C. Electron and Proton Transport Within the Cytochrome b 6 ƒ
ComplexThe cytochrome b 6 ƒ complex mediates the electron transport between photosystem II and photosystem I
via plastoquinol to plastocyanin and cyclic electron flow around PSI via ferredoxin to plastocyanin. The
binding of plastoquinol is a function of Cyt b 6 , subunit IV, and Rieske proteins [51]. The oxidation of
plastoquinol is described by the concept of the Q-cycle. According to this concept, cytochrome b 6 ƒ acts
as a plastoquinol oxidizing and plastoquinone reducing enzyme. Although the details of this dual func-
tion are still debated [66], in general it means that cytochrome b 6 ƒ binds plastoquinol and plastoquinone.
For every electron transported along the linear path from plastoquinol to plastocyanin, another one is used
for reduction of the plastoquinone. This process is coupled with injection of two protons in the thylakoid
lumen. The oxidation of a second molecule of plastoquinol leads to complete reduction of bound plasto-
quinone to plastoquinol via acceptance of two protons from the stromal site of thylakoid. In summary, the
linear transport of two electrons would lead to release of four protons in the thylakoid lumen, oxidation
of two molecules of plastoquinol, and coupled reduction of one molecule of plastoquinone to plasto-
quinol. Under natural conditions, depending on light intensities, the ratio can vary and a strong decrease
appears under high light [66]. Mechanistically, this could be explained by proton channels connecting the
plastoquinol binding site alternatively to the luminal or stromal side of the cytochrome b 6 ƒ complex, giv-
ing rise to a proton slip reaction at high transmembrane pH [66]. This scheme is supported by several
discoveries:
The appearance of an internal five-water chain, which has the properties of a proton wire and serves
as a long-distance proton thanslocation line, was suggested by Martinez et al. [60] and Cramer
et al. [51].
The existence of two binding places at the Qo-plastoquinol/plastoquinone binding site was proved
by use of inhibitors [67].
A flexible hinge was found in the Rieske protein that allows it to orient in two configurations: the
first close enough to cytochrome b 6 to allow electron transfers from the plastoquinol binding site
and the second close enough to the cytochrome ƒ in the lumen to allow electron transfer to its
heme was also found [65].IV. COMPLEX PHOTOSYSTEM I
A. Supramolecular Organization of Photosystem I Complex
Photosystem I (PSI) is a pigment-protein complex that functions as a plastocyanin:ferredoxin oxidore-
ductase [68]. The holo-PSI complex is composed of light-harvesting complex I (LHCI) and a core com-
plex. The holo-PSI contains up to 17 subunits, 100–200 molecules chlorophyll per P700 (chlorophyll a/b
ratio is greater than 5), 10–15 molecules -carotene, 2 phylloquinones, and 3 (4Fe-4S) clusters [68,69].
Some lipids may also be integral components of the PSI complex [68]. Limited information about LHCI
is available. It is accepted that two sets of light-harvesting pigment-protein complexes deliver energy to
PSI. LHCI is specific for PSI and probably bound to PSI in a fixed stoichiometry [68]. LHCII serves as a
light-harvesting complex for PSII as well as PSI and its association with PSI and PSII is variable [70].
Figure 5 summarizes the current knowledge of the PSI subunit structure.
The core complex drives the electron transfer from plastocyanin, which is located in the luminal
space, to ferredoxin, which is situated in the chloroplast stroma. The core complex combines both inte-
gral and peripheral subunits. It consists of 13 subunits—from A to N (gene products of psaAtopsaN)
without M, which was found only in cyanobacteria [69].
B. Subunit Organization of the PSI Reaction Center (Core) Complex
- PSI-A and PSI-B Subunits
The backbone of PSI is a heterodimer consisting of the PSI-A and PSI-B subunits. It is known as
P700–chlorophylla–protein1 and, together with PSI-C, builds all the core complex pigments and elec-
tron transfer subunits [68,69].
288 DENEV AND MINKOV