Ubiquinone-cytochrome

Reported applications of DST include the crosslinking of ubiquinone cytochrome C reductase (Smith et al., 1978), characterization of the cell surface receptor for colony-stimulating factor (Park et al., 1986), investigation of the Ca+2-, Mg+2-activated ATP of E. coli (Bragg and Hou, 1980), and characterization of human properdin polymers (Farries and Atkinson, 1989). 

Smith, R. J., Capaldi, R. A., Muchmore, D., and Dahlquist, F. (1978) Cross-linking of ubiquinone cytochrome c reductase (complex III) with periodate-cleavable bifunctional reagents. Biochemistry 17, 3719—3723. 

Respiratory Chain (Complex I, II, III, and IV, Ubiquinone, Cytochrome c, Proton Pump, Membrane Potential, Proton Motive Force)... 

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. 

The classic cytochrome h, expressed sometimes as cytochrome hn, b-561, or b-562, has a single symmetric a band at 561-562 nm in the reduced minus oxidized difference spectrum at room temperature at liquid nitrogen temperature (77°K) it has an a band at 558-559.5 nm, a /3 band at 529 nm, and a Soret band at 428 nm. Cytochrome b is readily reduced by succinate and NAD-linked substrate in both coupled and uncoupled mitochondria. This cytochrome is associated with complex III (ubiquinone-cytochrome c reductase) 24). Figure 1 shows the absorption spectra of reduced cytochrome b at different temperatures between liquid helium and room temperature 26). 

The spectral and potentiometric characteristics of type b cytochromes in a succinate-cytochrome c reductase, prepared from pigeon breast muscle mitochondria using a mixture of ionic and nonionic detergents, have shown a close resemblance to those in intact mitochondria 8,34,37). The succinate-cytochrome c reductase may be fractionated into two complexes, succinate-ubiquinone reductase and ubiquinone-cytochrome c reductase, so-called complexes II and III, respectively. Complex III contains type b cytochromes, cytochrome c, and nonheme iron protein in a stoichiometry of 2 1 1 (38). One of the type b cytochromes shows an a peak at 562 nm (559.5 nm at 77°K) by the reduction with succinate and is identified as cytochrome b. The other, with the a peak at 566 nm (562.5 and 554 nm at 77°K), is reduced by succinate only in the presence of antimycin or by dithionite and is identified as cytochrome bi- These two cytochromes do not combine with CO. 

Predict the oxidation-reduction states of NAD", NADH-Q reductase, ubiquinone, cytochrome Cl, cytochrome c, and cytochrome a in liver mitochondria that are amply supplied with isocitrate as substrate, Pj, ADP, and oxygen but are inhibited by... 

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)... 

There are other variations on this type of experiment. Protoplasts from Para-coccus denitrificans showed a similar type of response [204] except that there was a higher level of endogeneous cytochrome c reduction. Respiration was tapped oxidatively at + 395 mV under conditions of which anaerobiosis was marked by a sharp increase in current. Further increase was obtained by additions of succinate, which is a reductant for cytochrome c via the sequence of membrane-bound enzymes succinate dehydrogenase (Complex II) and ubiquinone-cytochrome c reductase (Complex III). By contrast, respiratory coupling could not be observed with protoplasts of Escherichia coli, an organism in which the terminal oxidase system is not reduced via cytochrome c. 

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

The respiratory chain can be separated by various techniques into three multienzyme complexes (Figure 16.3). Complex I is NADH-ubiquinone reductase. Complex III is known as ubiquinone-cytochrome c reductase and contains cytochromes b and Ci. Complex IV is cytochrome oxidase. (Succinate dehydrogenase is referred to as Complex II). Ubiquinone and cytochrome c are small molecules which do not form part of these complexes. Reconstitution of the isolated Complexes I-IV with cytochrome c and ubiquinone leads to recovery of the activity of the respiratory chain. 

Antimycin A, an antibiotic produced by Streptomyces spp. that is used as a fungicide against fungi that are parasitic on rice. It inhibits complex III (ubiquinone —> cytochrome c reduction). It inhibits the oxidation of both malate and succinate, as both require complex III, and the addition of the uncoupler has no effect. 

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