Rieske-type Fe:S proteins

E. Fe S Proteins with Non-thiolate Ligands Rieske-Type Fe S Proteins. . 269... 

Fig. 19. Sequence features of the Rieske-type Fe S proteins. C, Cysteine D, aspartic acid F, phenylalanine G, glycine H, histidine I, isoleucine L, leucine P, proline S, serine T, threonine V, valine, W, tryptophan Y, tyrosine.

The two protons from plastoquinol are released into the thylakoid lumen. This reaction is reminiscent of that catalyzed by ubiquinol cytochrome c oxidoreductase in oxidative phosphorylation. Indeed, most components of the enzyme complex that catalyzes the reaction, the cytochrome bf complex, are homologous to those of ubiquinol cytochrome c oxidoreductase. The cytochrome hf complex includes four subunits a 23-kd cytochrome with two Z>-type hemes, a 20-kd Rieske-type Fe-S protein, a 33-kd cytochrome/with a c-type cytochrome, and a 17-kd chain. 

One of the earliest recognized Fe S proteins was that associated with mitochondrial electron transport (Rieske et al., 1964). Even in the first partial in vivo characterization it was apparent that the protein had spectral properties that set it apart from the bacterial and plant-type ferredoxins which had just been discovered. Namely, the EPR spectrum had a gave near 1.91 and the high-held g value was shifted upheld. Furthermore, the protein had an Eq of approximately -t-250 mV, 600 mV more positive than the ferredoxins. Due to the instability of the protein, a more detailed analysis was not possible until the 1980s, when an analogous protein was isolated from bacterial sources (Fee etal., 1984). The ensuing... 

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 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. 

Application of the impure bef complex to the hydroxyapatite column yielded a similar profile as in Fig. 2b, however with a predominant cyt.bef-peak (not shown). The four subunits of about 38, 24, 19 and 15 kDa (Table I) are likely to be cytochrome f- and cytochrome be-binding subunit, the Rieske Fe-S-protein and subunit IV, respectively, consistent with the subunit composition of other cyanobacterial cyt.bef-complexes. The presence of b-type and c-type cytochromes in a ratio of 2 1 was further confirmed by reduced-minus-oxidised difference spectra (not shown). 

A wide range of soluble redox enzymes contain one or more intrinsic [2Fe-2S]2+ +, [3Fe-4S]+ , or [4Fe S]2+ + clusters that function in electron transport chains to transfer electrons to or from nonheme Fe, Moco/Wco, corrinoid, flavin, thiamine pyrophosphate (TPP), Fe S cluster containing, or NiFe active sites. Many have been structurally and spectroscopically characterized and only a few of the most recent examples of each type are summarized here. Dioxygenases that function in the dihydroxylation of aromatics such as benzene, toluene, benzoate, naphthalene, and phthalate contain a Rieske-type [2Fe-2S] + + cluster that serves as the immediate electron donor to the monomeric nonheme Fe active site see Iron Proteins with Mononuclear Active Sites). The xanthine oxidase family of molybdoenzymes see Molybdenum MPT-containing Enzymes) contain two [2Fe-2S] + + clusters that mediate electron transfer between the Moco active site and the Other soluble molybdoen-... 

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-... 

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