Zinc Interactions with a Soluble Mutated Rat Amylin to Mimic Whole Human Amylin: An Experimental and Simulation Approach to Understand Stoichiometry, Speciation and Coordination of the Metal Complexes

Islet amyloid polypeptide (IAPP) is a hormone co-secreted with insulin and zinc from pancreatic β-cells. To overcome the low solubility of human IAPP, we characterized zinc complexes species formed with 1) a mutated form of rat-IAPP(1–37; R18 H) able to mimic the human IAPP, 2) the r-IAPP(1–37) and the IAPP(1–8) fragment. Stoichiometry, speciation and coordination features of zinc(II) complexes were unveiled by ESI-MS, potentiometry and NMR measurements combined with DFT and free-energy simulations. Mononuclear species start to form around pH 6; Zn2+ binds both His18 and N-amino terminus in rat-IAPP(1–37; R18 H). The in silico study allows us to assess not only a structured turn compact domain in r-IAPP(1–37) and r-IAPP(1–37; R18 H) featured by a different free energy barrier for the transition from the compact to elongated conformation upon the coordination of Zn2+, but also to bring into light a coordination shell further stabilized by noncovalent interactions.