Molecular Characterization of Dimerization of Human Amylin Peptide In Solution

Islet amyloid polypeptide (IAPP, also known as amylin) is responsible for the formation of pancreatic amyloid deposits in type II diabetes. Such deposits, as well as small aggregates that arise as intermediates in their assembly, are cytotoxic to pancreatic β-cells and are believed to contribute to the loss of β-cell mass associated with type II diabetes. To better understand the mechanism and cause of such aggregation, we have used molecular simulations with explicit solvent models to identify early aggregation structures and the mechanism of aggregation. In past work on monomeric structures of amylin, we have shown that it can adopt three structures: α-helical, β-hairpin or an unstructured coil [1]. Here we show that of these, the β-hairpin is the most stable state and a primary cause for aggregation. We propose a novel enhanced sampling technique to investigate the free energy surface of the peptides in solution. Using free-energy maps generated through that technique, we examine the relative free energies of different possible dimer structures of the peptide. From simulations of β-hairpins and α-helices we identify a β-hairpin-catalyzed transition of the α-helical structure to a β-hairpin conformation. This phenomenon is examined in detail through transition path sampling (TPS) simulations. Our study provides a precise molecular mechanism for the early stage aggregation of amylin in solution. The various structures of the dimer predicted by these simulations are consistent with the final aggregate structure proposed by Luca et al. [2] and are consistent with our own 2DIR measurements of amylin fibrils [3].

References:

1. Reddy, A. S.; Wang, L.; Singh, S.; Ling, Y.; Buchanan, L.; Zanni, M. T.; Skinner, J. L.; De Pablo, J. J. Biophys. J. 2010, 99 (7), 2208–2216

2. Luca, S.; Yau, W.-M. ; Leapman, R. and Tycko R. Biochemistry 46, 13505 (2007)

3. Wang, L.; Middleton, C. T.; Singh, S.; Reddy, A. S.; Woys, A. M., Strasfeld, D. B.; Marek, P.; Raleigh, D. P.; de Pablo, J. J.; Zanni, M. T. and Skinner, J. L. Submitted