Multiple forms of copper(II) co-ordination occur throughout the disordered N-terminal region of the prion protein at pH 7.4

Overview & Synopsis

Although the physiological function of the prion protein remains unknown, in vitro experiments suggest that the protein may bind copper (II) ions and play a role in copper transport or homeostasis in vivo. The unstructured N-terminal region of the prion protein has been shown to bind up to six copper (II) ions, with each of these ions coordinated by a single histidine imidazole and nearby backbone amide nitrogen atoms. Individually these sites have micromolar affinities, which is weaker than would be expected of a true cuproprotein. Here we show that with sub-saturating levels of copper different forms of coordination will occur, which have higher affinity. We have investigated the copper binding properties of two peptides representing the known copper binding regions of the prion protein; residues 57-91, which contains four tandem repeats of the octapeptide GGGWGQPH, and residues 91-115. Using equilibrium dialysis and spectroscopic methods we unambiguously demonstrate that the mode of copper coordination in both of these peptides depends on the number of copper ions bound and that at low copper occupancy, copper ions are coordinated with sub micromolar affinity by multiple histidine imidazole groups. At pH 7.4, three different modes of copper coordination are accessible within the octapeptide repeats and two within the peptide comprising residues 91-115. The highest affinity copper (II) binding modes cause self-association of both peptides, suggesting a role for copper (II) in controlling prion protein self-association in vivo.