Ubiquinol oxidoreductase

Fig. 13.1.1. Schematic overview of mitochondrial oxidative phosphorylation. A part of the mitochondrion is represented, showing the outer mitochondrial membrane (OMM), inner mitochondrial membrane (IMM) and crista (an invagination of the inner membrane). Substrates for oxidation enter the mitochondrion through specific carrier proteins, e.g., the pyruvate transporter, (PyrT). Reducing equivalents from fatty acyl CoA dehydrogenases, pyruvate dehydrogenase and the TCA cycle are delivered to the electron transport chain through NADH, succinate ubiquinol oxidoreductase (SQO), electron transfer flavoprotein (ETF) and its ubiquinol-...

Electrons from NADH, together with two protons, are transferred to ubiquinone to form ubiquinol by complex I (NADH ubiquinone oxidoreductase). Complex I... 

Gardner et al. [165] have shown that the redox-cycling agent phenazine methosulfate (PMS), mitochondrial ubiquinol-cytochrome c oxidoreductase, or hypoxia inactivated aco-nitase in mammalian cells. It has been proposed that the inactivation of aconitase is mediated by superoxide produced by prooxidants because the overproduction of mitochondrial MnSOD protected aconitase from inactivation by the prooxidants mentioned above except hyperoxia. Later on, the reaction of superoxide with aconitases began to be considered as one of the most important ways to NTBI generation in vivo. 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

Complex III Ubiquinone to Cytochrome c The next respiratory complex, Complex III, also called cytochrome focx complex or ubiquinone icytochrome c oxidoreductase, couples the transfer of electrons from ubiquinol (QH2) to cytochrome c with the vectorial transport of protons from the matrix to the intermembrane space. The determination of the complete structure of this huge complex (Fig. 19-11) and of Complex IV (below) by x-ray crystallography, achieved between 1995 and 1998, were landmarks in the study of mitochondrial electron transfer, providing the structural framework to integrate the many biochemical observations on the functions of the respiratory complexes. 

These complexes are usually named as follows I, NADH-ubiquinone oxidoreductase II, succinate-ubiquinone oxidoreductase III, ubiquinol-cytochrome c oxidoreductase IV, cytochrome c oxidase. The designation complex V is sometimes applied to ATP synthase (Fig. 18-14). Chemical analysis of the electron transport complexes verified the probable location of some components in the intact chain. For example, a high iron content was found in both complexes I and II and copper in complex IV. 

Complex IV. Cytochrome c oxidase (ubiquinol-cytochrome c oxidoreductase). Complex IV from mammalian mitochondria contains 13 subunits. All of them have been sequenced, and the three-dimensional structure of the complete complex is known (Fig. 18-10).125-127 The simpler cytochrome c oxidase from Paracoccus denitrificans is similar but consists of only three subunits. These are homologous in sequence to those of the large subunits I, II, and III of the mitochondrial complex. The three-dimensional structure of the Paracoccus complex is also known. Its basic structure is nearly identical to that of the catalytic core of subunits I, II, and III of the mitochondrial complex (Fig. 18-10,A).128 All three subunits have transmembrane helices. Subunit III seems to be structural in function, while subunits I and II contain the oxidoreductase centers two hemes a (a and a3) and two different copper centers, CuA (which contains two Cu2+) and a third Cu2+ (CuB) which exists in an EPR-silent exchange coupled pair with a3. Bound Mg2+ and Zn2+ are also present in the locations indicated in Fig. 18-10. 

The third protein complex in this electron-transfer chain (complex 111) is ubiquinol cytochrome c oxidoreductase (E.C. 1.10.2.2), or commonly known as cytochrome be, complex named after the its b-type and c-type cytochrome subunits. Probably the best-understood one among the complexes, be, complex catalyses electron transfers between two mobile electron carriers the hydrophobic molecule ubiquinone (Q) and the small soluble haem-containing protein cytochrome c. Two protons are translocated across the membrane per quinol oxidized (Hinkel, 1991 Crofts, 1985 Mitchell, 1976). 

Gurbiel, R. J., Obnishi, T., Robertson, D. E., Daldal, F., and Hoffman, B. M., 1991, Q-band ENDOR spectra of the Rieske protein from Rhodobactor capsulatus ubiquinol-cytochrome c oxidoreductase show two histidines coordinated to the [2Fe-2S] cluster. Biochemistry 30 11579nll584. 

