59 巻 (2017) 2-3 号 p. 64-71
Photosystem II （PSII） is a multi-subunit protein complex embedded in the thylakoid membrane and splits water molecules, producing electrons, protons and molecular oxygen in photosynthesis. We started to purify and crystallize PSII in 1990, which led to the first crystal structure analysis at a resolution of 3.7 Å in 2003 using SPring-8. The resolution of the structure was raised to 1.9 Å in 2011, which was considered a breakthrough in the filed of photosynthesis because, on the basis of this atomic resolution structure, it became possible to discuss the molecular mechanism of the water-splitting reaction. We are continuing our studies to obtain much more precise information on the oxygen-evolving complex （OEC） at the S1-state of the Kok cycle and to analyze the intermediate state structures using femtosecond X-ray pulses from XFEL of SACLA.