Cytochrome be! complex

In contrast to common usage, the distinction between photosynthetic and respiratory Rieske proteins does not seem to make sense. The mitochondrial Rieske protein is closely related to that of photosynthetic purple bacteria, which represent the endosymbiotic ancestors of mitochondria (for a review, see also (99)). Moreover, during its evolution Rieske s protein appears to have existed prior to photosynthesis (100, 101), and the photosynthetic chain was probably built around a preexisting cytochrome be complex (99). The evolution of Rieske proteins from photosynthetic electron transport chains is therefore intricately intertwined with that of respiration, and a discussion of the photosynthetic representatives necessarily has to include excursions into nonphotosynthetic systems. 

The Rieske clusters observed in cytochrome be complexes from traditional sources (i.e., mitochondria, proteobacteria, plastids, and cy-... 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

The resulting hydroquinone (QH2) then diffuses to the cytochrome be complex, which oxidises QH2 back to Q, using the resulting reduction potential, via cytochrome c, to reduce the special pair and hence regenerate the reaction centre. 

CYSTATHIONINE /3-LYASE CYTIDINE DEAMINASE CYTIDYLATE KINASE Cytochrome be, complex,... 

In chemical terms the photoinduced electron transfer results in transfer of an electron across the photosynthetic membrane in a complex sequence that involves several donor-acceptor molecules. Finally, a quinone acceptor is reduced to a semiquinone and subsequently to a hydroquinone. This process is accompanied by the uptake of two protons from the cytoplasma. The hydroquinone then migrates to a cytochrome be complex, a proton pump, where the hydroquinone is reoxidized and a proton gradient is established via transmembrane proton translocation. Finally, an ATP synthase utilizes the proton gradient to generate chemical energy. Due to the function of tetrapyrrole-based pigments as electron donors and quinones as electron acceptors, most biomimetic systems utilize some... 

Heiss, B., Frunzke, K., and Zumft, W. G. (1989). Formation of the N-N bond from nitric oxide by a membrane-bound cytochrome be complex of nitrate-respiring (denitrifying) Pseudomortas stutzeri. ]. Bacterial. 171, 3288-3297. 

Zumft, W. G., Braun, C., and Cuypers, H. (1994). Nitric oxide reductase from Pseudo moruis stutzeri. Primary structure and gene organization of a novel bacterial cytochrome be complex. Eur. ]. Biochem. 219, 481-490. 

FIGURE 19-11 Cytochrome be, complex (Complex III). The complex is a dimer of identical monomers, each with 11 different subunits. (a) Structure of a monomer. The functional core is three subunits cytochrome b (green) with its two hemes (bH and foL, light red) the Rieske iron-sulfur protein (purple) with its 2Fe-2S centers (yellow) and cytochrome ci (blue) with its heme (red) (PDB ID 1BGY). (b) The dimeric functional unit. Cytochrome c, and the Rieske iron-sulfur protein project from the P surface and can interact with cytochrome c (not part of the functional complex) in the intermembrane space. The complex has two distinct binding sites for ubiquinone, QN and QP, which correspond to the sites of inhibition by two drugs that block oxidative phosphorylation. Antimycin A, which blocks electron flow from heme bH to Q, binds at QN, close to heme bH on the N (matrix) side of the membrane. Myxothiazol, which prevents electron flow from... 

Functions of iron-sulfur enzymes. Numerous iron-sulfur clusters are present within the membrane-bound electron transport chains discussed in Chapter 18. Of special interest is the Fe2S2 cluster present in a protein isolated from the cytochrome be complex (complex III) of mitochondria. First purified by Rieske et al.,307 this protein is often called the Rieske iron-sulfur protein 308 Similar proteins are found in cytochrome be complexes of chloroplasts.125 300 309 310 In... 

In contrast, the reaction centers of green sulfur bacteria resemble PSI of chloroplasts. Their reaction centers also receive electrons from a reduced quinone via a cytochrome be complex.245 However, the reduced form of the reaction center bacteriochlorophyll donates electrons to iron-sulfur proteins as in PSI (Fig. 23-17). The latter can reduce a quinone to provide cyclic photophosphorylation. Cyanobacteria have a photosynthetic apparatus very similar to that of green algae and higher plants. 

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

Cytochrome be, complex to cytochrome c to cytochrome oxidase Cytochrome c is a peripheral membrane protein that is loosely bound to the outer surface of the inner mitochondrial membrane. It binds to the cytochrome be, complex and accepts an electron via an Fe3 to Fe2+ transition. Then it binds to cytochrome oxidase and donates the electron, with the iron atom of the heme of cytochrome c then reverting to the Fe3+ state (Fig. 2). 

Engstrom G, Rajagukguk R, Saunders A, et al. Design of a ruthenium-labeled cytochrome c derivative to study electron transfer with the cytochrome be, Complex. Biochemistry 2003 42 2816-24. 

Iwata, S., et al. (1998). Complete Structure of the 11-Subunit Bovine Mitochondrial Cytochrome be, Complex. Science 281 64. 

Xia, D. et al. (1997). Crystal Structure of the Cytochrome be Complex from Bovine Heart Mitochondria. Science 277 60. 

Molik, S., Karnauchov, I., Weidlich, C., Herrmann, R. G., and Klosgen, R. B. (2001). The Rieske Fe/S protein of the cytochrome be,// complex in chloroplasts missing link in the evolution of protein transport pathways in chloroplasts J. Biol. Chem. 276, 42761-42766. 

The third protein complex in this electron-transfer chain (complex 111) is ubiquinol cytochrome c oxidoreductase (E.C. 1.10.2.2), or commonly known as cytochrome be, complex named after the its b-type and c-type cytochrome subunits. Probably the best-understood one among the complexes, be, complex catalyses electron transfers between two mobile electron carriers the hydrophobic molecule ubiquinone (Q) and the small soluble haem-containing protein cytochrome c. Two protons are translocated across the membrane per quinol oxidized (Hinkel, 1991 Crofts, 1985 Mitchell, 1976). 

Mitochondrial cytochrome be, complexes have been isolated from animals, plants, yeast, and fungus by the use of detergents. These complexes all have the common three redox-active subunits that contain prosthetic... 

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]. 

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