Chloroplasts iron-sulfur proteins

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). 

During the 1960s, research on proteins containing iron—sulfur clusters was closely related to the field of photosynthesis. Whereas the first ferredoxin, a 2[4Fe-4S] protein, was obtained in 1962 from the nonphotosynthetic bacterium Clostridium pasteurianum (1), in the same year, a plant-type [2Fe-2S] ferredoxin was isolated from spinach chloroplasts (2). Despite the fact that members of this latter class of protein have been reported for eubacteria and even archaebacteria (for a review, see Ref. (3)), the name plant-type ferredoxin is often used to denote this family of iron—sulfur proteins. The two decades... 

Chapter 6). Other iron-sulfur proteins, so named because they contain iron sulfur clusters of various sizes, include the rubredoxins and ferredoxins. Rubredoxins are found in anaerobic bacteria and contain iron ligated to four cysteine sulfurs. Ferredoxins are found in plant chloroplasts and mammalian tissue and contain spin-coupled [2Fe-2S] clusters. Cytochromes comprise several large classes of electron transfer metalloproteins widespread in nature. At least four cytochromes are involved in the mitrochondrial electron transfer chain, which reduces oxygen to water according to equation 1.29. Further discussion of these proteins can be found in Chapters 6 and 7 of reference 13. 

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

In the overall reaction catalyzed by the mitochondrial respiratory chain, electrons move from NADH, succinate, or some other primary electron donor through flavoproteins, ubiquinone, iron-sulfur proteins, and cytochromes, and finally to 02. A look at the methods used to determine the sequence in which the carriers act is instructive, as the same general approaches have been used to study other electron-transfer chains, such as those of chloroplasts. 

Cytochromes b, a, and o. Protoheme-containing cytochromes b are widely distributed.127,128 There are at least five of them in E. coli. Whether in bacteria, mitochondria, or chloroplasts, the cytochromes b function within electron transport chains, often gathering electrons from dehydrogenases and passing them on to c-type cytochromes or to iron-sulfur proteins. Most cytochromes b are bound to or embedded within membranes of bacteria, mitochondria, chloroplasts, or endoplasmic reticulum (ER). For example, cyto-... 

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. 

Photosystem I contains three iron-sulfur clusters firmly associated with the reaction center. These are designated Fe-Sx, Fe-SA, and Fe-SB in figure 15.17. The cysteines of Fe-Sx are provided by the two main polypeptides of the reaction center, which also bind P700 and its initial electron acceptors Fe-SA and Fe-SB are on a separate polypeptide. The quinone that is reduced in photosystem I probably transfers an electron to Fe-Sx, which in turn reduces Fe-SA and Fe-SB. From here, electrons move to ferredoxin, a soluble iron-sulfur protein found in the chloroplast stroma, then to a flavoprotein (ferredoxin-NADP oxidoreductase), and finally to NADP+. 

These are involved in a wide range of electron-transfer processes and in certain oxidation-reduction enzymes, whose function is central to such important processes as the nitrogen cycle, photosynthesis, electron transfer in mitochondria and carbon dioxide fixation. The iron-sulfur proteins display a wide range of redox potentials, from +350 mV in photosynthetic bacteria to —600 mV in chloroplasts. 

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. 

Iron-sulfur proteins can be classified on the basis of iron and sulfide content into "plant-type iron-sulfur proteins, and "bacterial-type iron-sulfur proteins. Plant-type iron-sulfur proteins contain just two Fe and two inorganic S atoms per protein molecule the name refers to the material first isolated from chloroplasts. The bacterial-type iron-sulfur proteins always contain more than two Fe (and S—) atoms per protein molecule in general there are eight Fe and eight S— atoms per protein molecule. 

De Vitry, C., Oyuang, Y., Finazzi, G., Wollman, F.A. and Kallas, T., 2004, The chloroplast Rieske iron-sulfur protein - at the crossroad of electron transport and signal transduction. J. Biol. Chem. 279, 44621-44627. 

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]. 

Chloroplast ferredoxin containing the [(2Fe-2S)-(S-Cys)4] cluster is one common type of iron-sulfur protein. Another [2Fe-2S]-type protein is the Rieske iron-sulfur protein, present in the Cyt >6/complex as well as the Cyt Ac, complex. The pair of iron atoms in the cluster ofthe Rieske iron-sulfur protein are bound to two cysteine and two histidine residues, in addition to two sulfur atoms. The three-dimensional structures of ferredoxins and that of the Rieske iron-sulfur protein have been determined by X-ray crystallography (see Chapters 34 and 35, respectively, for the structure ofthe chloroplast ferredoxin and the Rieske iron-sulfur protein). The sulfide ions in iron-sulfur proteins urt acid-labile this provides a simple means for detecting the iron-sulfur proteins, as the sulfide is released as H2S upon acidification. The oxidized and reduced states of iron-sulfur clusters differ by just one unit of formal charge, corresponding to and Fe. Iron-sulfurproteins are commonly characterized by optical absorption, circular-dichro-... 

The membrane-bound iron-sulfiir centers were discovered by Dick Malkin and Alan Bearden in 1971 in spinach chloroplasts using EPR spectroscopy. Since the EPR spectrum was found to resemble that of the iron-sulfur protein ferredoxin and since the soluble ferredoxin had already been removed from the chloroplast sample used in the measurement, the substance represented by the newly found EPR spectrum was initially called membrane-bound ferredoxin. And since the iron-sulfur center was also found to be photo-reducible at cryogenic temperature, it was therefore suggested that it was the primary electron acceptor of photosystem I. 

Fig. 3. Left EPR spectra of membrane-bound iron-sulfur proteins reduced by dithionite and methyl viologen. HP700 particles from spinach (A), soybean (B) and Anabaena chloroplasts (C). Spectra D and E are for chloroplasts prepared from mutant No. 8 of Scenedesmus obliquus and for TSF-IIa particles, respectively. Right plot of the EPR-signal amplitude at g=2.05 due to reduced iron-sulfur-protein vs. the P700 concentration in chloroplast and subchloroplast samples from various sources (1, unfractionated chloroplasts 2, D144 particles 3, TSF-1 particles 4. D144 from stroma 5, D144 from grana 6, HP700 particles 7, HP700 particles from soybean 8, chloroplasts from Anabaena 9, chloroplasts from mutant No. 8 of Scenedesmus obliquus-, and 10, TSF-2a particles. Unpublished results of B Ke and H Beinert (1973).

In an attempt to characterize the membrane-bound iron-sulfur proteins, Malkin, Aparicio and Amon isolated the iron-sulfur protein by acetone extraction of chloroplasts which had previously been freed of soluble ferredoxin. They estimated the molecular mass of the protein to be 8 kDa. They also found that its absorption spectrum displayed no characteristic features. The EPR spectrum of the isolated protein was, however, quite different from that of the protein in the native, membrane-bound state. 

MOW Evans SG Reeves and R Cammack (1974) Determination of the oxidation-reduction potential of the bound iron-sulfur proteins of the primary electron acceptor complex of photosystem I in spinach chloroplasts. FEBS Lett 49 111-114... 

R Cammack and MCW Evans (1975) EPR spectra of iron-sulfur proteins in dimethylsulfoxide solutions e vi-dence of chloroplast photosystem I particles contain 4Fe-4S centers. Biochem Biophys Res Commun 38 1114-11188... 

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. 

R Maikin and PJ Aparicio (1975) Identification of a g=1.90 high-potential iron-sulfur protein in chloroplasts. Biochem Biophys Res Commun 63 1157-1160... 

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