Researchers resolve structure of so-called photosystem

Crystal structure of plant PSI-LHCI supercomplex.

Crystal structure of plant PSI-LHCI supercomplex.

Researchers at Okayama University and the Chinese Academy of Sciences have resolved the structure of so-called photosystem I (PSI) to a spatial resolution of 0.28 nanometers and elucidated the mechanisms governing the conversion of carbon dioxide into sugars for high-efficiency energy transfer in photosynthetic organisms. The results, published in AAAS Science magazine, may offer ‘blueprints’ for establishing artificial photosynthetic systems.

Photosystem I (PSI) is one of the two photosystems found in the thylakoid membrane of oxygenic photosynthetic organisms. Its function is to harvest light energy that is utilised to drive a chain of electron transfer reactions, which leads to the production of the reduction power required for converting CO2 into sugars. In higher plants, the core of PSI is surrounded by a large light-harvesting complex I (LHCI), which forms a PSI-LHCI supercomplex with a total molecular mass of 600 kDa. The light energy captured by LHCI is transferred to the PSI core with an extremely high efficiency.

The crystal structure of plant PSI-LHCI supercomplex has been reported previously. However, the crystal structures reported so far lacked sufficient resolution to reveal the detailed organisation of the PSI-LHCI supercomplex with atomic precision, especially with respect to the positions and number of cofactors associated with LHCI.

Now, Michi Suga and Jian-Ren Shen at Okayama University, in collaboration with Tingyun Kuang and Xiaochun Qin at the Chinese Academy of Sciences, have solved the crystal structure of plant PSI-LHCI supercomplex to a resolution of 2.8 Å.

The research group purified and crystallised the PSI-LHCI supercomplex from the leaves of a pea plant and succeeded in improving the quality of the crystals dramatically. With these improved crystals the group was able to collect the X-ray diffraction data using the intense X-ray at the synchrotron facility SPring-8 in Japan. They then analysed the data using crystallographic approaches to determine the structure.

The improved structure revealed the detailed organisation of protein sub-units and cofactors. This enabled the mechanisms of energy transfer, regulation, and photoprotection within the PSI-LHCI supercomplex to be examined on a more robust structural basis.

This work provides structural insights into the energy absorption and transfer mechanisms in photosynthesis. In addition it may provide a blueprint for the design of light-harvesting setups with extremely high efficiencies that can be utilised in artificial photosynthetic systems.

More details about these findings and related research can be found in the September 2015 issue of Okayama University e-Bulletin: Ω

Posted 24 September 2015