A Disulfide-Linked Amyloid-β Peptide Dimer Forms a Protofibril-like Oligomer through a Distinct Pathway from Amyloid Fibril Formation

The conversion of the soluble, nontoxic amyloid-β (Aβ) peptide into an aggregated, toxic form rich in β-sheets is considered a key step in the development of Alzheimer’s disease. Whereas growing evidence indicates that the Aβ amyloid fibrils consist of in-register parallel β-sheets, little is known about the structure of soluble oligomeric intermediates because of their transient nature. To understand the mechanism by which amyloid fibrils form, especially the initial development of the “nucleus” oligomeric intermediates, we prepared covalently linked dimeric Aβ peptides and analyzed the kinetics of the fibril-forming process. A covalent bond introduced between two Aβ molecules dramatically facilitated the spontaneous formation of aggregates with a β-sheet structure and affinity for thioflavin T. Transmission electron microscopy revealed, however, that these aggregates differed in morphology from amyloid fibrils, more closely resembling protofibrils. The protofibril-like aggregates were not the most thermodynamically stable state but were a kinetically trapped state. The results emphasize the importance of the conformational flexibility of the Aβ molecule and a balance in the association and dissociation rate for the formation of rigid amyloid fibrils.