Scientists Catch Damage-Free Glimpse Of Photosynthesis Complex

Japanese researchers have solved a new crystal structure of photosystem II (PSII) using an X-ray free-electron laser (Nature 2014, DOI: 10.1038/nature13991). The structure, free of radiation-derived artifacts associated with previous structures of PSII, could help in the design of artificial water-oxidation catalysts for energy applications.

The structure of PSII—the protein complex in plants and other photosynthetic organisms that catalyzes the oxidation of water during photosynthesis—has previously been solved using conventional X-ray diffraction. But those methods are thought to damage the Mn₄CaO₅ cluster that is the catalytic heart of PSII. Free-electron lasers avoid such radiation damage.

Solved by a team led by Jian-Ren Shen of Okayama University and Masaki Yamamoto and Hideo Ago of RIKEN, the new 1.95-Å structure is of PSII from the cyanobacterium Thermosynechococcus vulcanus. It reveals distances between catalytic Mn atoms that are about 0.1 to 0.2 Å shorter than those found using conventional X-ray diffraction.

The new structure also suggests that two of the Mn atoms are in the +4 oxidation state, and the other two are in the +3 oxidation state. Earlier estimates suggest that the X-ray dose used in the conventional method may have caused some of the Mn atoms to be reduced to the +2 oxidation state.

The oxygen atoms bridge the Mn atoms. One of the oxygen atoms has significantly longer bonds to Mn atoms than do the other oxygen atoms. The new structure confirms that those longer bonds are not an artifact of radiation damage. The researchers think that that oxygen is part of a hydroxide ion from the substrate water.

The finding “represents significant progress” in understanding the structure and workings of PSII, says Michael Haumann, a physicist at the Free University of Berlin who has studied PSII’s structure. The structure confirms the overall geometry previously reported while revealing interesting differences that might be relevant to the catalytic mechanism. “This structure is expected to serve as an important input to theoretical studies aiming at the mechanism of water oxidation.”