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The Rieske Protein is found in cytochrome bc1 (shown in wireframe in the left screen) of the mitochondrial electron transport chain and also in the analogous b6f complex of the chloroplast electron transport chain. It contains a 2Fe-2S centre that receives, in the case of the bc1 complex, electrons from ubiquinol (QH2) and passes them on to either a cytochrome c docked at the surface of the bc1 complex via a cytochrome f centre or to the cytochrome b centres in the bc1 complex as part of the "Q cycle." As electrons are transferred, four protons flow from the matrix side of the membrane to the inter membrane space, setting up a proton concentration gradient that is eventually used to generate ATP from ADP.
1 complex consists of 11 subunits; the Rieske subunit with the 2Fe-2S centre is shown in green. The cytochrome-f heme group in the blue subunit and the two cyctochrome-b heme groups in the violet subunit are also displayed.
2-) ligands and terminal thiolate and histidine ligands. This [(cys)2FeIII(S)2FeIII(his)2] geometry is a rare example of an iron sulfur centre that has ligands other than cysteinate ligands. The neutral overall charge of the complex makes it easier to reduce (E = 0.1 V) compared to the 2- overall charge of typical ferredoxin (E = -0.4 V) with a [(cys)2FeIII(S)2FeIII(cis)2]2- active site geometry.
The 2Fe-2S centre accepts and then gives up one electron at a time:
[(cys)2FeIII(S)2FeIII(his)2] + e- [(cys)2FeIII(S)2FeII(his)2]-
[(cys)2FeIII(S)2FeII(his)2]- [(cys)2FeIII(S)2FeIII(his)2] + e-