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Figure 3.
Fig. 3. Stereo views of the intramembrane core and bound
molecules. (A) Side view. The two b hemes (gray) are
bis-histidine-coordinated on the n and p sides of the B and D
helices (blue). Chlorophyll a (green) is sandwiched between the
F and G helices of subunit IV; the 20-carbon phytyl chain
(green) extends normal to the figure into the p side of the
quinone exchange cavity. Heme x (red-brown), ligated by water
and the heme b[n] propionate, lines the quinone exchange cavity,
in contact with plastoquinone (magenta) near the n side of the
cavity. TDS (yellow) is near the p side. (B) Linkage and
coordination of heme x. Colors are as in (A); Cys35 (yellow) on
the n side of the A helix makes the single covalent thioether
bond with heme x. The fifth ligand is a water that is
hydrogen-bonded (dashed line) to a heme b[n] propionate. Phe^40,
on the n side of the E helix, is parallel to heme x and near (6
to 9 Å) plastoquinone (PQ, magenta) in the cavity.
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