Cytochrome Rieske iron sulfur protein

Rieske proteins are constituents of the be complexes that are hydro-quinone-oxidizing multisubunit membrane proteins. All be complexes, that is, bci complexes in mitochondria and bacteria, b f complexes in chloroplasts, and corresponding complexes in menaquinone-oxidizing bacteria, contain three subunits cytochrome b (cytochrome 6e in b f complexes), cytochrome Ci (cytochrome f in b(,f complexes), and the Rieske iron sulfur protein. Cytochrome 6 is a membrane protein, whereas the Rieske protein, cytochrome Ci, and cytochrome f consist of water-soluble catalytic domains that are bound to cytochrome b through a membrane anchor. In Rieske proteins, the membrane anchor can be identified as an N-terminal hydrophobic sequence (13). 

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... 

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. 

An unusual [2Fe-2S] ferredoxin with unique spectroscopic properties exists in association with cytochromes b and c, and is involved in respiratory electron transport in mitochondria, chloroplasts and certain bacteria. When isolated, the complex contains two b hemes, one c, heme and the 2Fe-2S protein. The 2Fe-2S protein from the bct complex (Sections 62.1.5.2.3 and 62.1.5.2.5) was purified from bovine mitochondria by Rieske et al.,162 and is referred to as the Rieske iron-sulfur protein. The properties of this protein have been reviewed763 and its topography in mitochondrial ubiquinol-cytochrome c reductase has been described.764 They have high redox potentials in the range+150-330 mV. 

Engstrom G, Xiao K, Yu C-A, Yu L, Durham B, Millett F. Photoinduced electron transfer between the Rieske iron-sulfur protein and cytochrome bcr complex. J Biol Chem 2002 277 31072-8. 

ISP , Rieske" iron-sulfur protein Cyt. c Cytochrome c Cyt. Ci Cytochrome c,... 

The be complexes from mitochondria, chloroplasts, and bacteria all contain three catalytic subunits harboring the four redox centers cytochrome b, the high-potential cytochrome C or /, and the Rieske iron sulfur protein. These subunits are required and sufficient to support electron transport since most bacterial bci complexes only consist of these three subunits. However, some bacterial bc complexes contain a fourth subunit with yet unknown function. Mitochondrial bc complexes contain in addition to the three catalytic subunits 7-8 subunits without redox centers two large core proteins which are peripherally located and which are members of the family of matrix proeessing peptidases (MPP), and 5-6 small subunits. In cytochrome complexes, cytochrome b is split into cytochrome b(, and subunit IV containing the C-terminal part of cytochrome b in addition, 3 small hydrophobic subimits are present [18]. 

The bcf complexes form dimers in the membrane with molecular masses of approximately 480 kDa (mitochondria) and 130 kDa (bacteria), respectively. Each monomer has 10-13 membrane spanning helices, depending on the number of noncatalytic subunits. The membrane spanning helices of cytochrome b are in the center of the structure and form the dimer interface while the other membrane spanning helices are located around cytochrome b. Cytochrome c and the Rieske iron sulfur protein both have water soluble domains containing the redox centers, heme ci and the [2Fe-2S] cluster, respectively. These domains are at the outside of the iimer mitochondrial membrane, i.e., in the intermembrane space, and bound to the membrane via membrane spanning helices acting as membrane anchors. 

Brandt, U., Yu, L., Yu, C. A., and Trumpower, B. L., 1993, The mitochondrial targeting presequence of the Rieske iron-sulfur protein is processed in a single step after insertion into the cytochrome bcl complex in mammals and retained as a subunit in the complex, J. Biol. Chem. 268 8387n8390. 

Cytochromes, as components of electron transfer chains, must interact with the other components, accepting electrons from reduced donor molecules and transferring them to appropriate acceptors. In the respiratory chain of the mitochondria, the ubiquinolxytochrome c oxidoreductase, QCR or cytochrome bc complex, transfers electrons coming from Complexes 1 and 11 to cytochrome c. The bc complex oxidises a membrane-localised ubiquinol the redox process is coupled to the translocation of protons across the membrane, in the so-called proton-motive Q cycle, which is presented in a simplified form in Figure 13.14. This cycle was first proposed by Peter Mitchell 30 years ago and substantially confirmed experimentally since then. The Q cycle in fact consists of two turnovers of QH2 (Figure 13.14). In both turnovers, the lipid-soluble ubiquinol (QH2) is oxidized in a two-step reoxidation in which the semiquinone CoQ is a stable intermediate, at the intermembrane face of the mitochondrial inner membrane. It transfers one electron to the Rieske iron—sulfur protein (ISP), one electron to one of the two cytochrome b haems (bi), while two protons are transferred to the intermembrane space. In both of the Q cycles, the cytochrome bi reduces cytochrome bfj while the Reiske iron—sulfur cluster reduces cytochrome c/. The cytochrome ci in turn reduces the water-soluble cytochrome c, which transfers its electrons to the terminal oxidase, cytochrome c oxidase, described above. In one of the two Q cycles, reduced cytochrome bf reduces Q to the semiquinone, which is then reduced to QH2 by the second reduced cytochrome bn- The protons required for this step are derived from the matrix side of the membrane. The overall outcome of the two CoQ cycles (10) (/ — matrix o — intermembrane space) is... 

