Photosystem II (PSII) is a large homodimeric protein-cofactor complex that acts as light-driven water:plastoquinone oxidoreductase and is located in the photosynthetic thylakoid membrane of plants, green algae and cyanobacteria. The principal function of PSII is to oxidize two water molecules at the unique Mn₄Ca cluster to molecular (atmospheric) oxygen, 4 protons and 4 electrons. The protons serve to drive ATP synthetase and the electrons reduce plastoquinone (Q_B) to plastoquinol (Q_BH₂) that is exported and delivers the electrons (through the cytochromeb₆f complex) to photosystem I. Here the electrons gain a high reducing potential and serve at NADP reductase to generate NADPH that together with ATP reduces CO2 to carbohydrates in the Calvin cycle.

The crystal structure of PSII from Thermosynechococcus elongatus at 2.9-Å resolution allowed the unambiguous assignment of all 20 protein subunits and complete modeling of all 35 chlorophyll a molecules and 12 carotenoid molecules, 25 integral lipids and 1 chloride ion per PSII monomer. The presence of a third plastoquinone Q_C and a second plastoquinone-transfer channel, which were not observed before, suggest mechanisms for plastoquinol-plastoquinone exchange, and we calculated other possible water or dioxygen and proton channels. Putative oxygen positions obtained from Xenon derivative crystals indicate a role for lipids in oxygen diffusion to the cytoplasmic side of PSII. The chloride position suggests a role in proton-transfer reactions because it is bound through a putative water molecule to the Mn₄Ca cluster at a distance of 6.5Å and is close to two possible proton transfer channels.

References and Footnotes

Guskov, A et al., Nature Structural and Molecular Biology, February 2009.