The anchoring ability of specific non-coordinating side chains in the rat amylin fragments

Human islet amyloid polypeptide (hIAPP) is a peptide hormone consisting of 37 amino acid residues which is co-secreted with insulin by pancreatic islet ?-cells. Although the physiological role of hIAPP is still not well established, it is assumed, that the formation of hIAPP aggregates is strongly associated with ?-cell degeneration in type 2 diabetes. Besides humans, only a few species of animals are known to develop islet amyloid, these include non-human primates, cats and raccoons. Rat amylin (rIAPP) does not show tendency for self-aggregation and is not toxic to islet cells. Rat amylin is highly homologous to human amylin, but differs in six amino acids resulting in a nonamyloidogenic peptide. The main difference is that histidine is not present in rat amylin. Despite the lack of any common strongly coordinating donor functions the fragments of rat amylin are able to bind copper(II) ions, and the -VRSSNN- sequence can be the main metal binding sequence. The previous studies of terminally protected mutants of rIAPP has shown the especially specific role of the polar seryl and asparaginyl side chains (SSNN) in metal binding. To clarify the exact binding mode of copper(II) in these complexes and understand the possible role of Arg, Ser and Asn side chains in copper(II) binding, however, required further studies in this subject. Thereby we synthesized and studied further mutants with terminally free amino group (NH2-VRSSNN-NH2, NH2-VRAANN-NH2, NH2-VRSSAA-NH2, NH2-VRSS-NH2, NH2-SSNN-NH2, NH2-SSNA-NH2 and NH2-AANN-NH2). At first, all data rule out the involvement of arginyl residues in copper(II) binding. The effect of the 19-22 domain SSNN sequence is, however, more complicated and largely depends on the length of the peptides and on the presence of other strongly coordinating groups. The data obtained for copper(II) complexes of these peptides unambiguously demonstrate the dominating role of the terminal amino group over the asparaginyl side chains in copper(II) binding, but the significant enhancement of the thermodynamic stability can be observed in some cases. At the same time the SSNN sequence can provide a secondary metal binding site for copper(II) ion resulting in formation of dinuclear species.