First-principles density functional theory calculations have been carried out for various models of oxidized and reduced Cu, Zn superoxide dismutase (SOD) active sites and the adduct with its substrate, the superoxide anion. The electronic structure of the first stages of the enzymatic reaction is described. The electron-transfer process between the SOD copper(II) ion and superoxide is discussed. A model potential energy functional derived from these calculations allows the interaction to be discussed for any configuration of the SOD Cu ion, the superoxide, and Arg141-the most important residue in terms of electrostatic effects. The model indicates that superoxide is bound to Cu and that Arg141 easily separates from the superoxide bonded to the active site.