Chemical structures of (A) ubiquinol, (B) ubiquinone, (C) plastoquinone, (D) menaquinone and (E) demethylmenaquinone. The number of isoprenoid units is denoted as n. Ubiquinol is the reduced form of ubiquinone. (F) Cytochrome (cyt) bc1 complex from Saccharomyces cerevisiae and its relative position in the inner mitochondrial membrane. The location of the quinol oxidation Qo site is marked by the inhibitor stigmatellin (SMA) which was co-crystallised with cyt bc1 complex (pdb 2ibz). The location of the quinone reduction Qi site is indicated by ubiquinone-6 (UQ-6) which was co-isolated with the enzyme (pdb 2ibz). Soluble cyt c is a substrate of cyt bc1 complex. Its docking position is illustrated based on the X-ray structure of the electron-transfer complex (pdb 3cx5). The three membrane-bound catalytic subunits of one protomer of the dimeric enzyme, namely cyt b, Rieske iron-sulfur protein (ISP) and cyt c1 as well as the substrate cyt c, are separately illustrated in (G) The extrinsic domain of Rieske ISP undergoes diffusional movement and its position close to cyt b (b-position, pdb 2ibz) and close to cyt c1 (c-position, pdb 1be3) are both indicated. The iron-sulfur cluster (FeS) is depicted in gray scale at the c-position and the extrinsic domain at the c-position is only outlined. P and N indicate the electropositive and -negative sides of the inner mitochondrial membrane. Iron atoms are depicted in brown, sulfur atoms are shown in yellow.