Zinc ions promote Alzheimer Aβ aggregation via population shift of polymorphic states

Although a key factor in Alzheimer's disease etiology is enrichment of Zn(2+) in aggregates, and there are data suggesting that zinc promotes aggregation, how Zn(2+)-A beta coordination promotes aggregation is elusive. Here we probe the structures and mechanisms through which Zn(2+) can affect amyloidosis. By covalently linking fragments (that have experiment-based coordinates) we observed that, in oligomeric Zn(2+)-A beta(42), Zn(2+) can simultaneously coordinate intra-and intermolecularly, bridging two peptides. Zinc coordination significantly decreases the solvation energy for large Zn(2+)-A beta(42) oligomers and thus enhances their aggregation tendency. Zn(2+) binding does not change the beta-sheet association around the C-terminal hydrophobic region; however, it shifts the relative population of the preexisting amyloid polymorphic ensembles. As a result, although a parallel beta-sheet arrangement is still preferred, antiparallel and other less structured assemblies are stabilized, also becoming major species. Overall, Zn(2+) coordination promotes A beta(42) aggregation leading to less uniform structures. Our replica exchange molecular dynamics simulations further reproduced an experimental observation that the increasing Zn(2+) concentration could slow down the aggregation rate, even though the aggregation rates are still much higher than in Zn(2+)-free solution.