Ubiquinol :cytochrome c oxidoreductase

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. 

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. 

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). 

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. 

The mitochondrial respiratory chain is composed of more than 80 proteins grouped into 5 distinct complexes that form an integrated electron transfer chain (ETC, Figure 4). Initiation of electron transport takes place either from complex I (reduced nicotinamide adenine diphosphate (NADH)—ubiquinone oxidor-eductase) or from complex II (succinate—ubiquinone oxidoreductase) to complex III (ubiquinol—cytochrome c oxidoreductase) by ubiquinone (UQ, coenzyme Q, 39). As shown in Scheme 1, ubiquinone is reduced to... 

In metazoans, the electron transport chain consists of four integral membrane complexes localized to the inner mitochondrial membrane complex I (NADH-ubiquinone oxidoreductase), complex II (succinate-ubiquinone oxidoreductase), complex III (ubiquinol-cytochrome c oxidoreductase) and complex IV (cytochrome c oxidase), plus coenzyme Q (ubiquinone) and cytochrome c. As first shown by Fry and Beesley (1991), the plasmodial electron transport chain differs from the metazoan system in lacking complex I however, a single subunit NADH dehydrogenase is present and is homologous to that found in plants, bacteria and yeast but not in animals (Krungkrai, 2004 Vaidya, 2004,2005 van Dooren et al., 2006). 

Many cytochromes in energy-transducing organelles are membrane-bound proteins. Present in mitochondria and in purple photosynthetic bacteria, the cyt bc complex (also called ubiquinol-cytochrome c oxidoreductase, or complex III) catalyzes the electron transfer from ubiquinol to ferricyt c and pumps protons from the matrix to the cytosol.The catalytic core of the cyt bci complex comprises three redox-active subunits they are a cyt b with two ft-type hemes bn and bf), a cyt Cl, and a Reiske iron sulfur protein. While this catalytic core has enzymatic activity in some a proteobacteria like Paracoccus, Rhodospirillum rubrum, and Rb. capsulatus, mitochondrial cyt bci complexes have an additional seven or eight subunits." ... 

Between Mitochondrial Succinate-Ubiquinone and Ubiquinol-Cytochrome C Oxidoreductase Probed by Sensitivity to Quinone-Related Inhibitors, II,... 

General Structural Description of Complex III (Ubiquinol cytochrome c Oxidoreductase)... 

The ubiquinol-cytochrome c oxidoreductase (cytochrome bc complex) of Rhodobacter sphaeroides is an integral component of the intracytoplasmic membrane (ICM) and functions in light-driven cyclic electron flow and the conservation of radiant energy as an electrochemical proton gradient. Previous studies on the assembly of electron transfer constituents in/ , sphaeroides have demonstrated that complete cycles of electron flow do not occur merely upon insertion of newly synthesized reaction centers at sites of initiation of ICM growth, but instead, subsequent synthesis and assembly of redox centers of the be complex are required [1]. To further characterize the assembly process and for detailed structural investigations, the complex was purified and antibodies were raised against the isolated polypeptide constituents. In this report, results on the localization and levels of the feCj complex in various membrane fractions are presented a detailed description of the structurd work will appear elsewhere [2]. 

Salerno JC. 1984. Cytochrome electron spin resonance Une shapes, ligand fields and components stochiometry in ubiquinol-cytochrome-c oxidoreductase. J Biol Chem 259(4) 2331-2336. 

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