Robertson, D. E., Prince, R. C., Bowyer, J. R., Matsuura, K., Dutton, P. L., and Ohnishi, T., 1984, Thermodynamic properties of the semiquinone and its binding site in the ubiquinol-cytochrome c (c2) oxidoreductase of respiratory and photosynthetic systems, J. Biol. Chem. 2S9 1758nl763. 

The first complex in the respiratory chain, NADH ubiquin-one oxidoreductase (complex 1), transfers two electrons from matrix NADH to ubiquinone to form ubiquinol. This... 

Protein-membrane association via a post-translational modification introduces the notion of dynamic association and partitioning of proteins between the membrane phase of the cells and the aqueous phase (cytosol or inner phase of organelles). Consequently, such proteins can be found both as membrane-associated and membrane-free, which is not the case with intrinsic membrane proteins which are strictly membrane embedded. Another type of association to membrane is mediated by protein-protein interactions with other membrane proteins. A typical example of this situation is provided by the respiratory complexes. In the case of ubiquinol-cytochrome c oxidoreductase, core proteins 1 and 2 does not show any interaction with the lipid membrane, but only with the protein subunits spanning the membrane (e.g. cytochrome b) (Iwata et al. 1998). 

Electrons Flow from Ubiquinol to Cytochrome c Through Q-Cytochrome c Oxidoreductase... 

The two protons from plastoquinol are released into the thylakoid lumen. This reaction is reminiscent of that catalyzed by ubiquinol cytochrome c oxidoreductase in oxidative phosphorylation. Indeed, most components of the enzyme complex that catalyzes the reaction, the cytochrome bf complex, are homologous to those of ubiquinol cytochrome c oxidoreductase. The cytochrome hf complex includes four subunits a 23-kd cytochrome with two Z>-type hemes, a 20-kd Rieske-type Fe-S protein, a 33-kd cytochrome/with a c-type cytochrome, and a 17-kd chain. 

Van Ark, G. (1980) Electron Transfer Through the Ubiquinol Ferricytochrome c Oxidoreductase Segment of the Mitochondrial Respiratory Chain, Ph.D. Thesis, University of Amsterdam. 

Due to the extension of the topics covered in this chapter, the reference list has been kept to a minimum and only a few representative research papers and reviews are mentioned per section. We wish to apologize for this to all our colleagues. For more detailed information the reader should refer to the following reviews For bacterial RC, Refs. 1, 2, 3, 5, 7, 292-294 for PSI-RC and PSII-RC, Refs. 1, 3-5, 295 for ubiquinol-cyt. c (plastocyanin) oxidoreductase, Refs. 93, 109, 116, 139, 140, 296 for the oxygen-evolving complex, 93, 297-301. 

Cytochromes, as components of electron transfer chains, must interact with the other components, accepting electrons from reduced donor molecules and transferring them to appropriate acceptors. In the respiratory chain of the mitochondria, the ubiquinolxytochrome c oxidoreductase, QCR or cytochrome bc complex, transfers electrons coming from Complexes 1 and 11 to cytochrome c. The bc complex oxidises a membrane-localised ubiquinol the redox process is coupled to the translocation of protons across the membrane, in the so-called proton-motive Q cycle, which is presented in a simplified form in Figure 13.14. This cycle was first proposed by Peter Mitchell 30 years ago and substantially confirmed experimentally since then. The Q cycle in fact consists of two turnovers of QH2 (Figure 13.14). In both turnovers, the lipid-soluble ubiquinol (QH2) is oxidized in a two-step reoxidation in which the semiquinone CoQ is a stable intermediate, at the intermembrane face of the mitochondrial inner membrane. It transfers one electron to the Rieske iron—sulfur protein (ISP), one electron to one of the two cytochrome b haems (bi), while two protons are transferred to the intermembrane space. In both of the Q cycles, the cytochrome bi reduces cytochrome bfj while the Reiske iron—sulfur cluster reduces cytochrome c/. The cytochrome ci in turn reduces the water-soluble cytochrome c, which transfers its electrons to the terminal oxidase, cytochrome c oxidase, described above. In one of the two Q cycles, reduced cytochrome bf reduces Q to the semiquinone, which is then reduced to QH2 by the second reduced cytochrome bn- The protons required for this step are derived from the matrix side of the membrane. The overall outcome of the two CoQ cycles (10) (/ — matrix o — intermembrane space) is... 

N Gabeiiini, JR Bowyer, E Hurt, BA Meiandri and G Hauska (1982) A cytochrome bci complex with ubiquinol-cytochromec2 oxidoreductase activity from Rps sphaeroides GA. EurJ Biochem 126 105-111... 

---