[Redox-dependent protonation of cytochrome oxidase hemes in submitochondrial particles of the bovine heart]

Abstract

Potentiometric titrations of cytochrome oxidase in bovine heart submitochondrial particles were carried out within the pH range 5.3-9.0 in the alpha-band of heme absorption. The data obtained were analyzed within a model of non-interacting hemes and in accordance with a "neoclassical" scheme implying heme--heme interactions between cytochromes a and a3. The individual pH-dependencies of half-reduction potentials of cytochromes a and a3 were determined. It was found that the redox-dependent protonation (the oxidation Bohr effect) is characteristic of the both cytochromes; however, the reduction of one of the hemes significantly diminishes the Bohr effect for the second heme. The redox transitions of cytochrome a are coupled to ionization of at least two heme-linked acid-base groups of the enzyme with pK1(red) in the acidic and pK2(ox) in the alkaline regions of pH, whereas the pH dependence of E0' for cytochrome a3 fits to the model containing one hemi-linked group with a pKred in the acidic region.