Prion diseases are caused by the misfolding and aggregation of the prion protein (PrP). Herein we provide evidence that the Cu-II adduct of the unstructured amyloidogenic fragment of the human PrP (PrP(91-126)) is redox active under physiological conditions. We have identified that the relevant high-affinity Cu-II binding region of PrP(91-126) is contained between residues 106 and 114. Both [Cu-II(PrP(91-126))] and [Cu-II(PrP(106-114))] have Cu-II K-d values of similar to 90 mu M. Furthermore, the smaller PrP fragment PrP(106-114) coordinates Cu-II producing an electronic absorption spectrum nearly identical with [Cu-II(PrP(91-126))] (lambda(max) similar to 610 nm (epsilon similar to 125 M-1 cm(-1))) suggesting a similar coordination environment for Cu-II. Cu K-edge X-ray absorption spectroscopy (XAS) reveals a nearly identical CuN(N/O)(2)S coordination environment for these two metallopeptides (2N/O at similar to 1.97 angstrom; 1S at similar to 2.30 angstrom; 1 imidazole N at similar to 1.95 angstrom). Both display quasireversible Cu-II/Cu-I redox couples at similar to-350 mV vs Ag/AgCl. ESI-MS indicates that both peptides will coordinate Cu-I. However, XAS indicates differential coordination environments between [Cu-I(PrP(91-126))] and [Cu-I(PrP(106-114))]. These data indicate that [Cu-I(PrP(91-126))] contains Cu in a four coordinate (N/O)(2)S-2 environment with similar (N/O)-Cu bond distances (Cu-(N/O) r = 2.048(4) angstrom), while [Cu-I(PrP(106-114))] contains Cu in a four coordinate (N/O)(2)S-2 environment with differential (N/O)-Cu bond distances (Cu-(N/O) r(1) = 2.057(6) angstrom; r(2) = 2.159(3) angstrom). Despite the differential coordination environments both Cu-metallopeptides will catalytically reduce O-2 to O-2(center dot-) at comparable rates.