The infectious isoform of the prion protein (PrPSc) is derived from cellular PrP (PrPC) in a conversion reaction involving a dramatic reorganization of secondary and tertiary structure. While our understanding of the pathogenic role of PrPSc has grown, the normal physiologic function of PrPC still remains unclear. Using recombinant Syrian hamster prion protein [SHaPrP(29-231)], we investigated metal ions as possible ligands of PrP. Near-UV circular dichroism spectroscopy (CD) indicates that the conformation of SHaPrP(29-231) resembles PrPC purified from hamster brain. Here we demonstrate by CD and tryptophan (Trp) fluorescence spectroscopy that copper induces changes to the tertiary structure of SHaPrP(29-231). Binding of copper quenches the Trp fluorescence emission significantly, shifts the emission spectrum to shorter wavelengths, and also induces changes in the near-UV CD spectrum of SHaPrP(29-231). The binding sites are highly specific for Cu2+, as indicated by the lack of a change in Trp fluorescence emission with Ca2+, Co2+, Mg2+, Mn2+, Ni2+, and Zn2+. Binding of Cu2+ also promotes the conformational shift from a predominantly alpha-helical to a beta-sheet structure. Equilibrium dialysis experiments indicate a binding stoichiometry of approximately 2 copper molecules per PrP molecule at physiologically relevant concentrations, and pH titration of Cu2+ binding suggests a role for histidine as a chelating ligand. NMR spectroscopy has recently demonstrated that the octarepeats (PHGGGWGQ) in SHaPrP(29-231) lack secondary or tertiary structure in the absence of Cu2+. Our results suggest that each Cu2+ binds to a structure defined by two octarepeats (PHGGGWGQ) with one histidine and perhaps one glycine carbonyl chelating the ion. We propose that the binding of two copper ions to four octarepeats induces a more defined structure to this region.