Comparing the structural properties of human and rat islet amyloid polypeptide by MD computer simulations

Conformational properties of the full-length human and rat islet amyloid polypeptide 1-37 (amyloidogenic hIAPP and non-amyloidogenic rIAPP, respectively) were studied at 310 and 330 K by MD simulations both for the cysteine (reduced IAPP) and cystine (oxidized IAPP) moieties. At all temperatures studied, IAPP does not adopt a well-defined conformation and is essentially random coil in solution, although transient helices appear forming along the peptide between residues 8 and 22, particularly in the reduced form. Above the water percolation transition (at 320 K), the reduced hIAPP moiety presents a considerably diminished helical content remaining unstructured, while the natural cystine moiety reaches a rather compact state, presenting a radius of gyration that is almost 10% smaller and characterized by intrapeptide H-bonds that form many β-bridges in the C-terminal region. This compact conformation presents a short end-to-end distance and seems to form through the formation of β-sheet conformations in the C-terminal region with a minimization of the Y/F distances in a two-step mechanism: the first step taking place when the Y37/F23 distance is ~ 1.1 nm, and subsequently Y37/F15 reaches its minimum of ~ 0.86 nm. rIAPP, which does not aggregate, also presents transient helical conformations. A particularly stable helix is located in proximity of the C-terminal region, starting from residues L27 and P28. Our MD simulations show that P28 in rIAPP influences the secondary structure of IAPP by stabilizing the peptide in helical conformations. When this helix is not present, the peptide presents bends or H-bonded turns at P28 that seem to inhibit the formation of the β-bridges seen in hIAPP. Conversely, hIAPP is highly disordered in the C-terminal region, presenting transient isolated β-strand conformations, particularly at higher temperatures and when the natural disulfide bond is present. Such conformational differences found in our simulations could be responsible for the different aggregational propensities of the two different homologues. In fact, the fragment 30-37, which is identical in both homologues, is known to aggregate in vitro, hence the overall sequence must be responsible for the amyloidogenicity of hIAPP. The increased helicity in rIAPP induced by the serine-to-proline variation at residue 28 seems to be a plausible inhibitor of its aggregation.