Fig. 1.

Prosthetic groups and subunit structures of the cyt bc₁ complex. (A) Arrangement of prosthetic groups in the dimeric bc₁ complex and illustration of the electron bifurcation at the Q_o site. The b_L, b_H, and c₁ heme groups are shown as ball-and-stick models, and the [2Fe2S] clusters are depicted as cpk models. Carbon atoms, black; nitrogen, blue; oxygen, red; sulfur, yellow; iron, brown. The Q_o pockets near the IMS side of the membrane and the Q_i pockets near the matrix side are labeled and shaded in gray. Cyt c is shown as a gray shaded oval. Distances between redox centers are given on the left half of the diagram, and the redox potential for each center is given on the right. The high- and low-potential ET paths are depicted with red and green arrows, respectively. Circles in pink and light green within the Q_o pockets are hypothesized distal-QH₂ and proximal-Q binding sites, respectively. (B) Ribbon diagram of the dimeric cyt b, cyt c₁, and ISP subunit in the mitochondrial bc₁ complex. Two symmetry-related cyt b subunits are shown (green and light green). The eight TM helices of cyt b are denoted with letters A–H. Helices A–E form one bundle in which the two b-type hemes (b_L and b_H in ball-and-stick models) reside; helices F–H form the other bundle. The ISP subunit (yellow and red for the symmetry pair) has an extrinsic soluble domain with a [2Fe2S] cluster at its tip, connecting to a TM segment by a flexible neck. The extrinsic domain of cyt c₁ (blue and magenta for the symmetry pair) with its heme group is rigidly attached to its TM helix. The locations for the two active sites (Q_o and Q_i) per monomer in cyt b are labeled. The surface depression in cyt b at the IMS side of the membrane is labeled as the ISP-docking crater.