Rieske centers

A decade after the discovery of the Rieske protein in mitochondria (90), a similar FeS protein was identified in spinach chloroplasts (91) on the basis of its unique EPR spectrum and its unusually high reduction potential. In 1981, the Rieske protein was shown to be present in purified cytochrome Sg/complex from spinach (92) and cyanobacteria (93). In addition to the discovery in oxygenic photosynthesis, Rieske centers have been detected in both single-RC photosynthetic systems [2] (e.g., R. sphaeroides (94), Chloroflexus (95)) and [1] (Chlo-robium limicola (96, 97), H. chlorum (98)). They form the subject of a review in this volume. 

Rieske centers distinguish themselves from common [2Fe-2S] and [4Fe-4S] clusters (a) by an unusual EPR spectrum characterized by a lowgav-value of 1.91 as compared to 1.96 for most 2Fe2S and 4Fe4S ferredoxins (102) and (b) by an electrochemical potential that is al-... 

As early as 1976, Prince and Dutton (94) showed that the values of the mitochondrial and proteobacterial Rieske centers decrease above pH 8, indicating the involvement in the redox transition of a deprotonatable group on the oxidized form of the cluster. A similar pK value of about 8 was subsequently found for Rieske proteins from both the PQ/UQ and the MK classes (97, 130-132, 136-138). It was furthermore shown that the implication of two protons rather than one (131) with differing pK values (138) can be distinguished above pH 8. 

It is noteworthy that except for the Rieske center in Complex III, Complexes I and 11 are home to all the iron-sulfur clusters in the mitochondrial electron transfer chain and consequently most of the iron-containing carriers in the entire sequence. Hibbs subsequently showed that CAM-injured cells lose a substantial portion of their total intracellular iron (Hibbs et al., 1984) [later studies specifically identified loss of mitochondrial iron (Wharton et al., 1988)] and Drapier and Hibbs (1986) showed that the activity of another iron-sulfur-containing enzyme, aconitase, is also lost. In early 1987 Hibbs reported that the cytostatic actions of CAMs requires the presence of only one component in culture medium, L-arginine (Hibbs et al., 1987b). Thus, the stage was set for the discovery of a unique reactive species that targets intracellular iron, produced by CAMs. 

Inhibitors binding at the Qo site exclusively to cytochrome b, e.g., myxothiazol as well as E-fi-methoxyacrylate (MOA) and related inhibitors. These inhibitors prevent the reduction of both the Rieske center and heme 6l through die Qo site but heme hn can still be reduced through the Qi site with hydroquinone acting as a reductant (reverse electron flow). 

At this point, Q resides in the Q site. A second molecule of QH2 binds to the site and reacts in the same way as the first. One of its electrons is transferred through the Rieske center and cytochrome c j to reduce a second molecule of cytochrome c. The other electron goes through cytochromes b l and Z) to Q bound in the Q site. On the addition of the second electron, this quinone radical anion takes up two protons from the matrix side to form QH2. The removal of these two protons from the matrix contributes to the formation of the proton gradient. At the end of the Q cycle, two molecules of QH2 are oxidized to form two molecules of Q, and one molecule of Q is reduced to QH2, two molecules of cytochrome c are reduced, four protons are released on the cytoplasmic side, and two protons are removed from the mitochondrial matrix. 

In addition to the hemes, the enzyme contains an iron—sulfur protein with an 2he-2S center. This center, termed the Rieske center, is unusual in that one ot the iron ions is coordinated by two histidine residues rather than two cysteine residues. This coordination stabilizes the center in its reduced form, raising its reduction potential so that it can readily accept electrons from... 

Figure 8 Model and proposed reaction cycle for arsenite oxidase from A.faecalis. Reaction steps are (1) binding of arsenite, AsOjOH, to the enzyme, (2) two-electron transfer to Mo, oxidizing As(III) to As(V) and reducing Mo(Vl) to Mo(lV), (3) release of arsenate oxyanion, (4) two-electron transfer from Mo(Vl) to [3Fe-4S] center, regenerating Mo(IV) reaction center, (5) two-electron transfer from [3Fe-4S] center in large subunit to [2Fe-2S] Rieske center of small subunit, and (6) electron transfer from the [2Fe-2S] center of arsenite oxidase to the associated small copper protein azurin. (Based on Refs. 63 and 64.)...

Tinberg CE, Tonzetich ZJ et al (2010) Characterization of iron dinitrosyl species formed in the reaction of nitric oxide with a biological Rieske center. J Am Chem Soc 132 18168-18176... 

For the tether to be responsible for the motion of the FeS center of the globular component of the Rieske center from the Q site to the heme Cl site it must have contracted the length of the tether must have shortened. This question... 

Rieske oxygenases are part of a superfamily of enzymes that share a characteristic structure consisting of an oxygenase component (a mononuclear non-heme iron(II) high spin center containing a 2-His-l-carboxylate facial triad motif in the active site) [31-33]. Besides, the active site contains a reductase component (an Fe2-S2 Rieske center) that delivers electrons from NAD(P)H to the oxygenase center [34]. 

Fig. 5,4. A scheme of the photosystem 1 and the bjf complexes arrangement in the thylakoid membrane. Photosystem 1 P700 is the reaction center Aq, and 42 are electron acceptors and FeSA and FeSg are two forms of bound ferredoxin. bjf complex / is the cytochrome / 6553 is cytochrome 65 3 FeS is the Rieske electron transport protein PQH2 is the plastoquinol molecule bound to Rieske center Red is ferredoxin-NADP-reductase Fd is water soluble ferredoxin and Pc is plastocyanin.

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