The cytochrome bc, complex

It has become clear in the recent years that electron transfer chains of mitochondria, chloroplasts and some bacteria all contain a cytochrome be complex with very similar structural and functional properties (see Refs. 87, 176-180). Although we focus here on the mitochondrial Complex III, much information has, in particular, come from studies on the bacterial chromatophore system [8,87,176,178]. 

The stoicheiometry of the polypeptides in the complex is somewhat controversial (Table 3.5). Complex III can apparently take two oligomeric forms. Hydrodynamic experiments of Tzagoloff et al. [197], carried out in the presence of taurocholate or taurodeoxycholate, suggested a monomeric complex. In contrast, when solubilised with Triton X-100 it appears to be a dimer. Both these preparations are enzymically active [see 178]. 

Cytochrome 6 is a very hydrophobic protein. A model of how it may be folded in the inner mitochondrial membrane may be obtained from its primary structure [204,204a,205] (Fig. 3.9). The haem-binding histidines are positioned pairwise in two segments that traverse the membrane. The haems may therefore be sandwiched between transmembranous helices (cf., cytochrome oxidase haem groups, Section 3.6). This agrees with the perpendicularity between the haem and the membrane planes [206], and with proposed transmembranous electron transfer catalysed by the cytochrome b haems [207,208]. 

In most potentiometric titrations only two cytochrome b species are distinguished [208-210], i.e., 6-562 and 6-566, with values of about -(-40 and —40 mV, respectively (both pH dependent). However, proposals have been made of up to four different species (see Refs. 202, 208, 211), i.e., two components 6-562 and separate identity of 6-558 and 6-566. Although it now seems clear that there are two haems 6 per monomer, a functional dimer [211] or different functional states of the monomer 

Cytochrome c, is an amphiphilic protein with a molecular weight of 28-31000. Weiss et al. [212] found that it may be isolated only with the help of a detergent. However, by mild proteolysis they could release the haem-binding domain from the rest of the protein. This segment is soluble in aqueous solutions. The amino acid sequence of the bovine protein [181] shows that it has only one continuous hydrophobic segment that is close to the C-terminus. This segment probably acts as an anchor to the membrane. The covalently bound haem is located in the water-soluble part, with its plane perpendicular to the membrane plane [206], The two-domain structure makes the architecture of cytochrome c, very similar to that of microsomal cytochrome [213]. 

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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 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. 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 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) ... 

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. 

Figure 1 The mitochondrial respiratory chain. Electron transfer (brown arrows) between the three major membrane-bound complexes (I, III, and IV) is mediated by ubiquinone (Q/QH2) and the peripheral protein c)dochrome c (c). Transfer of protons hnked to the redox chemistry is shown by blue arrows red arrows denote proton translocation. NAD+ nicotinamide adenine dinucleotide, FMN flavin mononucleotide, Fe/S iron-sulfur center bH,bi, and c are the heme centers in the cytochrome bc complex (Complex III). Note the bifurcation of the electron transfer path on oxidation of QH2 by the heme bL - Fe/S center. Complex IV is the subject of this review. N and P denote the negatively and positively charged sides of the membrane, respectively...

The cytochrome bc complex (cyt bci) is an integral membrane protein in the electron-transport chains of... 

The photochemistry of the reaction center takes place one electron at a time. However, one of the products of the electron transfer process is a reduced ubiquinone, which has taken up two electrons as well as two protons. To form this species, the reaction center must turn over twice, with electrons entering the complex by donation of cytochrome ci with the oxidized special pair. The electrons accumulate in the quinone acceptors and protons are taken up from the surrounding medium. Finally, a fidly reduced ubiquinol is formed, which is released from the complex into the hydrocarbon portion of the membrane. The quinol is subsequently reoxidized at the cytochrome bc complex (described below). 

Figure 18 Schematic structure of the cytochrome bc complex from mitochondria. The struemre of the complex from purple photosynthetic bacteria is thought to be similar. The pathway of electron and proton transfer (modified Q-cycle) is overlaid on the schematic structure. Movement of the Rieske FeS protein is shown by the semitransparent yellow areas ...

FIGURE 5. (A) Interaction of atovaquone with the Fe-S center (on left) in the cytochrome bc complex via H-bonding with the coordinated His (Plate VIII). (B) The active center of a class 2 dihydroorotate dehydrogenase (PDB ID 1UUM) with a hound inhibitor atovaquone (top baU-and-stick stmcture), orotic acid (green) and riboflavin 5 -(dihydrogen phosphate) (FMN, pink) (Plate IX)... 

In unimolecular ET, the rate can be controlled by large-scale cofactor motion, such as the quinone motion in the photosynthetic reaction centers, the Rieske subunit motion in the cytochrome bc complex (47), or the cytochrome fcs-domain in sulfite oxidase. Theoretical models for conformationally controlled ET reactions have been suggested by Hoffman and Rat-ner (48) and Bnmschwig and Sutin (49). Large-scale protein or domain motions are themselves linked to the movement of water molecules. 

Cytochrome c carries electrons from the cytochrome bc complex, but also from auxiliary redox enzymes (see e.g., Refs. 1,2) to cytochrome oxidase. Ubiquinone is the only non-protein member of the chain. This highly hydrophobic substituted... 

We already encountered the respiratory electron transfer chain in Chapter 5, and in the present context. Figure 13.8 serves as a reminder that the structures of many of the components have been determined (Hosier, Ferguson-Miller, Mills, 2006). Electrons flow from NADH/NAD" " and succinate (Complexes I and II) via Coenzyme Q to the cytochrome bc complex (Complex III) and are then transferred via cytochrome c to cytochrome c oxidase (CcO) (Complex IV). We will discuss the electron transport cytochromes in the next section. 

Based on the nature of the cytochromes, there are two kinds of photosynthetic bacterial reaction centers. The first kind, represented by that of Rhodobacter sphaeroides, has no tightly bound cytochromes. For these reaction centers, as shown schematically in Fig. 2, left, the soluble cytochrome C2 serves as the secondary electron donor to the reaction center the RC also accepts electrons from the cytochrome bc complex by way ofCytc2- The rate of electron transfer from cytochrome to the reaction center is sensitive to the ionic strength of the medium. Functionally, cytochrome C2 is positioned in a cyclic electron-transport loop. In Rb. sphaeroides, Rs. rubrum and Rp. capsulata cells, the two molecules of cytochromes C2 per RC are located in the periplasmic space between the cell wall and the cell membrane. When chromatophores are isolated from the cell the otherwise soluble cytochrome C2 become trapped and held by electrostatic forces to the membrane surface at the interface with the inner aqueous phase. These cytochromes electrostatically bound to the membrane can donate electrons to the photooxidized P870 in tens of microseconds at ambient temperatures, but are unable to transfer electrons to P870 at low temperatures. 

S Izrailev, AR Crofts, EA Berry and K Schulten (1999) Steered molecular simulation of the Rieske subunit motion in the cytochrome bc complex. Biophys J 77 1753-1768... 

The hydroquinone (QbH2), carrying in its chemical bonds some of the energy of the photons that originally excited P870, enters the pool of reduced quinone (QH2) dissolved in the membrane and moves through the hpid phase of the bilayer to the cytochrome bc complex. 

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