STUDIES OF CYTOCHROME C OXIDASE

Cytochrome c oxidase (CcO) accepts electrons from cytochrome c to reduce oxygen molecules to water in the  final step of cellular respiration as shown:   4H⁺ + O₂ + 4e^- ====>  2H₂O   The enzyme also pumps protons against an electrochemical gradient, and the energy stored in this gradient is used for the production of ATP    from ADP. The enzyme contains 12 subunits, of which subunits I,II, and III form the core of a functional enzyme. Within this core, the redox active metal centers are located.   Subunit I contains the ligands for heme  a, heme a₃, and Cu_B, which lie in a plane within the membrane.   The Cu_A is located in subunit II of CcO which is believed to be the primary acceptor of electrons from the one electron carrier cytochrome c.  Based on the crystal structure , Cu_A is found to be a binuclear center bridged by a di-thiolate ligand, and bound to di-histidyl, a methionine sulfur, and a carbonyl oxygen of Glu-198.   The site of molecular oxygen reduction is a binuclear metal center, heme a₃ and Cu_B. The Cu_A site, together with the other heme site, heme a, mediate electron transfer from cytochrome c to the oxygen binding site.  Recently we have made an attempt to break the coupling between Cu_B and heme a₃.  Upon incubating the CcO with sat. ammonium sulfate at room temperature overnight and centrifugation, a new signal was obtained.