New Insights into Metal Interactions with the Prion Protein: EXAFS Analysis and Structure Calculations of Copper Binding to a Single Octarepeat from the Prion Protein

Copper coordination to the prion protein (PrP) has garnered considerable interest for almost 20 years, due in part to the possibility that this interaction may be part of the normal function of PrP. The most characterized form of copper binding to PrP has been Cu²⁺ interaction with the conserved tandem repeats in the N-terminal domain of PrP, termed the octarepeats, with many studies focusing on single and multiple repeats of PHGGGWGQ. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used in several previous instances to characterize the solution structure of Cu²⁺ binding into the peptide backbone in the HGGG portion of the octarepeats. All previous EXAFS studies, however, have benefitted from crystallographic structure information for [Cu^II (Ac-HGGGW-NH₂)_−2H] but have not conclusively demonstrated that the complex EXAFS spectrum represents the same coordination environment for Cu²⁺ bound to the peptide backbone. Density functional structure calculations as well as full multiple scattering EXAFS curve fitting analysis are brought to bear on the predominant coordination mode for Cu²⁺ with the Ac-PHGGGWGQ-NH₂ peptide at physiological pH, under high Cu²⁺ occupancy conditions. In addition to the structure calculations, which provide a thermodynamic link to structural information, methods are also presented for extensive deconvolution of the EXAFS spectrum. We demonstrate how the EXAFS data can be analyzed to extract the maximum structural information and arrive at a structural model that is significantly improved over previous EXAFS characterizations. The EXAFS spectrum for the chemically reduced form of copper binding to the Ac-PHGGGWGQ-NH₂ peptide is presented, which is best modeled as a linear two-coordinate species with a single His imidazole ligand and a water molecule. The extent of <i>in situ</i> photoreduction of the copper center during standard data collection is also presented, and EXAFS curve fitting of the photoreduced species reveals an intermediate structure that is similar to the Cu²⁺ form with reduced coordination number