Hpn, one of Helicobacter pylori’s nickel accessory proteins, is an amazingly peculiar protein – almost half of its sequence consists of polyhistydyl residues. In this work, we try to understand the origin of this naturally occurring sequence, shedding some light upon the bioinorganic chemistry of Hpn’s numerous poly-His repeats. Using potentiometric, mass spectrometric and various spectroscopic techniques, we studied the Ni2+ and Cu2+complexes of the wild type Ac-THHHHYHGG-NH2 fragment of Hpn and of its six analogues, in which consecutive residues (His or Tyr) were replaced by Ala (Ala-substitution or Ala-scan approach), resulting in Ac-TAHHHYHGG-NH2, Ac-THAHHYHGG-NH2, Ac-THHAHYHGG-NH2, Ac-THHHAYHGG-NH2, Ac-THHHHAHGG-NH2 and Ac-THHHHYAGG-NH2 peptides, respectively. We found that the His-4 residue is critical for both Ni2+ and Cu2+ ion binding and the effectiveness of binding varies even if the substituted amino acid doesn’t take part in the direct binding
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