SPring-8, the large synchrotron radiation facility

Release Date

18 May, 2009

Okayama University

<Achievements>
Scientists in the joint research group of Okayama University and Osaka City University isolated and crystallized photosystem II, a membrane protein^*1 complex from blue-green algae,^*2 which are oxygenic phototrophs,^*3 and analyzed its structure using synchrotron radiation at Structural Biology I Beamline BL41XU of SPring-8. They successfully identified two chloride ions bound to the oxygen-evolving center and clarified the mechanism underlying their functions.

During photosynthesis, which involves the evolution of oxygen, energy from sunlight is converted into chemical energy that can be directly used by organisms. At the same time, water is split to produce oxygen vital for life. The involvement of chloride ions in photosynthesis was first suggested at least 60 years ago; however, details of the mechanism underlying their functions were not clear until now.

The research group crystallized the photosystem II complex with chloride ions replaced by bromine or iodine ions, and analyzed its structure (Fig. 1). The binding sites of chloride ions (Fig. 2) were thus visualized for the first time, revealing the mechanism underlying their functions.

<Expectations>
The oxygen-evolving reaction efficiently converts energy from sunlight into chemical energy, producing oxygen and hydrogen ions. The achievements of this research should lead to a detailed clarification of the mechanism underlying this reaction, which is expected to lead to the highly efficient use of energy from sunlight by humans.

<Figure>

Water splitting reaction during photosynthesis

Fig. 1 Crystal structure of photosystem II (PSII) complex.
Photosystem II complex is a giant complex comprising 17 types of transmembrane proteins and 3 types of peripheral membrane proteins. It exists as a dimer.

Fig. 2 Oxygen-evolving center and binding sites of chloride ions.
The oxygen-evolving center is composed of 4 manganese atoms and a calcium atom. This structure is supported by two chloride ions that bind to the sites near the center.

<Glossary>

*1 Membrane protein
A generic term for proteins present in biological membranes. Membrane proteins are difficult to crystallize because of their low water solubility; thus, it is difficult to determine their structure by crystallographic analysis, in contrast with water-soluble proteins.

*2 Blue-green algae
Also called cyanobacteria, and known as "orchid bacteria" in Japan. Blue-green algae are prokaryotes and are the most primitive organisms among oxygenic phototrophs from the evolutionary standpoint. Oxygenic blue-green algae first appeared on Earth approximately 2.7 billion years ago. In this research, thermophilic cyanobacteria, Thermosynechococcus vulcanus, which are found in hot springs and grow at 55°C, were used.

*3 Oxygenic phototroph (Oxygen-producing photosynthetic organism)
A generic term for organisms that absorb light energy and carry out photosynthesis, in which carbon dioxide (CO₂) is reduced to organic compounds and oxygen is produced by the splitting of water. Higher plants and algae are examples of oxygenic phototrophs.