Rieske proteins reduction potentials

When the second-site revertants were segregated from the original mutations, the bci complexes carrying a single mutation in the linker region of the Rieske protein had steady-state activities of 70-100% of wild-type levels and cytochrome b reduction rates that were approximately half that of the wild type. In all these mutants, the redox potential of the Rieske cluster was increased by about 70 mV compared to the wild type (51). Since the mutations are in residues that are in the flexible linker, at least 27 A away from the cluster, it is extremely unlikely that any of the mutations would have a direct effect on the redox potential of the cluster that would be observed in the water-soluble fragments. However, the mutations in the flexible linker will affect the mobility of the Rieske protein. Therefore, the effect of the mutations described is due to the interaction between the positional state of the Rieske protein and its electrochemical properties (i.e., the redox potential of the cluster). 

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. 

Ferredoxins and Rieske proteins employ a (Fe )2/Fe Fe redox couple for biological electron transfer reactions. Within the protein, the two iron atoms are rendered inequivalent, even in the hilly oxidized (Fe )2 state, by the surrounding protein environment Within a synthetic cluster, however, both iron atoms are typically equivalent, as may be expected from the symmetry of the overall complex. Table 4 shows reduction potentials for selected analog clusters. 

It is also noted that rusticyanin has three additional histidines (five in all). An increase in reduction potential for Rieske s [2Fe-2S] protein (350 mV) as compared to that for chloroplast [2Fe-2S] ferrodoxins (-400 mV) has been explained by the coordination of two histidines instead of two cysteines (76). In the case of the high-potential [4Fe-4S] protein, the reduction potential of 350 mV, compared to that for [4Fe-4S] centers in bacterial ferrodoxins (-400 mV), is accounted for by a different redox state change. This is made possible by H bonding and/or the more buried nature of the [4Fe-4S] cluster (77, 78). On present evidence, neither of these possibilities would seem to explain the high E° of rusticyanin. Another so far unexplained difference in the case of rusticyanin is its stability at pH 2, which is the working pH in its in vivo reaction with an acid-resistant cytochrome and aqua Fe (47, 48). An X-ray crystal structure of rusticyanin is required to help understand these different properties. 

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... 

Rieske protein is the electron-transfer site in the oxidation of plastoquinol (a hydroquinone) to plastosemi-quinone, during which protons are released. Rieske protein has a positive reduction potential (4-290 mV) for that isolated from spinach chloroplasts, contrasting with negative values for [2Fe-2S] ferredoxins. The difference must be attributed to the His versus Cys coordination of one Fe centre. 

Mechanistically, the cyt bej complex is thought to contain two distinct catalytic domains located on each side of the membrane (8,9,10). The quinol oxidation site (called Qz in bacterial and Qo in mitochondrial systems) is on the outer side of the membrane. It converts a quinol molecule to a quinone by transferring an electron to the Rieske FeS center and another to the lower potential cyt b heme ( l) This second electron is subsequently transferred to the cyt bn which then reduces a quinone trapped at the quinone reduction site (called Qc in bacterial, Qi in mitochondrial systems) located in the vicinity of the inner negative face of the membrane (8,14). Several classes of inhibitors are known to affect the reactions catalyzed at these active sites of the cyt bci complex (11). Myxothiazol, mucidin and stigmatellin interfere with the electron transfer between ubiquinol, Rieske FeS protein and cyt bi at the Qz site (28). Although stigmatellin also affects the Photosystem II of... 

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