Cytochrome bcx complex

Oxidation and reduction of UQ by the cytochrome bcx complex results in the uptake of protons from the solution... 

The main part of the electron transport chain consists of three large protein complexes embedded in the inner mitochondrial membrane, called NADH dehydrogenase, the cytochrome bcx complex and cytochrome oxidase. Electrons flow from NADH to oxygen through these three complexes as shown in Fig. 1. Each complex contains several electron carriers (see below) that work sequentially to carry electrons down the chain. Two small electron carriers are also needed to link these large complexes ubiquinone, which is also called coenzyme Q (abbreviated here as CoQ), and cytochrome c (Fig. 1). 

Sadowski RC, Engstrom G, Tian H, et al. Use of photoactivated ruthenium dimer complex to measure electron transfer between the Rieske iron-sulfur and cytochrome C in the cytochrome bcx complex. Biochemistry 2000 39 4231-6. 

Figure 18-8 Stereoscopic ribbon diagrams of the chicken bc1 complex (A) The native dimer. The molecular twofold axis runs vertically between the two monomers. Quinones, phospholipids, and detergent molecules are not shown for clarity. The presumed membrane bilayer is represented by a gray band. (B) Isolated close-up view of the two conformations of the Rieske protein (top and long helix at right) in contact with cytochrome b (below), with associated heme groups and bound inhibitors, stigmatellin, and antimycin. The isolated heme of cytochrome c, (left, above) is also shown. (C) Structure of the intermembrane (external surface) domains of the chicken bcx complex. This is viewed from within the membrane, with the transmembrane helices truncated at roughly the membrane surface. Ball-and-stick models represent the heme group of cytochrome cy the Rieske iron-sulfur cluster, and the disulfide cysteines of subunit 8. SU, subunit cyt, cytochrome. From Zhang et al.105...

The purification and characterization of individual cytochromes (simple or complex) and other redox centres in electron-transfer chains leads to a study of their interactions with each other, with the ultimate objective of reconstituting parts of the chain. Approaches to this problem will be illustrated724 with reference to the reaction of cytochrome c with ubiquinol cytochrome c reductase (complex III, cytochrome bcx) and cytochrome oxidase (complex IV) as shown in equation (2l). 

The Q-cycle hypothesis and other alternative versions of it were attempts to explain the two important reactions occurring during electron transport and proton translocation in the mitochondrial cytochrome be complex and also in the chloroplast cytochrome complex by the so-called oxidant-induced reduction of Cyt b and the interheme electron transport in the Cyts b. Abundant experimental work to obtain evidence for these two reactions as well as other aspects relating to the structure and function of the be complexes has been performed. In addition to what has been mentioned above, we will present several selected examples to elucidate the oxidant-induced reduction of Cvt b and the need for two quinone-binding sites, using the chloroplast CyX-b(f complex or the Cyi-bcx complex from photosynthetic bacteria as examples, all monitored by absorbance changes of cytochromes induced by either steady or flash illumination. 

Gennis, Barquera, Hacker, van Doren, Amaud, Crofts, Davidson, Gray and Daldal have demonstrated the value of flash spectroscopy as a valuable tool for elucidating the effects of inhibitors as well as mutagenesis on the mechanism of electron transfer in bcx complexes of photosynthetic bacteria. Fig. 12 shows the effects of these inhibitors on the redox reactions of cytochromes examined by flash-in-... 

Cytochromes of this group are widely distributed. Examples are listed in Table 17. Cytochrome b is a component of the respiratory chain in many aerobic organisms, but only as a complex bcx in mitochondria (and be/ hi chloroplasts). The separation of be, may involve a modification of cytochrome b. Amino acid sequences of a number of mitochondrial cytochromes b are available.712... 

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