Ubiquinone:cytochrome oxidoreductase

Nonetheless, photosynthesis did not evolve immediately at the origin of life. The failure to discover photosynthesis in the domain of Archaea implies that photosynthesis evolved exclusively in the domain of Bacteria. Eukaryotes appropriated through endosymbiosis the basic photosynthetic units that were the products of bacterial evolution. All domains of life do have electron-transport chains in common, however. As we have seen, components such as the ubiquinone-cytochrome c oxidoreductase and cytochrome hf family are present in both respiratory and photosynthetic electron-transport chains. These components were the foundations on which light-energy-capturing systems evolved. 

The subject matter of this chapter is confined to the role of cytochrome as a secondary electron donor, D, i.e., the interaction with the photooxidized primary electron donor formed during the photochemical charge-separation process in photosynthetic bacteria. Another cytochrome, present essentially as a ubiquinone-cytochrome c oxidoreductase in the cytochrome-6ci complex, is particularly important in energy conservation and the creation of a proton gradient for ATP synthesis in of photosynthetic bacteria. This cytochrome fee, complex, is discussed in Chapter 35 dealing with proton transport. 

Complex III Ubiquinone to Cytochrome c The next respiratory complex. Complex III, also called cytochrome bci complex or ubiquinone cytochrome 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. 

Figure 2. The effect of the novel inhibitor of ubiquinone cytochrome c oxidoreductase on the growth ofthe wild-type ( ) andG143A( ) strains of S. cerevisiae using lactic acid as the carbon-source.

Complex III Ubiquinone cytochrome c Oxidoreductase (The Cytochrome bci Complex)... 

Ill Iron-sulfur protein Cytochrome b (b by) Cytochrome c. Iron-sulfur centers Heme (noncovalent) Heme (covalently bound) Ubiquinone cytochrome c oxidoreductase... 

Darzon A, Vandenberg CA, Schonfeld M, Ellisman MH, Spitzer NC and Montal M (1980) Reassembly of protein-lipid complexes into large bilayer vesicles perspectives for membrane reconstitution, Proc.Natl.Acad.Sci USA 77, 239-243. Dutton PL, Mueller P, O Keefe DP, Packham NK, Prince RC and Tiede DM (1982) Electrogenic reactions of the photochemical reaction center and the ubiquinone-cytochrome b/c oxidoreductase, Curr.Top.Membr.Transp. 16, 323-343. Feher G and Okamura MY (1978) Chemical composition and properties of reaction centers. In Clayton RK and Sistrom WR, ed. The photosynthetic bacteria, pp. 349-386. New York Plenum Press. 

Another pathway is the L-glycerol 3-phosphate shuttle (Figure 11). Cytosolic dihydroxyacetone phosphate is reduced by NADFl to s.n-glycerol 3-phosphate, catalyzed by s,n-glycerol 3-phosphate dehydrogenase, and this is then oxidized by s,n-glycerol 3-phosphate ubiquinone oxidoreductase to dihydroxyacetone phosphate, which is a flavoprotein on the outer surface of the inner membrane. By this route electrons enter the respiratory chain.from cytosolic NADH at the level of complex III. Less well defined is the possibility that cytosolic NADH is oxidized by cytochrome bs reductase in the outer mitochondrial membrane and that electrons are transferred via cytochrome b5 in the endoplasmic reticulum to the respiratory chain at the level of cytochrome c (Fischer et al., 1985). 

Qrunones can accept one or two electrons to form the semiquinone anion (Q ") and the hydroquinone dianion (Q ). Single-electron reduction of a quinone is catalyzed by flavoenzymes with relatively low substrate selectivity (Kappus, 1986), for instance NADPH cytochrome P-450 reductase (E.C. 1.6.2.3), NADPH cytochrome b5 reductase (E.C. 1.6.2.2), and NADPH ubiquinone oxidoreductase (E.C. 1.6.5.3). The rate of reduction depends on several interrelated chemical properties of a quinone, including the single-electron reduction potential, as well as the number, position, and chemical characteristics of the substituent(s). The flavoenzyme DT-diphorase (NAD(P)H quinone acceptor oxidoreductase E.C. 1.6.99.2) catalyzes the two-electron reduction of a quinone to a hydroquinone. 

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). 

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. 

