Cu(II) site geometry of the octarepeat domain of Prion Protein: a XAS study.

The cellular prion protein, the precursor of the major protein component of prions, is a Cu(II) binding protein. Cooperative cation binding occurs in the flexible and dynamic N-terminal protein domain that is characterized by a highly conserved PHGGGWGQ octarepeat segment. The Cu(II) site geometry in synthetic peptides containing multiple copies of the octarepeat and in the a-like form of BoPrP (24-242) in complexes with the metal ion at various equivalences has been studied using X-ray absorption spectroscopy (XAS) at the Cu K-edge. Inspection of the near edge region of the spectra (XANES) confirms that upon binding Cu(II) preserves its oxidation state. Accurate analysis of the extended region of the XAS spectra (EXAFS) using a multiple-scattering approach shows the occurrence of two types of coordination geometries differing in the number of imidazole rings coordinated to Cu(II). For BoPrP forms containing more than 3 octarepeats at a half saturation of cation sites, Cu(II) is bonded to two imidazole rings through nitrogen atoms, whereas in synthetic peptides containing lower site multiplicity (1 or 2 octarepeats) at substoichiometric site saturation the first Cu(II) coordination shell contains only one imidazole nitrogen. Increasing Cu(II) site saturation from 0.5 to 0. 8 in synthetic fragments containing four copies of the octarepeat an intermediate situation is generated. The modeled geometries for alternative Cu(II) bindings provide a structural ground for a positive cooperativity mechanism underlying the cation binding process.