Three Different Midpoint Oxidation-Reduction Potentials of Cytochrome Oxidase from Bovine Heart Muscle

Abstract

The equilibrium concentrations of the oxidized and reduced forms in mixture of cytochrome oxidase [EC 1.9.3.1] from bovine heart muscle or cytochrome c from bovine heart muscle or cytochrome C₂ from Rhodospirillum rubrum were measured under anaerobic conditions. These measurements were carried out in the presence of an appropriately low concentration of ascorbate such that the ascorbate could reduce cytochrome c or cytochrome C₂ at an appropriately slow rate without affecting the cytochrome oxidase at all. Cytochrome oxidase showed three different E_m, 7 values, +0.360 volt (n=1.7), +0.300 volt (n=1.7) and +0.208volt (n=1.5), indicating that it possessed three different types of heme a moieties: these are called heme a-1, heme a-2 and heme a-3 moieties, respectively. The EE_{m, 7}, value of the carbon monoxide complex of the heme a moiety was +0.269 volt with n=3–4. For the binding of carbon monoxide, it was concluded that the heme a-2 moiety was the principal factor, the other heme a moieties the complementary factors.