Cyanobacterial photosystem II at 2.9-Å resolution and the role of quinones, lipids, channels and chloride

Photosystem II ( PSII) is a large homodimeric protein-cofactor complex located in the photosynthetic thylakoid membrane that acts as light-driven water: plastoquinone oxidoreductase. The crystal structure of PSII from Thermosynechococcus elongatus at 2.9-angstrom 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 monomer. The presence of a third plastoquinone QC and a second plastoquinone-transfer channel, which were not observed before, suggests mechanisms for plastoquinol-plastoquinone exchange, and we calculated other possible water or dioxygen and proton channels. Putative oxygen positions obtained from a Xenon derivative 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 6.5 angstrom and is close to two possible proton channels. (c) 2009 Nature America, Inc. All rights reserved.