Within the thylakoid membranes of the chloroplast, are two photosystems. Photosystem I optimally absorbs photons of a wavelength of 700 nm. Photosystem II optimally absorbs photons of a wavelength of 680 nm. The numbers indicate the order in which the photosystems were discovered, not the order of electron transfer. Under normal conditions electrons flow from PSII through cytochrome bf (a membrane bound protein analogous to Complex III of the mitochondrial electron transport chain) to PSI. Photosystem II uses light energy to oxidize two molecules of water into one molecule of molecular oxygen. The 4 electrons removed from the water molecules are transferred by an electron transport chain to ultimately reduce 2NADP+ to 2NADPH. During the electron transport process a proton gradient is generated across the thylakoid membrane. This proton motive force is then used to drive the synthesis of ATP. This process requires PSI, PSII, cytochrome bf, ferredoxin-NADP+ reductase and chloroplast ATP synthase. I. Photosystem II Photosystem II transfers electrons from water to plastoquinone and in the process generates a pH gradient. O H3 C H3 C CH3 C H2 O C H CH3 C C H2 H
n = 6-10
2e- + 2H+ Plastoquinone
Plastoquinone (PQ) carries the electrons from PSII to the cytochrome bf complex. Plastoquinone is an analog of Coenzyme Q. The only differences are the methyl groups replacing the methoxy groups of Q and a variable isoprenoid tail. Plastoquinone can functions as a one or two electron acceptor and donor. When it is fully reduced to PQH2 it is called plastoquinol. Like CoQ, PQ is a lipophilic mobile electron carrier carrying electrons from PSII to cytochrome bf.
Photosystem II is homologous to the purple bacterial photoreaction center we talked about previously. PSII is an integral membrane CH CH H C protein. The core of this membrane protein is formed by two subunits H C C C C C H D1 and D2. These two subunits span the membrane and are H H H n homologous to subunits L and M of the bacterial photosystem. Of OH course PSII is more complicated than its prokaryotic counterpart. PSII Plastoquinol contains a lot more subunits and additional chlorophylls to achieve a lot higher efficiency than bacterial systems. OH
3 3 3 3 2 2
The overall reaction of PSII is shown below. 2PQ + 2H2O O2 +2PQH2
The ribbon diagram of the crystal structure of PSII is shown below. The D1 subunit is shown in red and is homologous to the L subunit of the bacterial photosystem. The D2 subunit is shown in blue and is homologous to the M subunit of the bacterial photosystem. The structure of a bound cytochrome molecule is shown in yellow. The chlorophyll molecules are shown in green. The manganese center is shown in purple.
Just as in the bacterial photosystem, there is a special pair of chlorophylls in PSII bound by D1 and D2 that are in close proximity of each other. This special pair is analogous to the special pair of bacteriochlorophylls in the bacterial photosystem. The PSII special pair consists of 2 chlorophyll a molecules that absorb light at an optimal wavelength of 680 nm. This special pair of chlorophylls is called P680. On excitation-either by the absorption of a photon or HC H C CH exciton transfer-P680* rapidly transfers an electron to a nearby pheophytin a. Pheophytin a is a chlorophyll a molecule with the Magnesium replaced by two protons. N CH H C H The electron is then transferred to a tightly bound N N plastoquinone at the QA site. The electron is then O H transferred to an exchangeable plastoquinone located at CH N the QB site of the D2 subunit. The arrival of a second H H C P O electron to the QB site with the uptake of two protons from O the stroma produces plastoquinol, PQH2. 3 2 3 3 2 3
H3 C HC CH2
H3 C N O
CH3 N CH2 H 3C
When the electron is rapidly transferred from P680* to pheophytin a, a positive charge is formed on...
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