Structure of the mobile ubiquinone QB binding site. (a) Superposition of ubiquinone observed here (black) with earlier proximal (red) and distal (blue) QB binding sites in RCSph (PDB entries 1aig and 1aij). (b) Superposition of ubiquinone observed here (black) with an earlier proximal (red) QB binding site in RCvir (PDB entry 2i5n). (c) Electron density for the LCP SFX structure. (d) Electron density showing the presence of monoolein in an earlier LSP crystal form (PDB entry 2wjn). Fobs − Fcalc omit electron-density maps (green) are contoured at 3σ. 2Fobs − Fcalc electron-density maps (blue) are contoured at 1σ.