Copper-zinc superoxide dismutase (CuZnSOD) catalyzes the disproportionation of superoxide to hydrogen peroxide and dioxygen at diffusion controlled rates, Previous mechanistic studies have focused on the dramatic electrostatic guidance mechanism by which superoxide is drawn into the active site of this enzyme. Another striking bur : less understood feature of this enzyme is its ability to dismutate superoxide over a wide range of pH (5-9.5) without any change in rate-determining step or structural changes at the active site copper. To investigate the explanation for this pH independence, we have redetermined the rate : of superoxide disproportionation, k(cat), catalyzed by the zinc-deficient form (Cu-apoSOD) of the enzyme as a function of pH and have found that it is pH-dependent, in contrast to the native enzyme, even under conditions in which the topper ion does not leave the native copper-binding site, In addition, we have determined the rare of reduction, k(1), of Cu-apoSOD by superoxide and have found that this step of the catalytic cycle is pH-independent. We conclude that the reoxidation rate, k(2), of the catalytic cycle is pH-dependent for Cu-apoSOD. These results have led us to propose that the key role of the zinc and of the hist idyl imidazolate that bridges copper and zinc in CuZnSOD is to aid in the rapid dissociation of the product peroxide.