Role of Aspartate-1 in Cu(II) Binding to the Amyloid-β Peptide of Alzheimer's Disease

Abstract

Considerable effort is being directed at unraveling the molecular mechanisms of Alzheimer's disease (AD), including the participation of metal ions. The extracellular proteinaceous plaques associated with AD are composed predominantly of the amyloid-beta (Aβ) peptide, which has been shown to bind copper. Because coordination environment is an important determinant of copper reactivity, work to establish the copper binding site in Aβ is central to understanding copper's role in AD. A major controversy centers on the coordination environment of Cu(II) when bound to Aβ. We have shown previously that tyrosine is not the O-atom donor to Cu(II) and that N-terminal deletions to the Aβ peptide disrupt the native Cu(II) binding site [Karr, J. W.; Akintoye, H.; Kaupp, L. J.; Szalai, V. A. Biochemistry, 2005, 44, 5478−5487]. Here, we present low-temperature electron paramagnetic resonance (EPR) spectra of Cu(II) bound to N-terminal mutants of Aβ16 as a means to assess the role these amino acids play in creating the Cu(II) binding site at pH 7.2. Mutation of glutamate-3 to glutamine produces no effect on the EPR spectrum of Cu(II) bound to the Aβ peptide, indicating that this residue is not involved in Cu(II) binding. In contrast, mutation of aspartate-1 to asparagine produces a spectrum with two components. The spectrum of the first component is identical to that observed for wild-type peptide in the presence of Cu(II) at pH 7.2. The second component has very different EPR parameters (A∥ = 156 ± 1 G and g∥ = 2.226). Our data are consistent with a model in which the carboxylate of aspartate-1 creates the native binding site through hydrogen-bonding interactions rather than through direct ligation to Cu(II) in Aβ. Our results clearly influence research conducted on copper and Aβ, but our model also impacts work on the interaction of copper with α-synuclein (Parkinson's disease) and prion proteins involved in other neurodegenerative diseases.