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Figure 1.
Fig. 1. Prosthetic groups and subunit structures of the cyt
bc[1] complex. (A) Arrangement of prosthetic groups in the
dimeric bc[1] complex and illustration of the electron
bifurcation at the Q[o] site. The b[L], b[H], and c[1] 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-QH[2] and proximal-Q binding
sites, respectively. (B) Ribbon diagram of the dimeric cyt b,
cyt c[1], and ISP subunit in the mitochondrial bc[1] 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 c[1] (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.
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