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Exploring the aggregation free energy landscape of the amyloid-β protein (1–40)

Significance Protein aggregation and amyloid formation seem to be at the heart of the pathology of multiple neurodegenerative diseases, including Alzheimer’s disease. Aβ protein has long been considered one of the protein components that contributes to the pathogenesis and the progression of the disease. The concepts of energy landscape analysis established in the theory of protein folding are applied here to create a quantitative image of the aggregation energy landscape of Aβ. The resulting “amyloid funnel” not only helps visualize the complexity of the early stages of aggregation of WT Aβ but also, predicts the effects of mutations at specific sites on aggregation behavior. A predictive coarse-grained protein force field [associative memory, water-mediated, structure, and energy model for molecular dynamics (AWSEM)-MD] is used to study the energy landscapes and relative stabilities of amyloid-β protein (1–40) in the monomer and all of its oligomeric forms up to an octamer. We find that an isolated monomer is mainly disordered with a short α-helix formed at the central hydrophobic core region (L17-D23). A less stable hairpin structure, however, becomes increasingly more stable in oligomers, where hydrogen bonds can form between neighboring monomers. We explore the structure and stability of both prefibrillar oligomers that consist of mainly antiparallel β-sheets and fibrillar oligomers with only parallel β-sheets. Prefibrillar oligomers are polymorphic but typically take on a cylindrin-like shape composed of mostly antiparallel β-strands. At the concentration of the simulation, the aggregation free energy landscape is nearly downhill. We use umbrella sampling along a structural progress coordinate for interconversion between prefibrillar and fibrillar forms to identify a conversion pathway between these forms. The fibrillar oligomer only becomes favored over its prefibrillar counterpart in the pentamer where an interconversion bottleneck appears. The structural characterization of the pathway along with statistical mechanical perturbation theory allow us to evaluate the effects of concentration on the free energy landscape of aggregation as well as the effects of the Dutch and Arctic mutations associated with early onset of Alzheimer’s disease.