imidazole of histidine are the most common metal binding sites of proteins. As a consequence, huge number of papers has already been published in this field focused on the characterization of metal binding1-3. Metal complexes of (multi)histidine peptides have been widely studied because of the possible role in neurodegeneration4. The studies on the cysteine containing peptides are much less available, most important examples are related to nickel(II) homeostasis of Helicobacter pylori and to the zinc(II) transporter and finger proteins. The presence of the thiolate group, however, significantly enhances the metal binding abality towards the borderline and soft metal ions. Typically toxic metal ions have soft Lewis character such as cadmium(II), lead(II) or palladium(II), which are able to substitute the essential metal ions. This irreversible connection may change or hinder the participation of metalloenzymes and metalloproteins in biochemical processes. In our lab at the Bioinorganic Research Group at the University of Debrecen, systematic studies have been performed for the synthesis and structural characterization of metal complexes of peptides with thiolate and other binding sites. By using of solid phase peptide synthesis two N-terminally free but C-terminally amidated peptides (AHAAAC-NH2 andAAHAAC-NH2) have been synthesized and their transition metal complexes (copper(II), nickel(II), zinc(II), cadmium(II) and palladium(II)) have been characterized by equilibrium (pH-potentiometry, UV/Vis), spectroscopic (NMR, EPR, UV/Vis, CD, ESI-TOF-MS) and computational (DFT) methods. Our results indicate that both synthesized peptides have outstanding metal binding affinity but the speciation of the systems shows significant specificity. In the case of AAHAAC-NH2, the N-terminal part of the peptides is the primary nickel(II) and copper(II) binding site with (NH2,N-,N-,Nim)-coordination mode. This binding mode corresponds well to that of albumin with an outstanding copper(II) and nickel(II) binding affinity (ATCUN motif). In the presence of palladium, unusual thermodynamic behaviour has been observed, namely, the formation of fused-chelate system with (NH2,N-,N-,Nim) donor set is not prefered because of the thiolate group which is able to hinder a deprotonation and coordination of the second amide group resulting in a (NH2,N-,Nim) coordinated species supporting via machrochelation from the distant thiolate group. In the case of AHAAAC-NH2 all investigated metal ions, except cadmium(II), can induce the deprotonation and coordination of the first amide bond in the sequence. This coordination environment, however, results in an unsaturated coordination sphere that can be completed via the coordination of the thiolate group resulting in macrochelate supported species.