Oral Presentation 2014 International Biophysics Congress

  Photosynthetic water-splitting generates electrons, protons, and oxygen, which are the sources for energy and oxygen indispensible for maintaining almost all life forms on the earth. This reaction is catalyzed by photosystem II (PSII), a large membrane-protein complex consisting of 20 subunits with a total molecular mass of 350 kDa for a monomer. We have solved the crystal structure of dimeric PSII from a thermophilic cyanobacterium Thermosynechococcus vulcanus at 1.9 Å resolution (Umena Y. et al. Nature, 2011, 473, 55-60), which revealed a clear picture of the catalytic center for photosynthetic water-splitting, namely, a Mn₄CaO₅-cluster organized into a distorted chair form. This feature of the distorted shape suggested a remarkable flexibility in the structure, which would be needed for structural changes expected to occur during the catalytic water-splitting cycle (S-state cycle). Some of the inter-atomic distances within the metal cluster revealed by the X-ray structural analysis, however, were shown to be slightly longer than those obtained by previous EXAFS studies as well as theoretical studies. Among these, the position of O5 has received particular attention, since it connects Mn1 and Mn4 with bond distances unusually longer compared with typical Mn-O bond distances. This unusual property implied that this oxo-bridge may participate in the O-O bond formation during O₂ release. In order to reveal the role of the Ca²⁺ ion in the Mn₄CaO₅-cluster, we replace it with Sr²⁺ and solved the Sr²⁺-substituted PSII structure at 2.1 Å resolution (Koua HMK, et al. PNAS, 2013, 110, 3889-3894), which had an oxygen-evolving activity half of native PSII. Based on the results obtained, I will discuss the possible mechanism for photosynthetic water-splitting.