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X-ray Information without the Damage

The oxygen-evolving complex of photosystem II uses energy derived from light to oxidize water and create oxygen. Information on the structure of the site of water oxidation is emerging slowly but steadily. Recent crystal structures of the enzyme from cyanobacteria have been used to produce models for the site, a tetranuclear manganese cluster and a calcium atom. Unfortunately, it is now clear that the x-ray fluxes used for the diffraction experiments are sufficient to produce severe damage to the cluster with accompanying modifications of ligand-metal interactions. However, the EXAFS (extended x-ray absorption fine structure) technique, which uses lower energy x-rays and cryogenic conditions, has been used successfully to probe the manganese cluster without damage.

Models for the manganese cluster in the oxygen-evolving complex of photosystem II.

In this Paper of the Week, Yulia Pushkar and colleagues use the EXAFS technique on oriented preparations of the enzyme from spinach to generate new structural information on the undamaged metal cluster. By comparing their results to the proposed manganese cluster models based on spectroscopic and diffraction data, the authors are able to provide input for refining and selecting among these models. These studies thus make an important contribution to the fundamental problem of the mechanism of photosynthetic splitting of water to molecular oxygen.

FOOTNOTES

See referenced article, J. Biol. Chem. 2007, 282, 7198-7208

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