The influence of a single octa repeat expansion on the Cu-II and Zn-II coordination environments within the octarepeat domain of the human prion protein is examined. Using X-ray absorption spectroscopy and diethyl pyrocarbonate labeling studies, we find that at low copper concentrations the "normal" octarepeat domain (four PHGGGWGQ repeats) coordinates Zn-II in an (N/O)(6) coordination environment with two histidine residues and Cu-II in a redox-inactive (N/O)(4) coordination environment using one imidazole residue. Expansion of the octarepeat region by one repeat (five PHGGGWGQ repeats) yields a three-histidine (N/O)(6) coordination environment for Zn-II and a two-histidine (N/O)(4) coordination environment for Cu-II at low copper concentrations. This Cu-II[(N/O)(2)-histidine(2)] coordination motif is redox-active and capable of generating H2O2 under reducing aerobic conditions.