PMID: 23426624

Authors:

Koua FH, Umena Y, Kawakami K, Shen JR

Title:

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

Journal:

Proc Natl Acad Sci U S A. 2013 Mar 5;110(10):3889-94. doi:, 10.1073/pnas.1219922110. Epub 2013 Feb 20.

Abstract:

Oxygen-evolving complex of photosystem II (PSII) is a tetra-manganese calcium penta-oxygenic cluster (MnCaO) 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 in this cluster and the possible location of water substrates in this process, we crystallized Sr-substituted PSII from Thermosynechococcus vulcanus, analyzed its crystal structure at a resolution of 2.1 A, and compared it with the 1.9 A structure of native PSII. Our analysis showed that the position of Sr was moved toward the outside of the cubane structure of the MnCaO-cluster relative to that of Ca, 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, W3, whereas that of the Sr-W4 distance was not much changed. This result may contribute to the decrease of oxygen evolution upon Sr-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 A structure of native PSII, was found in the Sr-PSII structure.