The cytochrome c oxidase from Paracoccus denitrificans does not change themetal center ligation upon reduction

Cytochrome c oxidase catalyzes the reduction of oxygen to water. This proce ss is accompanied by the vectorial transport of protons across the mitochon drial or bacterial membrane ("proton pumping"). The mechanism of proton pum ping is still a matter of debate. Many proposed mechanisms require structur al changes during the reaction cycle of cytochrome c oxidase. Therefore, th e structure of the cytochrome c oxidase was determined in the completely ox idized and in the completely reduced states at a temperature of 100 K. No l igand exchanges or other major structural changes upon reduction of the cyt ochrome c oxidase from Paracoccus denitrificans were observed. The three hi stidine Cu, ligands are well defined in the oxidized and in the reduced sta tes. These results are hardly compatible with the "histidine cycle" mechani sms formulated previously.