Capturing a reactive state of amyloid aggregates: NMR-based characterization of copper-bound Alzheimer disease amyloid β-fibrils in a redox cycle.

The interaction of redox-active copper ions with misfolded amyloid β (Aβ) is linked to production of reactive oxygen species (ROS), which has been associated with oxidative stress and neuronal damages in Alzheimer disease. Despite intensive studies, it is still not conclusive how the interaction of Cu(+)/Cu(2+) with Aβ aggregates leads to ROS production even at the in vitro level. In this study, we examined the interaction between Cu(+)/Cu(2+) and Aβ fibrils by solid-state NMR (SSNMR) and other spectroscopic methods. Our photometric studies confirmed the production of ~60 μM hydrogen peroxide (H2O2) from a solution of 20 μM Cu(2+) ions in complex with Aβ(1-40) in fibrils ([Cu(2+)]/[Aβ] = 0.4) within 2 h of incubation after addition of biological reducing agent ascorbate at the physiological concentration (~1 mM). Furthermore, SSNMR (1)H T1 measurements demonstrated that during ROS production the conversion of paramagnetic Cu(2+) into diamagnetic Cu(+) occurs while the reactive Cu(+) ions remain bound to the amyloid fibrils. The results also suggest that O2 is required for rapid recycling of Cu(+) bound to Aβ back to Cu(2+), which allows for continuous production of H2O2. Both (13)C and (15)N SSNMR results show that Cu(+) coordinates to Aβ(1-40) fibrils primarily through the side chain Nδ of both His-13 and His-14, suggesting major rearrangements from the Cu(2+) coordination via Nε in the redox cycle. (13)C SSNMR chemical shift analysis suggests that the overall Aβ conformations are largely unaffected by Cu(+) binding. These results present crucial site-specific evidence of how the full-length Aβ in amyloid fibrils offers catalytic Cu(+) centers.