The Role of Side Chains in the Fine-Tuning of the Metal-Binding Ability of Multihistidine Peptides.

The systematic studies of copper(II), nickel(II) and zinc(II) ion complexes of protected multihistidine peptides containing amino acids with different side chains (Ac-SarHAH-NH2, Ac-HADH-NH2, Ac-HDAH-NH2, Ac-HXHYH-NH2 X, Y = A, F, D or K, Ac-HXHAHXH-NH2, X = F or D) have provided information about the metal ion and protein interaction and have made it possible to draw conclusions regarding general trends in the coordination of metal complexes of multihistidine peptides. The stability of the metal complexes significantly depends on the position of the histidines and amino acids, which are present in the neighbourhood of the histidine amino acids as well. The most significant effect was observed on peptides containing aspartic acid or phenylalanine. The redox parameters of complexes, however, depend on the number and position of histidines, and the other side chain donor atoms have practically no effect on the electrochemical properties of imidazole-coordinated species. However, the presence of aspartic acid side chains results in a more distorted geometry of amide-coordinated species and increases the reducibility of these complexes.

Thermodynamics and structural characterization of the nickel(II) and zinc(II) complexes of various peptide fragments of tau protein.

Nickel(II) and zinc(II) complexes of various peptide fragments of tau protein have been investigated by potentiometric, UV-Vis, CD and ESI-MS techniques. The peptides include the native fragment tau(9-16) (Ac-EVMEDHAG-NH2), and the Gln/Lys and Tyr/Ala mutated peptides (Ac-KGGYTMHK-NH2 and Ac-KGGATMHK-NH2) of tau(26-33). Similar to copper(II) the complexes of a chimeric peptide containing both His14 and His32 residues in one molecule (Ac-EDHAGTMHQD-NH2) were also studied. The metal binding ability of the R3 domain was studied by using the native fragment tau(326-333) (Ac-GNIHHKPG-NH2), and its two mutants (Ac-GNIHHKAG-NH2) and (Ac-GNGHHKPG-NH2) and the corresponding 1-histidine mutants (Ac-GNGAHKPG-NH2 and Ac-GNGHAKPG-NH2). The results of this study reveal that the histidyl residues of the N-terminal and R3 regions of tau protein can effectively bind nickel(II) and zinc(II) ions. In the case of nickel(II) and zinc(II) the M-Nim coordinated complexes are the major species in the physiological pH range and their stability is significantly enhanced by the presence of Glu and Asp residues in the neighbourhood of the His14 site. For all studied peptides, nickel(II) ions are able to promote the deprotonation and coordination of amide groups preceding histidine resulting in the exclusive formation of square planar (Nim,3N-) complexes in alkaline solutions. The native fragment of the R3 region and its mutants containing two adjacent histidine moieties also bind only one nickel(II) ion with the His330 residue being the primary metal binding site. Exclusive binding of the independent imidazole side chains (His14 and His32 sites) cannot prevent the hydrolysis of zinc(II) in a slightly basic solution but the adjacent histidines of the R3 domain can promote the formation of amide coordinated zinc(II) complexes.

Copper (II) binding properties of an octapeptide fragment from the R3 region of tau protein: A combined potentiometric, spectroscopic and mass spectrometric study.

The copper(II) complexes of a peptide fragment of the R3 domain of tau protein (tau(326-333) Ac-GNIHHKPG-NH2) and its mutants (Ac-GNGHHKPG-NH2, Ac-GNIHHKAG-NH2, Ac-GNGAHKPG-NH2 and Ac-GNGHAKPG-NH2) have been studied by potentiometric and spectroscopic (UV-Vis, CD) methods. ESR spectroscopy and mass spectrometry were also used to prove the coordination mode of the mononuclear complexes and the formation of dinuclear species, respectively. It has been demonstrated that the (326-333) fragment of tau protein is a versatile and effective ligand for copper(II) coordination. The versatility of copper(II) binding is related to the presence of two adjacent histidyl residues in the sequence, which results in the coexistence of mononuclear, bis(ligand) and dinuclear complexes at different metal to ligand ratios. The 1:1 mononuclear complexes are, however, the dominant species with all peptides and the imidazole-N and one to three deprotonated amide nitrogen atoms towards the N-terminal side of the histidyl residue have been suggested as metal binding sites. This binding mode allows the formation of coordination isomers because any of the two histidine moieties can be the primary anchoring site. It is evident from the CD spectroscopic measurements that the isomers are present in almost equal concentration. The copper(II) binding affinity of the native fragment of tau protein is comparable to that of a similar 2-histidine fragment of amyloid-ß mutant, Ac-SGAEGHHQK-NH2 but the comparison with an independent histidyl residue (H32) from the N-terminal region of the protein reveals the predominance of H32 over the histidines in the R3 domain.