Zinc-Induced Conformational Transitions in Human Islet Amyloid Polypeptide and Their Role in the Inhibition of Amyloidosis

Type-2 diabetes mellitus (T2DM) is a disease hallmarked by improper homeostasis within the islets of Langerhans of the pancreas. The most critical species affected is insulin, which is produced by the β-cells of the islets, but there are a number of other species copackaged and cosecreted within the insulin granules. This includes zinc, which exists in high (millimolar) concentrations within the β-cells, and islet amyloid polypeptide (IAPP), which is an amyloid peptide thought to induce β-cell apoptosis through self-association into toxic amyloid oligomers. Zinc is essential in the packaging of crystalline insulin within the vesicles but it can also bind and interact with IAPP. This implies a complex relationship between all three species and diabetes, particularly in the structure and function of toxic IAPP aggregates. Atypical (low or high) concentrations of zinc generally appear to correlate with increased hIAPP aggregation, whereas physiological zinc concentrations have an inhibitory effect. To better understand how zinc ions alter the monomer and oligomer structure of hIAPP in vitro, we employ a combination of ion mobility mass spectrometry and atomic force microscopy. We observe an increase in the extended β-hairpin conformation of hIAPP when it is bound to zinc. With sufficiently low concentrations of zinc this could result in an association site for zinc-free hIAPP, promoting amyloid aggregation. At high zinc concentrations, we see the appearance of a secondary zinc association site whose coordination could account for the loss of inhibition at high zinc concentrations. Generally, it appears that zinc preferentially stabilizes the β-hairpin conformation of hIAPP and the population of zinc-bound hIAPP in solution determines what effect this has on amyloid aggregation.