The proton/electron coupling ratio at heme a and Cu-A in bovine heart cytochrome c oxidase

Measurements of the H+/heme a, Cu-A ratios for proton-electron coupling at these centers (redox Bohr effect) in CO-inhibited cytochrome c oxidase puri fied from bovine heart mitochondria, both in the soluble state and reconsti tuted in Liposomes, are presented. In the soluble oxidase, the H+/heme a, C u-A ratios were experimentally determined upon oxidation by ferricyanide of these centers as well as upon their reduction by hexammineruthenium(II). T hese measurements showed that in order to obtain H+/heme a, Cu-A ratios app roaching 1, one-step full oxidation of both metal centers by ferricyanide h ad to be induced by a stoicheiometric amount of the oxidant. Partial stepwi se oxidation or reduction of heme a and Cu-A did produce H+/heme a, Cu-A ra tios significantly lower or higher than 1, respectively. The experimental H +/heme a, Cu-A ratios measured upon stepwise reduction/oxidation of the met als were reproduced by mathematical simulation based on the coupling of oxi de-reduction of both heme a and Cu-A to pK shifts of common acid-base group s. The vectorial nature of the proton-electron coupling at heme a/Cu-A was analyzed by measuring pH changes in the external bulk phase associated with oxidoreduction of these redox centers in the CO-inhibited oxidase reconsti tuted in liposomes. The results show that the proton release associated wit h the oxidation of heme a and Cu-A takes place in the external aqueous phas e. Protons taken up by the oxidase upon rereduction of the centers derive, on the other hand, from the inner space. These results provide evidence sup porting the view that cooperative proton-electron coupling at heme a/Cu-A i s involved in the proton pump of the oxidase.