Ubiquinone binding sites cytochrome

Unlike the other ubiquinol binding sites presented above, the ubiquinol binding site in cytochrome bo% stabilizes a semiquinone during the oxidation of ubiquinol. A previous model placed the ubiquinone binding site of cytochrome bo within the extrinsic domain of subunit II, replacing the Cua site (Murray et al., 1999). However, several lines of evidence are available to refute this model. 

In the h positional state , tiie distance is > 30 A between the Rieske cluster and cytochrome Cl whereas in the ci positional state , tiie Rieske [2Fe-2S] cluster is far away from tiie ubiquinone binding site and points in tiie opposite direction (Figure 3). Therefore, tiie Rieske protein has to move between these positional states in order to transfer an electron fiom ubihydroquinone to cytochrome Ci. The movement involves a rotation of the entire water soluble domain of 57 this rotation requires stretching of the flexible linker. 

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... 

In both structures the putative binding site for a mobile carrier (for ubiquinone or cytochrome c in bovine or only cytochrome c in S. cerevisiae) of each individual complex is in close proximity with the corresponding binding site of the neighboring complex (Fig. 8.4) supporting a substrate channeling mechanism in the supercomplex. 

---