Amyloid-β Binds Cu~(2+) in a Mononuclear Metal Ion Binding Site

摘要

Amyloid-β (Aβ) peptide is the principal constituent of plaques associated with Alzheimer's disease and is thought to be responsible for the neurotoxicity associated with the disease. Metal ions have been hypothesized to play a role in the formation and neurotoxicity of aggregates associated with Alzheimer's disease (Bush, A. I.; et al. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 11934). Elucidation of the chemistry through which transition-metal ions participate in the assembly and toxicity of Aβ oligomers is important to drug design efforts if inhibition of Aβ containing bound metal ions becomes a treatment for Alzheimer's disease. In this paper, we report electron paramagnetic resonance (EPR) spectroscopic characterization of Cu~(2+) bound to soluble and fibrillar Aβ. Addition of stoichiometric amounts of Cu~(2+) to soluble Aβ produces an EPR signal at 10 K with observable Cu~(2+) hyperfine lines. A nearly identical spectrum is observed for Aβ fibrils assembled in the presence of Cu~(2+). The EPR parameters are consistent with a Type 2 Cu~(2+) center with three nitrogen donor atoms and one oxygen donor atom in the coordination sphere of Cu~(2+): g_(||) = 2.26 and A_(||) = 174 +- 4 G for soluble Aβ with Cu~(2+), and g_(||) = 2.26 and A_(||) = 175 +- 1 G for Aβ fibrils assembled with Cu~(2+). Investigation of the temperature dependence of the EPR signal for Cu~(2+) bound to soluble Aβ or Cu~(2+) in fibrillar Aβ shows that the Cu~(2+) center displays normal Curie behavior, indicating that the site is a mononuclear Cu~(2+) site. Fibrils assembled in the presence of Cu~(2+) contain one Cu~(2+) ion per peptide. These results show that the ligand donor atom set to Cu~(2+) does not change during organization of Aβ monomers into fibrils and that neither soluble nor fibrillar forms of Aβ(1-40) with Cu~(2+) contain antiferromagnetically exchange-coupled binuclear Cu~(2+) sites in which two Cu~(2+) ions are bridged by an intervening ligand.