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 Cu(2+). Various spectroscopic techniques, complemented with transmission electron microscopy, were used to determine the properties of the Cu(2+)-Aβ(11-40/42) interaction and how Cu(2+) influences amyloid fiber formation. We show that Cu(2+)-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 Cu(2+) binding to Aβ(1-40/42) and more than an order of magnitude tighter than that of serum albumin, the extracellular Cu(2+) 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 Cu(2+) drastically perturbs Aβ(11-40/42) assembly, stabilizing much longer fibers. The very tight fm affinity of Cu(2+) for Aβ(11-40/42) explains the high levels of Cu(2+) observed in Alzheimer disease plaques.