Cytochrome bc1 complex

Cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bct in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria it can also carry electrons from the cytochrome b f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin. 

L., Yu, L., Deisenhofer, J. (1997) Crystal structure of the cytochrome bc1 complex from bovine heart mitochondria. Science 277, 60-66. 

Cyclic photophosphorylation in purple bacteria. QH2 is eventually dehydrogenated in the cytochrome bc1 complex, and the electrons can be returned to the reaction center by the small soluble cytochrome c2, where it reduces the bound tetraheme cytochrome or reacts directly with the special pair in Rhodobacter spheroides. The overall reaction provides for a cyclic photophosphorylation (Fig. 23-32) that pumps 3-4 H+ across the membrane into the periplasmic space utilizing the energy of the two photoexcited electrons. 

Complex III The Cytochrome hcl Complex. The cytochrome bc1 complex contains two b-type cytochromes b, and bH. Both of the b hemes are on a single 30-kd polypeptide. The complex also contains cytochrome ct, an iron-sulfur protein, and between four and six additional subunits. 

This scheme for electron transport through the cytochrome bc1 complex is known as the Q cycle. The role of the iron-sulfur protein was confirmed by extracting this protein from the cytochrome bc1 complex. If the iron-sulfur protein is removed, electron transfer from UQH2 to cytochrome c is prevented. This shows that the iron-sulfur protein operates upstream of cytochrome cj, even though its E° is more positive than that of the cytochrome (see fig. 14.7 and table 14.1). 

Complex III (ubiquinol-cytochrome c oxido-reductase or cytochrome bct complex). Mitochondrial complex III is a dimeric complex, each subunit of which contains 11 different subunits with a total molecular mass of 240 kDa per monomer.104-107 However, in many bacteria the complex consists of only three subunits, cytochrome b, cytochrome c , and the high potential ( 0.3 V) Rieske iron-sulfur protein, which is discussed in Chapter 16, Section A,7. These three proteins are present in all bc1 complexes. 

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...

Bcl-2 and related cytoplasmic proteins are key regulators of apoptosis [26], Anti-apoptotic proteins such as Bcl-2 and Bc1-Xl prevent apoptosis in response to numerous stimuli. During the apoptotic process, cytochrome c is released from mitochondria, but the release can be inhibited by the presence of Bcl-2 on the organelles [27]. The released cytochrome c forms an essential part of die apoptosome, which is composed of cytochrome c, Apaf-1, and procaspase-9 [28]. The complex formation results in activation of caspase-9, which leads to the stimulation of caspase-3. Bcl-XL has recently been reported to bind to Apaf-1 [29], It may inhibit the association of Apaf-1 with procaspase-9 and thereby prevent caspase activation. 

The Bcl-2 family of oncoproteins is known to play an important role in apoptosis through their ability to regulate cytochrome c release from mitochondria [11,15]. The antiapoptotic proteins Bcl-2 and Bc1-Xl prevent cytochrome c release, whereas the proapoptotic family members (e.g.. Bad, Bid, Bik, Bax) facilitate cytochrome c efflux or block the protective effects of Bcl-2 and Bc1-Xl. The mechanism involved is unclear the Bcl-2 family proteins may interact directly with the MTP (mitochondrial permeability transition) protein complex (the PTPC) or form independent ionic pores in the outer mitochondrial membrane (Fig. 3). Nonetheless, cytochrome c-depen-dent caspase-3 activation and changes in the expression or phosphorylation state of Bcl-2 family proteins are taken as indicative of mitochondria-dependent apoptotic pathways. It is important to remember that other apoptogenic proteins are also present in the mitochondrial intermembrane space, including smac/ DIABLO and flavoprotein (AIF) [10,11,60]. The release stimuli for the latter factors, which are currently being elucidated, may also involve the permeability transition or the Bcl-2 family proteins [37]. 

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