Cytochrome c oxidase (CcO) is the terminal oxidase of cell respiration which reduces molecular oxygen (O2) to H2O coupled with the proton pump. For elucidation of the mechanism of CcO, the three-dimensional location and chemical reactivity of each atom composing the functional sites have been extensively studied by various techniques, such as crystallography, vibrational and time-resolved electronic spectroscopy, since the X-ray structures (2.8 Å resolution) of bovine and bacterial CcO have been published in 1995.
X-ray structures of bovine CcO in different oxidation and ligand binding states showed that the O2 reduction site, which is composed of Fe (heme a3) and Cu (CuB), drives a non-sequential four-electron transfer for reduction of O2 to water without releasing any reactive oxygen species. These data provide the crucial structural basis to solve a long-standing problem, the mechanism of the O2 reduction.
Time-resolved resonance Raman and charge translocation analyses revealed the mechanism for coupling between O2 reduction and the proton pump: O2 is received by the O2 reduction site where both metals are in the reduced state (R-intermediate), giving the O2-bound form (A-intermediate). This is spontaneously converted to the P-intermediate, with the bound O2 fully reduced to 2 O2−. Hereafter the P-intermediate receives four electron equivalents from the second Fe site (heme a), one at a time, to form the three intermediates, F, O, and E to regenerate the R-intermediate. Each electron transfer step from heme a to the O2 reduction site is coupled with the proton pump.
X-ray structural and mutational analyses of bovine CcO show three possible proton transfer pathways which can transfer pump protons (H) and chemical (water-forming) protons (K and D). The structure of the H-pathway of bovine CcO indicates that the driving force of the proton pump is the electrostatic repulsion between the protons on the H-pathway and positive charges of heme a, created upon oxidation to donate electrons to the O2 reduction site. On the other hand, mutational and time-resolved electrometric findings for the bacterial CcO strongly suggest that the D-pathway transfers both pump and chemical protons. However, the structure for the proton-gating system in the D-pathway has not been experimentally identified. The structural and functional diversities in CcO from various species suggest a basic proton pumping mechanism in which heme a pumps protons while heme a3 reduces O2 as proposed in 1978.
cell respiration cytochrome c oxidase heme/copper terminal oxidase membrane protein O2 reduction without forming ROS proton pump
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This work is supported by a Grant-in-Aid for the Global Center of Excellence Program (to S. Yoshikawa) and for Scientific Research (A) 2247012 (to S. Yoshikawa), each provided by the Japanese Ministry of Education, Culture, Sports, Science and Technology, and supported by CREST. S. Yoshikawa is “Senior Visiting Scientist in the Riken Harima Institute”.
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