From Aβ Filament to Fibril: Molecular Mechanism of Surface-Activated Secondary Nucleation from All-Atom MD Simulations

Secondary nucleation pathways in which existing amyloid fibrils catalyze the formation of new aggregates and neurotoxic oligomers are of immediate importance for the onset and progression of Alzheimer's disease. Here, we apply extensive all-atom molecular dynamics simulations in explicit water to study surface-activated secondary nucleation pathways at the extended lateral beta-sheet surface of a preformed A beta(9-40) filament. Calculation of free-energy profiles allows us to determine binding free energies and conformational intermediates for nucleation complexes consisting of 1-4 A beta peptides. In addition, we combine the free-energy profiles with position-dependent diffusion profiles to extract complementary kinetic information and macroscopic growth rates. Single monomers bind to the beta-sheet surface in a disordered, hydrophobically collapsed conformation, whereas dimers and larger oligomers can retain a cross-beta conformation resembling a more ordered fibril structure. The association processes during secondary nucleation follow a dock/lock mechanism consisting of a fast initial encounter phase (docking) and a slow structural rearrangement phase (locking). The major driving forces for surface-activated secondary nucleation are the release of a large number of hydration water molecules and the formation of hydrophobic interface contacts, the latter being in contrast to the elongation process at filament tips, which is dominated by the formation of stable and highly specific interface hydrogen bonds. The calculated binding free energies and the association rates for the attachment of A beta monomers and oligomers to the extended lateral beta-sheet surface of the filament seed are higher compared to those for elongation at the filament tips, indicating that secondary nucleation pathways can become important once a critical concentration of filaments has formed.