C-Oxidoreductase

L-lactate-cytochrome c-oxidoreductase (flavocytochrome was isolated for the first time from the thermo-tolerant yeast H. polymorpha. The mentioned above enzyme preparations were used for construction of the biorecognition elements of electrochemical sensors. 

Figure 12-7. Proposed sites of inhibition (0) of the respiratory chain by specific drugs, chemicals, and antibiotics. The sites that appear to support phosphorylation are indicated. BAL, dimercaprol. TTFA, an Fe-chelating agent. Complex I, NADHiubiquinone oxidoreductase complex II, succinate ubiquinone oxidoreductase complex III, ubiquinohferricytochrome c oxidoreductase complex IV, ferrocytochrome ctoxygen oxidoreductase. Other abbreviations as in Figure 12-4.

The subcellular location of PG was studied in cells disrupted by osmotic lysis through formation and disruption of sphaeroplasts from self-induced anaerobically-grown cells. A discontinuous sucrose-density gradient produced four bands labelled I, II, III and IV. Band I included many vesicles and a peak of alkaline phosphatase activity (a vacuolar marker in yeasts), NADPH cytochrome c oxidoreductase activity, an endoplasmic reticulum marker, and... 

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. 

Complex III (CoQ cytochrome c oxidoreductase) transfers electrons from CoQ to cytochrome c, through a sequence of cytochrome and iron-sulfur cofactors. Here, Alf for the couple CoQ/cytochrome c is 0.19 V, corresponding to a AG° of —36.7 kJ/mol, again enough to power the synthesis of an ATP molecule and to ensure that protons are pumped across the inner mitochondrial membrane. 

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). 

Capeillere-Blandin, C., Barber, M. J., Bray, R. C. Differences in electron transfer rates among prosthetic groups between two homologous flavocytochrome b2 (L-lactate cytochrome c oxidoreductase) from different yeasts. In Flavins and flavoproteins (Massey, V., Williams, C. H. eds.) pp. 838-843, New York, Amsterdam, Oxford, Elsevier/North Holland 1982... 

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. 

Beardmore-Gray, M., O Keeffe, D. T., and Anthony, C., 1983, The methanokcytochrome c oxidoreductase activity of methylotrophs. Journal of General Microbiology 129 9239933. 

Flavocytochromes 2 2-hydroxyacid dehydrogenases found in the inter-membrane space of yeast mitochondria where they couple oxidation of the substrate to reduction of cytochrome c. Examples include the enzymes from Saccharomyces cerevisiae and Hansenula anomala, both of which are l-lactate dehydrogenases (Chapman et al., 1998), and the enzyme from Rhodotorula graminis which is a L-mandelate dehydrogenase (Ilias et al., 1998). This article will concentrate on the flavocytochrome 2 (L-lactate cytochrome c oxidoreductase) from S. cerevisiae (Bakersi yeast), since this is by far the most studied of these enzymes (Chapman et al., 1991). Therefore, throughout this article, the term flavocytochrome 2 will refer specifically to the enzyme from S. cerevisiae unless otherwise stated. 

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). 

Crofts, A. R., 1985, The mechanism of ubiquinokcytochrome c oxidoreductases of mitochondria and of Rhodopseudomonas sphaeroides, in The Enzymes of Biological Membranes, Volume 4 (A. N. Martonosi, ed.). Plenum Press, New York, pp. 347n382. 

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