Although the number of proteins that utilize copper as an essential cofactor is not exceptionally large, the roles played by many of these proteins appear central to the success of the organism. For example, the ubiquitous copper/iron protein cytochrome oxidase functions in oxidative phosphorylation, catalyzing the final reduction of dioxygen to water coupled to the generation of a proton gradient and ATP formation (cf., review by Ferguson-Miller and Babcock, in this issue). A second biologically essential protein interconverting dioxygen and its reduced forms is the copper/zinc superoxide dismutase, which catalyzes a reduction of 1 mol of superoxide to hydrogen peroxide coupled to an oxidation of a second mole of superoxide to molecular oxygen. Copper is found to be especially important in the nervous system of higher eukaryotes, appearing in proteins responsible for the biogenesis of both catecholamine and …