Ubiquinone binding sites membrane proteins

To begin with, the proposed ubiquinone binding site was unlike any of the previous sites that have been described in membrane proteins that bind ubiquinone. As presented above the photosynthetic reaction center (Deisenhofer etal., 1995), the bc complex (Xia etal., 1987 Zhang et al., 1998 Iwata et al., 1998 Hunte et al., 2000), and fumarate reductase (Iverson et al., 1999 Lancaster et al., 1999) bind the ubiquinone molecule within membrane-spanning helices where the ring of the ubiquinone molecule is oriented near the phospholipid head group of the membrane. Second, a large mutational study of residues at or around... 

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... 

Fig. 10, taken from the exceptional review of Sitming, illustrates some recently obtained insights into the effect of protein structure of the L-subunit on herbicide action. Fig. 10 (A) shows how Qb is situated in the binding site ofthe L-subunit ofwild-typeRp. viridis. The figure includes transmembrane helices D and E and the membrane-parallel helix de. Here His 190 and His230 in the L-subunit of Rp. viridis and the corresponding His215 and His272 in D1 are the known ligands to Qb and the Fe-atom. Ubiquinone Qb in the bacterial RC fills the entire pocket formed by helices D and E and the de loop. Hydrogen bonds are...

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