Cyanobacterial photosystem II at 2.9-A 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-A 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 Q(C) 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 Mn(4)Ca cluster at a distance of 6.5 A and is close to two possible proton channels.
Guskov
Albert
A
Institut für Chemie und Biochemie/Kristallographie, Freie Universität Berlin, Takustrasse 6, D-14195 Berlin, Germany.
Kern
Jan
J
Gabdulkhakov
Azat
A
Broser
Matthias
M
Zouni
Athina
A
Saenger
Wolfram
W
D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D002712:Chlorides / Q000737:chemistry; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000737:chemistry*; D008055:Lipids / Q000737:chemistry; D008958:Models, Molecular; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D020836:Protein Structure, Quaternary; D021122:Protein Subunits / Q000737:chemistry; D011809:Quinones / Q000737:chemistry
D001426:Bacterial Proteins; D002712:Chlorides; D008055:Lipids; D045332:Photosystem II Protein Complex; D021122:Protein Subunits; D011809:Quinones