Photosystem II - Oxygen-Evolving Complex (OEC)

Oxygen-Evolving Complex (OEC)

The oxygen-evolving complex is the site of water oxidation. It is a metallo-oxo cluster comprising four manganese ions (in oxidation states ranging from +3 to +5) and one divalent calcium ion. When it oxidizes water, producing dioxygen gas and protons, it sequentially delivers the four electrons from water to a tyrosine (D1-Y161) sidechain and then to P680 itself. The structure of the oxygen-evolving complex is still contentious. The structures obtained by X-ray crystallography are particularly controversial, since there is evidence that the manganese atoms are reduced by the high-intensity X-rays used, altering the observed OEC structure. However, crystallography in combination with a variety of other (less damaging) spectroscopic methods such as EXAFS and electron paramagnetic resonance have given a fairly clear idea of the structure of the cluster. One possibility is the cubane-like structure. In 2011 the OEC of PSII was resolved to a level of 1.9 angstroms revealing five oxygen atoms serving as oxo bridges linking the five metal atoms and four water molecules bound to the Mn4CaO5 cluster; more than 1,300 water molecules were found in each photosystem II monomer, some forming extensive hydrogen-bonding networks that may serve as channels for protons, water or oxygen molecules.