Abstract
Photosystem II (PSII) is the site of photo-induced water-splitting, providing us with an unlimited amount of molecular oxygen required for sustaining life on the earth. The structure of PSII has been analyzed at 3.8-2.9 Å resolutions. However, these resolutions were not high enough to reveal the atomic structure of the oxygen-evolving complex (OEC). We have succeeded in analyzing the structure of PSII at a 1.9 Å resolution, allowing us to identify all of the 5 metal ions as well as 5 oxygen atoms that form the oxo-bridges unambiguously. The shape of the Mn4CaO5-cluster resembled a distorted chair form, with 3 Mn atoms, 1 Ca atom and 4 oxygen atoms forming a distorted chair base, and the 4th Mn and another oxygen atom forming the chair back. Four water molecules were found to be bound to the Mn4CaO5-cluster; at least some of the bound water molecules may function as substrate water for the oxygen-evolving reaction. All of the ligands for the Mn4CaO5-cluster were determined, which showed that each of the 4 Mn atoms are coordinated by 6 ligands whereas the Ca atom has 7 ligands. These structural features may have important implications on the mechanism of water-splitting.
