Analysis of xenon binding to photosystem II by X-ray crystallography
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In order to investigate oxygen binding and hydrophobic cavities in photosystem II (PSII), we have introduced xenon under pressure into crystals of PSII isolated from Thermosynechococcus elongatus and used X-ray anomalous diffraction analyses to identify the xenon sites in the complex. Under the conditions employed, 25 Xe-binding sites were identified in each monomer of the dimeric PSII complex. The majority of these were distributed within the membrane spanning portion of the complex with no obvious correlation with the previously proposed oxygen channels. One binding site was located close to the haem of cytochrome b559 in a position analogous to a Xe-binding site of myoglobin. The only Xe-binding site not associated with the intrinsic subunits of PSII was within the hydrophobic core of the PsbO protein.
KeywordsPhotosystem II Xenon-binding X-ray crystallography Oxygen channel Cytochrome b559 PsbO protein
Chlorophyll-binding PsbC protein
Reaction centre PsbA protein
We thank Elspeth Garman for the generous loan of a xenon cylinder and the pressurisation cell. We acknowledge financial support for this work from the Biotechnology and Biological Science Research Council (BBSRC) and from The Royal Society UK–Japan exchange programme. Preliminary data were collected at the Diamond synchrotron. We wish to acknowledge our access to the facilities and staff of the Swiss Light Source, particularly Clemens Schulze-Briese.
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