title: Structure of Sr-substituted photosystem II at 2.1 A resolution and its implications in the mechanism of water oxidation.
abstract: Oxygen-evolving complex of photosystem II (PSII) is a tetra-manganese calcium penta-oxygenic cluster (Mn4CaO5) catalyzing light-induced water oxidation through several intermediate states (S-states) by a mechanism that is not fully understood. To elucidate the roles of Ca(2+) in this cluster and the possible location of water substrates in this process, we crystallized Sr(2+)-substituted PSII from Thermosynechococcus vulcanus, analyzed its crystal structure at a resolution of 2.1 Å, and compared it with the 1.9 Å structure of native PSII. Our analysis showed that the position of Sr was moved toward the outside of the cubane structure of the Mn4CaO5-cluster relative to that of Ca(2+), resulting in a general elongation of the bond distances between Sr and its surrounding atoms compared with the corresponding distances in the Ca-containing cluster. In particular, we identified an apparent elongation in the bond distance between Sr and one of the two terminal water ligands of Ca(2+), W3, whereas that of the Sr-W4 distance was not much changed. This result may contribute to the decrease of oxygen evolution upon Sr(2+)-substitution, and suggests a weak binding and rather mobile nature of this particular water molecule (W3), which in turn implies the possible involvement of this water molecule as a substrate in the O-O bond formation. In addition, the PsbY subunit, which was absent in the 1.9 Å structure of native PSII, was found in the Sr-PSII structure.
authors: Division of Bioscience, Graduate School of Natural Science and Technology/Faculty of Science, Okayama University, Okayama 700-8530, Japan.
authors: Faisal Hammad Mekky
authors: FH
authors: Koua
authors: Yasufumi
authors: Y
authors: Umena
authors: Keisuke
authors: K
authors: Kawakami
authors: Jian-Ren
authors: JR
authors: Shen
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D055592:Biophysical Phenomena; D002118:Calcium / Q000737:chemistry; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000235:genetics / Q000378:metabolism; D008024:Ligands; D008345:Manganese / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D013324:Strontium / Q000737:chemistry; D014867:Water / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D008024:Ligands; D045332:Photosystem II Protein Complex; D014867:Water; D008345:Manganese; D002118:Calcium; D013324:Strontium