394 IRON-CONTAINING PROTEINS AND ENZYMES
(Nε 2 – Fe = 2.03 Å ) and met160 (S δ – Fe = 2.37 Å ). The heme is covalently anchored
to the protein through aa residues cys37 and cys40. The cysteine residues occur
in a typical cytochrome c CXXCH pattern — in this case cys37 – ser38 – ser39 –
cys40 – his41. The propionate groups of heme c 1 participate in salt bridges with
arg120 of the cytochrome c 1 protein and with his161 of the Rieske iron – sulfur
protein (of the opposite monomer). The [2Fe – 2S] center of the ISP is held in
place by bonds to four ISP aa residues: his141 (Fe 2 – N δ 1 = 2.18 Å ), his161
(Fe 2 – N δ 1 = 2.17 Å ), cys139 (Fe 1 – S γ = 2.21 Å ), and cys158 (Fe 1 – S γ = 2.25 Å ).
The cytochrome b protein contains two hemes, heme b L (L = low potential,
also called b 565 in the literature) and heme b H (H = high potential, also called
b 562 in the literature). Cytochrome b is composed of eight transmembrane
helices (A to H). There are seven extra - membrane loops connecting the trans-
membrane helices. Of these, loops AB (with short helix ab), CD (featuring
short helices cd1 and cd2 and a hairpin turn) and EF (with short helix ef) pass
through the intermembrane space and are important participants in forming
the Q o cavity. This cavity extends into the membrane from the intermembrane
space, and it sits between the [2Fe – 2S] center of the ISP and heme b L. The
long DE loop is on the matrix side of cytochrome b, helping to form the bound-
ary of the Q i pocket that is located near the matrix extending upward into the
membrane. (See Figure 7.28 and Figure 1 of reference 86 .)
Figure 7.28 Secondary structure of the cytochrome bc 1 complex.intermembrane space side246E220PEWY sequence-residues 270- 273 in bovine cyto b
11 a 18
N61
ab 72252
257EF loop
CD loopEF loopAB loop 137 cd1 152165 cd2 15654 274 ef 283A2875B104DE loopmatrix side22–24133CD100171204F1F2286304G340318H346379
C305
308219–217his83
heme bL
his182heme bH
his196his97