Rieske proteins function

Therefore, although the function of the helix-loop insertion in mitochondrial Rieske proteins appears to be the same as that of the C-terminal extension in chloroplast Rieske proteins, both structures show no structural similarity or sequence homology. 

In addition to this large movement of the Rieske protein, small but nevertheless significant conformational differences within the functional domain are observed. The structure of the functional domain of the Rieske subunit in the PGi22 crystal form showing the ci positional state is the same as that of the water soluble fragment... 

Because of the exposed histidine ligands of the [2Fe-2S] cluster, the Rieske is capable of binding quinones in a redox-dependent manner. The variation of the hydrogen bond strength and of the electrostatic properties will control the movement of the catalytic domain of the Rieske protein. Therefore, the function depends on the unique structural and electrochemical properties of the Rieske cluster. 

D. A Unique Functional Mechanism Involving the Rieske Protein... 

X-ray structures of mitochondrial 6ci complexes from three different sources (113, 124, 125) have found the b- and c-type hemes at roughly identical positions, whereas the Rieske protein was seen in different places as a function of crystal space group and presence or absence of inhibitors of the enzyme. This fact was interpreted to suggest a long-range conformational movement of the Rieske protein during turnover of the complex. The range of observed positions of the Rieske protein indicated that the soluble domain can move like a... 

Figure 13.13 The structure of the yeast cytochrome bct complex (a) the homodimeric complex of the catalytic subunits cytochrome b (blue), Rieske protein (green) and cytochrome c, (yellow) with their cofactors and the six additional subunits. Part b shows the catalytic subunits of one functional unit in the same orientation. (From Hunte et al., 2000. Copyright 2003, with permission from Elsevier.)...

The general composition of the complex in all systems studied so far is also universal they always contain two h-type cytochromes, one cytochrome of c type and a high potential Fe-S protein (the Rieske protein, so called after its discoverer in Complex III of the respiratory chain of beef heart mitochondria). In addition to these functions in electron transfer, the h/cj complexes also play a role in energy transduction, since they represent an essential part of the proton translocating apparatus of photosynthetic electron transfer chains. 

Comment on the similarities and differences between a [2Fe-2S] ferredoxin and Rieske protein, in terms of both structure and function. 

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... 

QH2 to the Rieske iron-sulfur protein, binds at QP, near the 2Fe-2S center and heme bL on the P side. The dimeric structure is essential to the function of Complex III. The interface between monomers forms two pockets, each containing a QP site from one monomer and a QN site from the other. The ubiquinone intermediates move within these sheltered pockets. 

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... 

The simpler cytochrome bc] complexes of bacteria such as E. coli,102 Paracoccus dentrificans,116 and the photosynthetic Rhodobacter capsulatus117 all appear to function in a manner similar to that of the large mitochondrial complex. The bc] complex of Bacillus subtilis oxidizes reduced menaquinone (Fig. 15-24) rather than ubiquinol.118 In chloroplasts of green plants photochemically reduced plastoquinone is oxidized by a similar complex of cytochrome b, c-type cytochrome /, and a Rieske Fe-S protein.119 120a This cytochrome b6f complex delivers electrons to the copper protein plastocyanin (Fig. 23-18). 

The action of some inhibitors is indicated in Figure 17.4. It is sometimes difficult to pinpoint exactly where an inhibitor may act, however, because our knowledge of the composition and function of the four complexes is far from complete. Complex I inhibitors, such as rotenone, piericidin A, and the barbiturates, are believed to inhibit the transfer of elctrons from the Fe-S centers to UQ. In complex III, antimycin appears to inhibit the reduction of UQ by cytochrome b. Myxothiazol and 2,3-dimercaptopropanol (BAL) inhibit the transfer of electrons from UQH2 to Rieske s protein, because they destroy the Fe-S centers. The action of cyanide and azide on complex IV is also unclear, but it is believed that these substances combine with the Fe3+ moiety of the a3 heme prosthetic group. 

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]. 

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