Copper is implicated in the in vitro formation and toxicity of Alzheimer's disease amyloid plaques containing the beta-amyloid (Abeta) peptide (Bush, A. I., et al. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 11934). By low temperature electron paramagnetic resonance (EPR) spectroscopy, the importance of the N-terminus in creating the Cu(2+) binding site in native Abeta has been examined. Peptides that contain the proposed binding site for Cu(2+)-three histidines (H6, H13, and H14) and a tyrosine (Y10)-but lack one to three N-terminal amino acids, do not bind Cu(2+) in the same coordination environment as the native peptide. EPR spectra of soluble Abeta with stoichiometric amounts of Cu(2+) show type 2 Cu(2+) EPR spectra for all peptides. The ligand donor atoms to Cu(2+) are 3N1O when Cu(2+) is bound to any of the Abetapeptides (Abeta16, Abeta28, Abeta40, and Abeta42) that contain the first 16 amino acids of full-length Abeta. When a Y10F mutant of Abeta is used, the coordination environment for Cu(2+) remains 3N1O and Cu(2+) EPR spectra of this mutant are identical to the wild-type spectra. Isotopic labeling experiments show that water is not the O-atom donor to Cu(2+) in Abeta fibrils or in the Y10F mutant. Further, we find that Cu(2+) cannot be removed from Cu(2+)-containing fibrils by washing with buffer, but that Cu(2+) binds to fibrils initially assembled without Cu(2+) in the same coordination environment as in fibrils assembled with Cu(2+). Together, these results indicate (1) that the O-atom donor ligand to Cu(2+) in Abeta is not tyrosine, (2) that the native Cu(2+) binding site in Abeta is sensitive to small changes at the N-terminus, and (3) that Cu(2+) binds to Abetafibrils in a manner that permits exchange of Cu(2+) into and out of the fibrillar architecture.

Amyloid-beta (Abeta) 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 Abeta oligomers is important to drug design efforts if inhibition of Abeta 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 Abeta. Addition of stoichiometric amounts of Cu(2+) to soluble Abeta produces an EPR signal at 10 K with observable Cu(2+) hyperfine lines. A nearly identical spectrum is observed for Abetafibrils 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( parallel) = 2.26 and A( parallel) = 174 +/- 4 G for soluble Abeta with Cu(2+), and g( parallel) = 2.26 and A( parallel) = 175 +/- 1 G for Abeta fibrils assembled with Cu(2+). Investigation of the temperature dependence of the EPR signal for Cu(2+) bound to soluble Abetaor Cu(2+) in fibrillar Abeta 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 Abetamonomers into fibrils and that neither soluble nor fibrillar forms of Abeta(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.

Copper has been proposed to play a role in Alzheimer's disease through interactions with the amyoid-beta (Abeta) peptide. The coordination environment of bound copper as a function of Cu:Abeta stoichiometry and Abeta oligomerization state are particularly contentious. Using low-temperature electron paramagnetic resonance (EPR) spectroscopy, we spectroscopically distinguish two Cu(II) binding sites on both soluble and fibrillar Abeta (for site 1, A parallel = 168 +/- 1 G and g parallel = 2.268; for site 2, A parallel = 157 +/- 2 G and g parallel = 2.303). When fibrils that have been incubated with more than 1 equiv of Cu(II) are washed, the second Cu(II) ion is removed, indicating that it is only weakly bound to the fibrils. No change in the Cu(II) coordination environment is detected by EPR spectroscopy of Cu(II) with Abeta (1:1 ratio) collected as a function of Abeta fibrillization time, which indicates that the Cu(II) environment is independent of Abeta oligomeric state. The initial Cu(II)-Abeta complexes go on to form Cu(II)-containing Abeta fibrils. Transmission electron microscopy images of Abeta fibrils before and after Cu(II) addition are the same, showing that once incorporated, Cu(II) does not affect fibrillar structure; however, the presence of Cu(II) appears to induce fibril-fibril association. On the basis of our results, we propose a model for Cu(II) binding to Abeta during fibrillization that is independent of peptide oligomeric state.