Prions are the agents responsible for a variety of neurodegenerative diseases, including mad cow disease. Because they consist uniquely of protein and their only apparent pathogenic defect is a protein misfold, prions represent an extraordinary discovery in biology and medicine. The correctly folded, normal isoform of prion (PrPC) exists in healthy brain tissue but its function there is unknown. It was recently demonstrated that PrPC binds Cu(II) in vivo, and it has been suggested that the normal function of the protein involves copper metabolism or storage. Chemical and spectroscopic techniques have begun to probe the copper-binding properties of both fragments of and full-length recombinant prion. Cu(II)-to-peptide binding stoichiometries and individual Cu(II) binding constants are known under a veriety of conditions, and a pH dependence for binding is well described. Copper is believed to be localized to a defined region of the peptide, but the precise binding sites within the 208 amino acid sequence are unknown. We propose a series of Cu K-edge X-ray absorption experiments to elucidate the binding sites and local site-structure of the Cu(II) ions in prions.
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