Alzheimer’s disease (AD) is the main cause of age-related dementia and currently affects approximately 5.7 million Americans. Major brain changes associated with AD pathology include accumulation of amyloid beta (Aβ) protein fragments and formation of extracellular amyloid plaques. Redox-active metals mediate oligomerization of Aβ, and the resultant metal-bound oligomers have been implicated in the putative formation of harmful, reactive species that could contribute to observed oxidative damage. In isolated plaque cores, Cu­(II) is bound to Aβ via histidine residues. Despite numerous structural studies of Cu­(II) binding to synthetic Aβ in vitro, there is still uncertainty surrounding Cu­(II) coordination in Aβ. In this study, we used X-ray absorption spectroscopy (XAS) and high energy resolution fluorescence detected (HERFD) XAS to investigate Cu­(II) coordination in Aβ(1–42) under various solution conditions. We found that the average coordination environment in Cu­(II)­Aβ(1–42) is sensitive to X-ray photoreduction, changes in buffer composition, peptide concentration, and solution pH. Fitting of the extended X-ray absorption fine structure (EXAFS) suggests Cu­(II) is bound in a mixture of coordination environments in monomeric Aβ(1–42) under all conditions studied. However, it was evident that on average only a single histidine residue coordinates Cu­(II) in monomeric Aβ(1–42) at pH 6.1, in addition to 3 other oxygen or nitrogen ligands. Cu­(II) coordination in Aβ(1–42) at pH 7.4 is similarly 4-coordinate with oxygen and nitrogen ligands, although an average of 2 histidine residues appear to coordinate at this pH. At pH 9.0, the average Cu­(II) coordination environment in Aβ(1–42) appears to be 5-coordinate with oxygen and nitrogen ligands, including two histidine residues.

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Highlights d PrP C consists of a flexible N-terminal domain and a structured C-terminal domain d The two domains interact to produce physiological and pathological effects d Cross-linking/MS and NMR define residue-level contacts between the two domains d Cu 2+ ions and mutations affect domain interactions in functionally relevant ways

Although Alzheimer’s disease (AD) was first described over a century ago, it remains the leading cause of age-related dementia. Innumerable changes have been linked to the pathology of AD; however, there remains much discord regarding which might be the initial cause of the disease. The “amyloid cascade hypothesis” proposes that the amyloid β (Aβ) peptide is central to disease pathology, which is supported by elevated Aβ levels in the brain before the development of symptoms and correlations of amyloid burden with cognitive impairment. The “metals hypothesis” proposes a role for metal ions such as iron, copper, and zinc in the pathology of AD, which is supported by the accumulation of these metals within amyloid plaques in the brain. Metals have been shown to induce aggregation of Aβ, and metal ion chelators have been shown to reverse this reaction in vitro. 8-Hydroxyquinoline-based chelators showed early promise as anti-Alzheimer’s drugs. Both 5-chloro-7-iodo-8-hydroxyquinoline (CQ) and 5,7-dichloro-2-[(dimethylamino)­methyl]-8-hydroxyquinoline (PBT2) underwent unsuccessful clinical trials for the treatment of AD. To gain insight into the mechanism of action of 8HQs, we have investigated the potential interaction of CQ, PBT2, and 5,7-dibromo-8-hydroxyquinoline (B2Q) with Cu­(II)-bound Aβ(1–42) using X-ray absorption spectroscopy (XAS), high energy resolution fluorescence detected (HERFD) XAS, and electron paramagnetic resonance (EPR). By XAS, we found CQ and B2Q sequestered ∼83% of the Cu­(II) from Aβ(1–42), whereas PBT2 sequestered only ∼59% of the Cu­(II) from Aβ(1–42), suggesting that CQ and B2Q have a higher relative Cu­(II) affinity than PBT2. From our EPR, it became clear that PBT2 sequestered Cu­(II) from a heterogeneous mixture of Cu­(II)­Aβ(1–42) species in solution, leaving a single Cu­(II)­Aβ(1–42) species. It follows that the Cu­(II) site in this Cu­(II)­Aβ(1–42) species is inaccessible to PBT2 and may be less solvent-exposed than in other Cu­(II)­Aβ(1–42) species. We found no evidence to suggest that these 8HQs form ternary complexes with Cu­(II)­Aβ(1–42).

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Bacterially expressed proteins used in NMR studies lack glycans, and proteins from other organisms are neither 15 N labeled nor glycosylated homogeneously. Here, we add two artificial glycans to uniformly 15 N labeled prion protein using a buffer system that evolves over a pH range to accommodate the conflicting pH requirements of the substrate and enzymes without the need to fine-tune buffer conditions. NMR and CD spectroscopy of the protein indicates that the glycans do not influence its fold.