Cytochrome c oxidase (ferricytochrome c: oxygen oxidoreductase, EC 1.9.3.1) is a complex enzyme that can catalyze the transfer of four electrons from its substrate ferricytochrome c to molecular oxygen and at the same time pump protons across the inner mitochondrial membrane from the matrix to the cytosol space (Capaldi and Briggs, 1976; Wikstrom and Krab, 1979; Azzi and Casey, 1979; Azzi, 1981). In this process a proton gradient is generated in which the free energy of the redox process is conserved (Wikstrom, 1977; Casey et al., 1979a; Sigel and Carafoli, 1979; Mitchell, 1966). The heme content of cytochrome c oxidase indicates a minimum molecular weight for the enzyme of approximately 140,000. The same molecular weight can be calculated for the monomer structure by summing the molecular weights of the seven subunits of the enzyme. It contains 4 one-electron redox centers, namely two a hemes and two coppers.