Structure of Sr-substituted photosystem II at 2.1 Å resolution and its implications in the mechanism of water oxidation

Faisal Hammad Mekky Koua, Yasufumi Umena, Keisuke Kawakami, Jian Ren Shen

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132 Citations (Scopus)


Oxygen-evolving complex of photosystem II (PSII) is a tetramanganese 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 Sr2+-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 Srwasmoved toward the outside of the cubane structure of the Mn4CaO5-cluster relative to that of Ca2+, 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 Ca2+,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.

Original languageEnglish
Pages (from-to)3889-3894
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number10
Publication statusPublished - Mar 5 2013


  • Artificial photosynthesis
  • Cyanobacteria
  • Membrane protein
  • Structural changes
  • Water-splitting

ASJC Scopus subject areas

  • General


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