Abstract
The location, structure and protein environment of the Mn4Ca2+ cluster, which catalyses the light-driven, water-splitting reaction of photosystem II, has been revealed by X-ray crystallography. However, owing to the low resolutions of the crystal structures reported to date, and the possibility of radiation damage at the catalytic centre, the precise position of each metal ion remains unknown. To some extent, these problems have been overcome by applying spectroscopic techniques like extended X-ray absorption fine structure. Taking into account the most recent results obtained with these two X-ray-based techniques, we have attempted to refine models of the structure of the Mn4Ca2+ cluster and its protein environment.