Photosystem II: light-dependent oscillation of ligand composition at its active site

Recently, the conclusions drawn from crystallographic data about the number of oxygen ligands associated with the CaMn₄ cofactor in the oxygen-evolving center (OEC) of Thermosynechococcus vulcanus photosystem II (PSII) have been called into question. Here, using OEC-omit, metal ion-omit and ligand-omit electron-density maps, it is shown that the number of oxygen ligands ranges from three in the functional OEC of monomer B following dark adaption (0F), i.e. in its ground state (PDB entry 6jlj/0F and PDB entry 6jlm/0F), to five for both monomers of PSII in photo-advanced states following exposure to one and two flashes of light. For a significant fraction of the 0F OECs in monomer A, the number is four (PDB entry 6jlj/0F). Following one flash it increases to five (PDB entry 6jlk/1F), where it remains after a second flash (PDB entry 6jlj/2F). Following a third flash (3F), it decreases to three (PDB entry 6jlp/3F), suggesting that an O₂ molecule has been produced. These observations suggest a mechanism for the reaction that transforms the O atoms of the water molecules bound at the O3 and O1 sites of the OEC into O₂.