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Figure 3.
(a) Schematic view of the PQ-PQH[2] exchange cavity and the
two entry and exit portals connecting the Q[B] and Q[C] sites to
the PQ pool in the thylakoid membrane. Approximate dimensions
are given in angstroms; Q[B] and Q[C] are colored cyan and
yellow, respectively, the Q[B] site is highlighted in pink, and
the three lipids forming the cork (the head groups for PG22,
SQDG4 and MGDG18 are shown as red, green and white squares)
nearly closing the cavity toward the cytoplasm are indicated.
(b) Calculated channels (I and II, gray) for PQ-PQH[2] transfer
between the PQ pool and the Q[B] and Q[C] sites, viewed from the
cytoplasmic side. Shown are the PQs in the Q[B] site (light
blue) and Q[C] site (yellow), non-heme Fe^2+ (blue sphere),
Car15 (orange), Chl37 (green), SQDG4 (gray), cyt b-559 heme
(dark blue) and the surrounding proteins (pink). Car[D2],
Chl[D2] and MGDG7 are not shown. (c) Possible mechanisms for the
PQ-PQH[2] exchange between the Q[B] site of PSII and the PQ pool
in the thylakoid membrane, viewed from the cytoplasm. Channels I
and II open toward the PQ pool. PQ is shown with a red and
PQH[2] with a blue head group. The Q[B] site is highlighted
pink, the Q[C] site in green and labeled; the yellow patch
indicates a hydrophobic region formed by the fatty acids of
MGDG7, MGDG18 and the phytol chain of Chl[D2]. Small arrows
symbolize movements of PQ molecules. See text for explanations
concerning the alternating, wriggling and single-channel
mechanisms.
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