Fig. 5.2. Possible metabolic pathways in facultative anaerobic mitochondria. Shaded boxes show components of the electron-transport chain used during hypoxia, open boxes are components used during aerobiosis, and the hatched boxes (complex I and ATP-synthase) are components used under aerobic as well as anaerobic conditions. ASCT acetate succinate CoA-transferase, C cytochrome c, Cl, CIII and CIV complexes I, III and IV of the respiratory chain, CITR citrate, ECR enoyl-CoA reductase (such as present in Ascaris suum), ETF electron-transfer flavoprotein, ETF RQ OR electron-transfer flavoproteimrhodoquinone oxidoreductase, FRD fumarate reductase, FUM fumarate, MAE malate, OXAC oxaloacetate, PYR pyruvate, RQ rhodoquinone, SDH succinate dehydrogenase, SUCC succinate, Succ-CoA succinyl-CoA, TER trans-2-enoyl-CoA reductase (such as present in E. gracilis), UQ ubiquinone...

Figure 17.4 The electron transport chain of mitochondria. Triangles indicate sites of inhibition by various compounds. Cyt, cytochrome ETF, electron transfer flavoprotein. (Reproduced with permission from Moreadith RW, Batshaw ML, Ohnishi T, Kerr D, Knox B, Jackson D, Hruben R, Olson J, Reynafarje B, Lehninger AL. Deficiency of the iron-sulfur clusters of mitochondrial reduced nicotinamide-adenine dinucleotide-ubiquinone oxidoreductase (complex I) in an infant with congenital lactic acidosis J Clin Invest 74 685-697, 1984.)...

Figure 7-1. Pathways of fuel metabolism and oxidative phosphorylation. Pyruvate may be reduced to lactate in the cytoplasm or may be transported into the mitochondria for anabolic reactions, such as gluconeogenesis, or for oxidation to acetyl-CoA by the pyruvate dehydrogenase complex (PDC). Long-chain fatty acids are transported into mitochondria, where they undergo [ -oxidation to ketone bodies (liver) or to acetyl-CoA (liver and other tissues). Reducing equivalents (NADH, FADII2) are generated by reactions catalyzed by the PDC and the tricarboxylic acid (TCA) cycle and donate electrons (e ) that enter the respiratory chain at NADH ubiquinone oxidoreductase (Complex 0 or at succinate ubiquinone oxidoreductase (Complex ID- Cytochrome c oxidase (Complex IV) catalyzes the reduction of molecular oxygen to water, and ATP synthase (Complex V) generates ATP fromADP Reprinted with permission from Stacpoole et al. (1997).

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). 

Robertson, D. E., Daldal, F., and Dutton, P. L., 1990, Mutants of ubiquinol-cytochrome c2 oxidoreductase resistant to Qo site inhibitors consequences for ubiquinone and ubiquinol affinity and catalysis. Biochemistry 29 11249nll260. 

The reaction centre can also be viewed as an enzyme, namely a cytochrome C2 ubiquinone oxidoreductase (Figure 3B). This enzyme uses light to power an energetically-unfavourable reaction, the reduction of UQjq by reduced cytochrome C2 (cytc ). The products of reaction centre enzymatic activity, oxidised cytochrome C2 (cyt c ). and reduced ubiquinone (UQjjH2) are the substrates for cytochrome bCj complex, which catalyses the reduction of cyt by UQ1QH2. 

Complex II is usually measured in two ways either as succinate ubiquinone oxidoreductase or as succinate cytochrome c oxidoreductase. The most commonly used assay for succinate ubiquinone oxidoreductase (or isolated complex II) uses DCIP in the same way as described above for the new complex I assay, only in this case succinate is added as a substrate (instead of NADH). The specificity of DCIP reduction can be determined by measuring in the presence or absence of mal-onate, a specific inhibitor of complex II. The assay for succinate cytochrome c oxidoreductase (or complex II 4- HI) uses succinate and oxidized cytochrome c as substrates and measures the reduction of cytochrome c, which can be followed spectrophotometrically at 550 nm. The assay is also suitable to screen for coenzyme Q deficiencies, as it is dependent on the endogenously present CoQio- In case of a CoQ deficiency, a reduced succinate cytochrome c oxidoreductase activity will be... 

The electron carriers in the respiratory assembly of the inner mitochondrial membrane are quinones, flavins, iron-sulfur complexes, heme groups of cytochromes, and copper ions. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers. The electrons emerge in QH2, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate dehydrogenase is a component of the succinate-Q reductase complex (Complex II), which donates electrons from FADH2 to Q to form QH2.This highly mobile hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes h and c j and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a 3 and three copper ions. A heme iron ion and a copper ion in this oxidase transfer electrons to O2, the ultimate acceptor, to form H2O. 

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