From Peptide Fragments to Whole Protein: Copper(II) Load and Coordination Features of IAPP

The copper binding features of rat islet amyloid polypeptide (r-IAPP) are herein disclosed through the determination of the stability constants and spectroscopic properties of its copper complex species. To mimic the metal binding sites of the h-IAPP, a soluble, single point mutated having a histidine residue in place of Arg18 was synthesized, i.e. r-IAPP(1-37;R18H). The peptide IAPP(1-8) was also characterized to have deeper insight into the N-terminus copper(II) binding features of r-IAPP as well as of its mutated form. A combined experimental, (thermodynamic and spectroscopic) and computational approach allowed us to assess the metal loading and the coordination features of the whole IAPP. At physiological pH, the N-terminal amino group is the Cu2+ main binding site both of entire r-IAPP and of the its mutated form that mimics h-IAPP. The histidine residue presents in this mutated polypeptide accounts for the second Cu2+ binding. We can speculate that the copper driven toxicity of h-IAPP in comparison to that of r-IAPP can be attributed to the different metal loading and the presence of a second metal anchoring site, the His18, whose role is usually invoked in the process of h-IAPP aggregation.