The Amyloid-β Peptide of Alzheimer’s Disease Binds CuI in a Linear Bis-His Coordination Environment: Insight into a Possible Neuroprotective Mechanism for the Amyloid-β Peptide

Abstract

Oxidative stress has been suggested to contribute to neuronal apoptosis associated with Alzheimer’s disease (AD). Copper may participate in oxidative stress through redox-cycling between its +2 and +1 oxidation states to generate reactive oxygen species (ROS). In vitro, copper binds to the amyloid-β peptide of AD, and in vivo, copper is associated with amyloid plaques characteristic of AD. As a result, the AβCu^I complex may be a critical reactant involved in ROS associated with AD etiology. To characterize the AβCu^I complex, we have pursued X-ray absorption (XAS) and electron paramagnetic resonance (EPR) spectroscopy of AβCu^II and AβCu^I (produced by ascorbate reduction of AβCu^II). The AβCu^II complex Cu K-edge XAS spectrum is indicative of a square-planar Cu^II center with mixed N/O ligation. Multiple scattering analysis of the extended X-ray absorption fine structure (EXAFS) data for AβCu^II indicates that two of the ligands are imidazole groups of histidine ligands, indicating a (N_Im)₂(N/O)₂ Cu^II ligation sphere for AβCu^II. After reduction of the AβCu^II complex with ascorbate, the edge region decreases in energy by ∼4 eV. The X-ray absorption near-edge spectrum region of AβCu^I displays an intense pre-edge feature at 8984.1(2) eV. EXAFS data fitting yielded a two-coordinate geometry, with two imidazole ligands coordinated to Cu^I at 1.877(2) Å in a linear geometry. Ascorbate reduction of AβCu^II under inert atmosphere and subsequent air oxidation of AβCu^I to regenerate AβCu^II was monitored by low-temperature EPR spectroscopy. Slow reappearance of the AβCu^II EPR signal indicates that O₂ oxidation of the AβCu^I complex is kinetically sluggish and Aβ damage is occurring following reoxidation of AβCu^I by O₂. Together, these results lead us to hypothesize that Cu^I is ligated by His13 and His14 in a linear coordination environment in Αβ, that Aβ may be playing a neuroprotective role, and that metal-mediated oxidative damage of Aβ occurs over multiple redox cycles.