Revealing the structure of the Mn-cluster of photosystem II by X-ray crystallography

X-ray crystallography has provided details of the location, structure and protein environment of the Mn4Ca2+ cluster which catalyses the light driven, water splitting reaction of photosystem II. However, because of 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 a matter of debate. To some extent these problems have been overcome by applying spectroscopic techniques such as Extended X-ray Absorption Fine Structure (EXAFS). The present state of knowledge is reviewed and used to construct two new models for water splitting site. These new models are discussed in terms of the earlier structures.