Respiratory Conservation of Energy with Dioxygen: Cytochrome c Oxidase

Abstract

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 O₂reduction site, which is composed of Fe (heme a 3) and Cu (CuB), drives a non-sequential four-electron transfer for reduction of O₂to water without releasing any reactive oxygen species. These data provide the crucial structural basis to solve a long-standing problem, the mechanism of the O₂reduction.Time-resolved resonance Raman and charge translocation analyses revealed the mechanism for coupling between O₂reduction and the proton pump: O₂is received by the O₂reduction 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 O₂fully 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 O₂reduction 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 O₂reduction 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 a 3 reduces O₂as proposed in 1978.

Effect of uric acid on mitochondrial function and oxidative stress in hepatocytes

“…The results of our study suggested that high levels of UA in hepatocytes significantly reduced the SDH activity in the mitochondria, indicating that UA damages the mitochondrial membrane structure and inhibits mitochondrial energy metabolism. CCO is an essential enzyme that produces a proton gradient for ATP production during mitochondrial oxidative phosphorylation (Yoshikawa et al, 2015). The results of our study suggest that high levels of UA in hepatocytes significantly reduced CCO activity in the mitochondria.…”

Snapshot of an oxygen intermediate in the catalytic reaction of cytochromecoxidase

“…However, its functional role has been controversial, despite the fact that it was supported by mutagenesis studies of a hybrid bovine/human construct, demonstrating that mutations of residues along the Hchannel blocked proton translocation, whereas those along the D-channel did not (33). In addition, several detailed models have been proposed for proton translocation via the Hchannel, such as those involving the changes in the redox state of heme a (Redox Bohr effect) (34,35) and proton accumulation mechanisms (1,14,36,37), although their role in proton translocation has been disputed (38). Recently, a detailed analysis of the H-channel by molecular dynamics simulations was reported by Sharma et al (39).…”

Uncovering the Transmembrane Metal Binding Site of the Novel Bacterial Major Facilitator Superfamily-Type Copper Importer CcoA

“…Copper (Cu) is a transition metal that is required for many important cellular processes exploiting its redox properties, such as aerobic respiration (e.g., cytochrome c oxidases [Cox]) ( 1 ), oxidation of chemicals (e.g., multicopper oxidases) ( 2 ), and elimination of free oxygen radicals (e.g., Cu-Zn superoxide dismutases) ( 3 ). However, like many essential metals, excessive concentrations of Cu are toxic to cells.…”