2018 Volume 72 Pages 3-14
Photosystem II (PSII) is a huge membrane-protein complex catalyzing light-induced electron transfer and water oxidation reactions, leading to the conversion of light energy into chemical energy and the evolution of molecular oxygen, two products indispensable for sustaining almost all life forms on the earth. This account describes the structure of PSII with the focus on that of the catalytic center for water oxidation, a Mn4CaO5-cluster embedded in the protein matrix of PSII, revealed by conventional highresolution X-ray crystallography and femtosecond X-ray free electron laser (XFEL) crystallography. Latest results on the structural changes of the Mn4CaO5-cluster during the water oxidation reaction cycle obtained by the femtosecond XFEL crystallography are also described. Based on these results as well as those obtained with other approaches, the mechanism of water oxidation and oxygen evolution is discussed. Other functions of PSII such as proton release pathways and light energy absorption and transfer are also discussed briefly.