Please be patient while the structures in the left frame load. In order to display all of the structures in the tour properly, press 'View' buttons below in order (from 1 to the end).
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.
The bc1 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.
The Rieske protein is spacefilled in this view. The mobile head of the protein containing the 2Fe-2S centre moves depending on whether cytochrome-c is docked to receive electrons. This causes an alternate transfer of electrons from the 2Fe-2S centre first to cyt-f and next to cyt-b.
The active site has two distorted tetrahedral iron centres coordinated by two bridging sulfido (S2-) 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-
A disulfide bond between cysteines 144 and 160 serves to lock the Fe2S2 group in place. This may be necessary to stabilize the weak Fe-N bonds in this complex.
This is the display of the bc1 complex. Feel free to play around with it and all the other molecules displayed in the previous pages.