Complexation of Zn(II) to a native sequence tripeptide of human serum albumin studied by 13C nuclear magnetic resonance

Abstract

Serum albumins from several species, which specifically bind and transport several divalent metals in blood, contain an Asp-X-His or Glu-X-His amino acid triad as their NH₂-terminal sequence. We have synthesized a tripeptide, Asp-Ala-His-N-methylamide, corresponding to the native sequence of the human serum albumin N-terminus, and examined its interaction with Zn(II) ions in aqueous solution using carbon-13 nmr techniques. Variations in carbon chemical shifts and spin–lattice relaxation times (T₁'s) as a function of pH and increments of added Zn(II) were used to delineate sites of Zn(II):peptide interactions. Trends in T₁ values indicated that local motions of several of the tripeptide carbons were restricted by an induced order accompanying metal binding. Analysis of spectral data suggested that Zn(II) is coordinated to the His imidazole nitrogen atom and the Asp β-carboxylate oxygen atom(s) in each peptide molecule. Similar experiments using a tripeptide analogue, Gly-Gly-His-N-methylamide, demonstrated that in the absence of a side chain carboxylate group, Zn(II) ions will complex the peptide through the His imidazole group and the N-terminal Gly α-amino group. These results support the possibility of a biologically functional role for the Asp-X-His triad as a liganding template for Zn(II)/protein binding.