The Cyt f complex lying between PS II and PS I in the electron-transport system resembles the Cyt be complex of mitochondria and photosynthetic bacteria. These cytochrome complexes possess one Rieske iron-sulfur protein R-FeS (a [2Fe-2S] protein discovered by John Rieske) and a so-called subunit IV. The two fc-hemes of Cyt b(, and the subunit IV span the thylakoid membrane, while the R-FeS and Cyt/ are located near the lumen side. As previously noted, the placement of the i>-hemes across the thylakoid membrane helps form a redox chain across the membrane. The function of the Cyt complex in green-plant thylakoids is to oxidize the plastohydroquinone formed by PS II and to transfer these electrons to plastocyanin. Accordingly, the Cyt ig/ complex has therefore also been called the plastohydroquinone-plastocyanin-oxidoreductase. ... 

The first electron from the reduced quinone are transferred to a -type cytochrome and a second electron to the Rieske iron-sulfur protein and cytochrome c, in the cytochrome-6Cj complex. These electrons are subsequently transferred to cytochrome C2 on the periplasmic side ofthe plasma membrane and finally go to reduce the oxidized primary electron donor P870 to complete the cyclic reaction. 

The presence of an iron-sulfur protein in the mitochondrial CyX-bc complex was discovered by Rieske, Zaugg and Hansen in 1964 from its low-temperature EPR spectrum characteristic of reduced [2Fe 2S] with aprominentpeak with ag-value near 1.90. This iron-sulfur protein has since been called the Rieske iron-sulfur protein, which we will abbreviate as R-ISP or R-[2Fe 2S]. Because of its unique spectroscopic properties, R-[2Fe 2S] in the mitochondrial Cyt bc complex has been used as a diagnostic protein for characterizing the cytochrome-ZiCi complexes. The subsequent discovery of an R-[2Fe 2S] in the... 

Oxidant-induced reduction of cytochrome b The oxidant-induced reduction of Cyt b means the reduction ofCyt-Z>6 linked to the oxidation ofquinol is mediated by the Rieske iron-sulfur protein. This reaction has long been documented for the mitochondrial and photosynthetic-bacterial Cyt-icj complex. The same reaction is expected to occur in the Cyt-f)6/complex of higher plants and cyanobacteria. Oxidant-induced reduction of Cyt b was illustrated in the early reaction steps in Fig. 11 (C) above. We describe here, with the help of Fig. 12, the work of Hurt and Hauska detailing the spectrophotometric evidence for the reaction steps in the Cyt-b(f complex isolated from spinach chloroplasts. 

Now that it is substantiated that the [2Fe 2S] domain of the Rieske iron-sulfur protein is not static but moves between domains of cytochrome-c, and cytochrome-/ subunits, and that it is likely that such movement may provide a novel mechanism to allow catalysis of all the reactions involved in the oxidation of hydroquinone at the Qo site and the subsequent bifurcated pathway of electron transfer. It has been found that during the movement, the mobile [2Fe 2S] domain retains essentially the same tertiary structure, and the anchoring N-terminal tail of the R-ISP molecule remains in the same fixed position. The movement occurs through an extension of a helical segment in the short linking span. 

T Kallas, S Spiller and R Malkin (1988) Primary structure of cotranscribed genes encoding the Rieske iron-sulfur protein and cytochrome f proteins of the cyanobacterium Nostoc PCC 7906. Proc Nat Acad Sci, USA 85 5794-5798... 

S Iwata, M Saynovits, TA Link and H Michel (1996) Structure of a water soluble fragment of the Rieske iron-sulfur protein of the bovine heart mitochondrial cytochrome bc complex determined by MAD phasing at 1.5 resolution. Structure 4 567-579... 

Color Plate 14. (A) A Rieske iron-sulfur protein fragment of the bovine-heart mitochondrial cytochrome be, complex. Iron is shown in red, sulfur in yellow, and the ligands to the cluster are shown in grey ball-and-stick models. (Courtesy of Dr. S. Iwata and Dr. H. Michel). (B) A Rieske iron-sulfur protein fragment of spinach chloropiast. (Courtesy Dr. W. A. Cramer, Dr. J. Smith, and Dr. C. J. Carrell). [See Chapter 35, Fig. 4.]... 

The Saccharomyces cerevisiae ubiquinolxytochrome c oxidoreductase or cytochrome bc complex (Complex III), which is highly homologous to the mammalian complex, is a component of the inner mitochondrial membrane. The b subunit, which is encoded by mtDNA, and the C and Rieske iron-sulfur protein subunits, which are encoded by nuclear DNA, make up the catalytic core of the complex. An additional 7 non-identical and non-catalytic nuclear encoded subunits, three heme groups, and two quinones make up the remainder of the complex (see (Hunte et al., 2008) for review). Complex III exists as a dimer in which fourteen phospholipid molecules have been identified in the... 

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