Structure of the amyloid-beta (1-42) monomer absorbed to model phospholipid bilayers: a molecular dynamics study

The amyloid-beta (Abeta) peptide, the 39 to 43 amino acid peptide that plays a substantial role in Alzheimer's disease, has been shown to interact strongly with lipids both in vitro and in vivo. Abeta-lipid interactions have been proposed as a considerable factor in accelerating Abeta aggregation through the templating role of membranes in aggregation disorders. Previous work has shown that anionic lipids are able to significantly increase Abeta aggregation rate and induce a structural conversion in Abeta from a random coil to a beta-structure that is similar to the monomer structure observed in mature fibrils. However, it is unclear if this structural change occurs with the Abeta monomer because of direct interactions with the lipids or if the structural change results from protein-protein interactions during oligomerization. We use extensive replica exchange molecular dynamics simulations of an Abeta monomer bound to a homogeneous model zwitterionic or anionic lipid bilayer. From these simulations, we do not observe any significant beta-structure formation except for a small, unstable beta-hairpin formed on the anionic dioleylphosphatidylserine bilayer. Further, we see that the Asp23-Lys28 salt bridge that plays a role in beta-hairpin formation is not substantially formed on the bilayer surface and that Lys28 preferentially interacts with lipids when bound to the bilayer. These results suggest that the structural conversion seen in experiments are not due to the ordering of monomeric Abeta on the bilayer surface but are a result of protein-protein interactions enhanced by Abeta binding to the cell membrane.