Cytochrome bc complex

The structure of the UQ-cyt c reductase, also known as the cytochrome bc complex, has been determined by Johann Deisenhofer and his colleagues. (Deisenhofer was a co-recipient of the Nobel Prize in Chemistry for his work on the structure of a photosynthetic reaction center [see Chapter 22]). The complex is a dimer, with each monomer consisting of 11 protein subunits and 2165 amino acid residues (monomer mass, 248 kD). The dimeric structure is pear-shaped and consists of a large domain that extends 75 A into the mito-... 

It has long been assumed that azurin is an in vivo electron donor to cytochrome cdi of P. aeruginosa. The construction of mutants of P. aeruginosa in which one or both of the genes for azurin and cytochrome C551 have been deleted has led to the conclusion that in vivo cytochrome C551 is essential for the donation of electrons to the nitrite reductase and that azurin is ineffective (24). The discrepancy between in vivo and in vitro observations either could be reconciled if it is the failure of azurin to accept electrons from the cytochrome bc complex or another donor that is responsible for its ineffectiveness in vivo. 

The simpler cytochrome bc] complexes of bacteria such as E. coli,102 Paracoccus dentrificans,116 and the photosynthetic Rhodobacter capsulatus117 all appear to function in a manner similar to that of the large mitochondrial complex. The bc] complex of Bacillus subtilis oxidizes reduced menaquinone (Fig. 15-24) rather than ubiquinol.118 In chloroplasts of green plants photochemically reduced plastoquinone is oxidized by a similar complex of cytochrome b, c-type cytochrome /, and a Rieske Fe-S protein.119 120a This cytochrome b6f complex delivers electrons to the copper protein plastocyanin (Fig. 23-18). 

The multisubunit complexes of the respiratory chain. Complexes I (NADH dehydrogenase) and II (succinate dehydrogenase) transfer electrons from NADH and succinate to UQ. Complex III (the cytochrome bc complex) transfers electrons from UQH2 to cytochrome c, and complex IV (cytochrome oxidase), from cytochrome c to 02. The arrows represent paths of electron flow. NADH and succinate provide electrons from the matrix side of the inner membrane, and 02 removes electrons on this side. Cytochrome c is reduced and oxidized on the opposite side of the membrane, in the lumen of a crista or in the intermembrane space. 

Fig. 7. Diheme binding by integral membrane protein four helix bundles (Lancaster, 2002e). (a) Two-helix motif His ligands from two transmembrane helices (mitochondrial cytochrome bc complex), (b) Three-helix motif His ligands from three transmembrane helices (e.g., hydrogenase, formate dehydrogenase), (c) Four-helix motif His ligands from four transmembrane helices (dihemeic succinate quinone oxidoreduc-tases). Modified from Lancaster (2002). Biochim. Biophys. Acta Biomembr. 1565, 215-231, with permission from Elsevier Science.

II. Structure of Cytochrome bc Complex from Bovine Heart Mitochondria. .. 153... 

All known cytochrome bc complexes have three essential subunits, which contain all four redox centers in common. These subunits are cytochrome b (cyt. b), cytochrome ( (cyt. c ), and the Rieske [2Fe-2S] protein (ISP). The redox centers are heme b i and heme b in cytochrome b (L and H represent low and high potential, respectively), heme C in cytochrome ci, and Rieske [2Fe-2S] (FeS) cluster in ISP. 

The reaction mechanism of cytochrome bc complex is known as the proton motive Qcycle originally proposed by Peter Mitchell (Mitchell, 1976). This mechanism is the basis of his chemiosmotic theory for which he was awarded the Nobel prize in 1978. Since then, the enzyme has been characterized extensively using various techniques. Redox centers have been characterized spectroscopically (for review, see Trumpower and Gennis, 1994), electron transfer pathways have been determined using kinetic experiments with specific inhibitors (De Vries 1986 Zhu et al., 1984), and the positions of quinone binding sites and redox centers have been determined using biochemical and mutational analysis (for review, see Esposti et al, 1993 Brasseur et al, 1996). As a result of these efforts, the latest modified Qcycle has been widely accepted by researchers in the field (for reviews, see Crofts et al, 1983 Trumpower, 1990 Berry et al, 2000). 

Fig. 5. Schematic view of the transmembrane section of the cytochrome bc complex looking down from the intermembrane space. Transmembrane helices, hemes, bound inhibitors, and phospholipids are shown.

Figure 7.27 The electron transport chain showing the three phosphorylation sites and points where insecticides inhibit this process. Flavoprotein represents complex I (NADH dehydrogenase). Coenzyme Q also accepts electrons from succinate dehydrogenase (complex II). Cytochrome b represents complex III consisting of cytochrome bc complex. Cytochrome oxidase represents complex IV.

Figure 1. The Q-cycle mechanism of the cytochrome bc complex (cf. section 1.1). The points of inhibition of three classes of inhibitors are indicated.

The cytochrome bc complex is inhibited by a variety of organic compounds that can be... 

In the X-ray structures of the cytochrome bc complex, tiie water soluble catalytic domain of the Rieske protein was found in different locations which have been grouped into three positional states [4-6,23] (Figure 3b) ... 

FIGURE 1. Respiratory chain the enzymes of the mitochondrial iimer membrane involved in oxidative phosphorylation. From complex I to V, they are NADH-dehydrogenase, succinate dehydrogenase, cytochrome bc complex, and cytochrome c oxidase. Protons are translocated across the membrane while electrons are transferred to Oj through the chain. The proton gradient is used by ATP synthase (complex V) to make ATP. (Reprinted with permission from Saraste, 1999, American Association for the Advancement of Science.)... 

Davidson, E., Ohinshi, T., Atta-Asafo-Adjei, E., and Daldal, F., 1992, Potential ligands of the [2Ee-2S] Rieske cluster of the cytochrome bc complex of Rhodobacter capsulatus probed by site-directed mutagenesis, Biochemistry 31 3342n3351. 

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