Cytochrome c oxidase is the last component of the respiratory chain in mitochondria and many bacteria, where it catalyses the reduction of O2 to water. The structures of cytochrome c oxidases from a number of organisms such as bacteria and cow, are known and show remarkable similarities in both structure and function.

The enzyme is bound to the inner mitochondrial membrane or in the cell membrane of bacteria. During reduction of dioxygen, protons are taken up from the inside of the membrane. In addition, the excess energy released in the oxygen-reduction reaction is used to pump protons across the membrane. The energy conserved by generation of the proton gradient across the membrane is utilised for synthesis of ATP by ATP synthase. The reduction of O2 in the enzyme must be carefully controlled in order to utilise the free energy of the reaction for proton pumping and to avoid the release of partially-reduced O2 intermediates which, if released, may be harmful to the cell.

In this work, the mechanisms controlling the electron- and proton-transfer events during reduction of dioxygen in cytochrome c oxidase have been studied. The main focus has been on cytochrome c oxidase from bovine heart and from the bacterium Rhodobacter sphaeroides. Comparative studies have also been made with a similar enzyme from the thermophilic bacterium Rhodothermus marinus. The results from this work have been summarised in a model that describes the oxygen-reduction chemistry and its coupling to the proton-pumping machinery.