Stroma
cyt b-559
Lumen
of light energy and its transfer to the reaction centre core; the reaction centre which contains the redox components involved in the photochemistry of charge separation and including the oxygen - evolving complex (OEC) which is concerned with water oxidation; the regulatory cap which is composed of polypeptides bound lumenally to the thylakoid membrane.The antenna is made up of proximal and distal antennae. The former encloses the reaction centre core and consists of two pigment - protein complexes, CP47 and CP43, which bind Chi a and P - carotene molecules, but not Chi b. The distal antenna consists of the light - harvesting complexes (LHCII) and binds Chi a, Chi b and xanthophylls. A 6 Â electron diffraction
structure of LHCII (Kuhlbrandt 1991) is available and from this it is clear that the
i and Wang,
complex is a trimer with each monomeric subunit binding 15 chlorophylls and some carotenoids. The LHCII can dissociate itself from PSII and associate itself more closely with PSI, so it plays a part in energy distribution by mediating between the two photosystems. The process is regulated by the phosphorylation and dephosphorylation of certain amino acid residues.
The regulatory cap is made up of three hydrophilic extrinsically bound proteins, BP33, BP24 and BP17. BP33 separates the OBC from the contents of the lumen. Cross - linking studies have shown that it is in contact with the reaction centre heterodimer. It is thought to have a role in water oxidation as its removal by Tris - washing results in reduced oxygen - evolving activity. It is sometimes referred to as the manganese - stabilising polypeptide because it is thought to be involved in the stable assembly of the manganese cluster and might even provide ligands to the manganese atoms.
The PSU core is the minimal functional unit. It is capable of carrying out the photoinducible charge separation reactions but is unable to evolve oxygen. The PSII core can be routinely isolated and has the following composition : 4 to 6 Chi a molecules, 2
phaeophytins, 2
(3
- carotenes, 1 heterodimer, 1 CP47, 1 CP43, 1 or 2 cytochrome bggg and 1 110 kDa I polypeptide per P680. The core preparation contains no quinone (Q^) and no OEC. PSII preparations that evolve oxygen also contain and a closely associated non -haem iron atom.
Kinetic studies have been carried out on the PSII reaction centre preparations using optical absorption techniques. The primary charge separation reaction which produces the radical pair P680^Phe" occurs in 3 ps. Charge recombination is prevented by the rapid transfer of the electron from Pheo" to in 200 to 500 ps and the rapid rereduction of P680^by the redox active tyrosine residue Y^. Qa" is oxidised in 1 0 0 to 2 0 0 ps by a second quinone, Qg
to give the semiquinone radical. This semiquinone is reduced by a second Qa to give Qg^ . By analogy with the purple sulphur bacteria it is assumed that Qg^" is doubly protonated to plastoquinol which is released from its binding and migrates to the bgf complex. The vacant Qg niche in the Dj polypeptide is filled by another Qg drawn from the cell’s plastoquinone pool.
PSII contains a second redox active tyrosine residue, Y^, in the D2 polypeptide present.
This tyrosine residue is known to interact with the the oxygen evolving complex in various S states. For example, in the dark the OEC adopts the state from Sq by donating an electron to Y^^, or from 8 3/ S3 by accepting electrons from Yg. Y^ has also been implicated
in the light - activated assembly of the manganese cluster of the OEC.
Cytochrome b^^g is a heterodimer consisting of a 9 kDa CLsub-unit and a 4 kDa p- subunit which coordinate a haem prosthetic group. This cytochrome may play a role in reducing the likelihood of photoinhibition and / or proton pumping during cyclic electron transfer in PSII.
for water oxidation is thought to be a tetrad of manganese atoms. This manganese cluster is thought to provide the means of storing the oxidising equivalents needed to oxidise water, as well as containing the substrate binding site and site of oxygen evolution. As the reaction centre is turned over by the absorption of successive light quanta, electrons are stripped from the OEC, leading to the accumulation of oxidising equivalents by a process known as the Kok cycle. According to this cycle the OEC goes through 5 distinct redox states, designated Sq to S4. It is known that oxygen gas is liberated at the S3 to Sq transition. The actual mechanism
of water oxidation, the identity of the S - state that binds water, which S - state transitions involve manganese oxidation are the subjects of ongoing investigation. The structure of the manganese cluster has been studied using X-ray absorption fine structure (XAFS) spectroscopy and should soon be known through X - ray diffraction studies of PSII crystals. The chemistry of the Kok cycle is being studied using ESR, XAFS and X - ray absorption near edge structure (XANES) spectroscopy.
1.2.2 Photosystem I (PSI)
Thanks to improvements in preparations/biochemical manipulation, the continuing application of spectroscopic techniques and more recently site directed mutagenesis, the structural and functional elucidation of PSI continues apace though several difficult questions remain to be answered. A schematic diagram of higher plant PSI is shown in figure 1.5.
The PSI reaction centre can be viewed as a complex integral membrane enzyme system, a light dependent plastocyanin : ferredoxin oxidoreductase to be exact. This reaction centre is found embedded in the thylakoid membranes of higher plants, green algae and the prokaryotic cyanobacteria. Essentially an electron donated by lumenally bound plastocyanin,
Figure 1.5 Model of PSI viewed in the plane of the membrane, showing the arrangement of the important polypeptides. The various redox components are arranged linearly between the two core polypeptides for the sake of clarity, the actual arrangement of the acceptors is not known with certainty. This schematic does not take into accoun that the redox chain is branched, there being two A,s and two AqS per P700.