Cytochrome c oxidase

The enzyme cytochrome c oxidase or Complex IV, (was , now reclassified as a translocase EC 7.1.1.9) is a large transmembrane protein complex found in bacteria, archaea, and mitochondria of eukaryotes. It is the last enzyme in the respiratory electron transport chain of cells located in the membrane. It receives an electron from each of four cytochrome c molecules and transfers them to one oxygen molecule and four protons, producing two molecules of water. In addition to binding the four protons from the inner aqueous phase, it transports another four protons across the membrane, increasing the transmembrane difference of proton electrochemical potential, which the ATP synthase then uses to synthesize ATP. The complex is a large integral membrane protein composed of several metal prosthetic sites and 14 protein subunits in mammals. In mammals, eleven subunits are nuclear in origin, and three are synthesized in the mitochondria. The complex contains two hemes, a cytochrome a and cytochrome a_3, and two copper centers, the Cu_A and Cu_B centers. In fact, the cytochrome a_3 and Cu_B form a binuclear center that is the site of oxygen reduction. Cytochrome c, which is reduced by the preceding component of the respiratory chain (cytochrome bc1 complex, Complex III), docks near the Cu_A binuclear center and passes an electron to it, being oxidized back to cytochrome c containing Fe^3+. The reduced Cu_A binuclear center now passes an electron on to cytochrome a, which in turn passes an electron on to the cytochrome a_3>-Cu_B binuclear center. The two metal ions in this binuclear center are 4.5 Å apart and coordinate a hydroxide ion in the fully oxidized state. Crystallographic studies of cytochrome c oxidase show an unusual post-translational modification, linking C6 of Tyr(244) and the ε-N of His(240) (bovine enzyme numbering). It plays a vital role in enabling the cytochrome a_3- Cu_B binuclear center to accept four electrons in reducing molecular oxygen and four protons to water.