Truncated amyloid-β(11-40/42) from Alzheimer disease binds Cu2+ with a femtomolar affinity and influences fiber assembly

Abstract

Alzheimer disease coincides with the formation of extracellular amyloid plaques composed of the amyloid-β (Aβ) peptide. Aβ is typically 40 residues long (Aβ(1-40)) but can have variable C and N termini. Naturally occurring N-terminally truncated Aβ(11-40/42) is found in the cerebrospinal fluid and has a similar abundance to Aβ(1-42), constituting one-fifth of the plaque load. Based on its specific N-terminal sequence we hypothesized that truncated Aβ(11-40/42) would have an elevated affinity for Cu2+. Various spectroscopic techniques, complemented with transmission electron microscopy, were used to determine the properties of the Cu2+-Aβ(11-40/42) interaction and how Cu2+ influences amyloid fiber formation. We show that Cu2+-Aβ(11-40) forms a tetragonal complex with a 34 ± 5 fM dissociation constant at pH 7.4. This affinity is 3 orders of magnitude tighter than Cu2+ binding to Aβ(1-40/42) and more than an order of magnitude tighter than that of serum albumin, the extracellular Cu2+ transport protein. Furthermore, Aβ(11-40/42) forms fibers twice as fast as Aβ(1-40) with a very different morphology, forming bundles of very short amyloid rods. Substoichiometric Cu2+ drastically perturbs Aβ(11-40/42) assembly, stabilizing much longer fibers. The very tight fM affinity of Cu2+ for Aβ(11-40/42) explains the high levels of Cu2+ observed in Alzheimer disease plaques.