A research team led by KUANG Tingyun of the Institute of Botany, Chinese Academy of Sciences, described the crystal structure of higher plant PSI-LHCI super-complex at 2.8 ？ resolution in an article published May 29 in the prestigious journal Science. The article, offered the first atomic view of PSI-LHCI and was highlighted on the cover of the same issue of Science. 

Photosynthesis is the largest energy conversion process on earth, and utilizes light energy from the Sun to convert carbon dioxide and water into carbohydrates and oxygen, thus sustaining all aerobic life forms on the earth. Some consider photosynthesis “the most important chemical reaction on Earth.” The central problems of photosynthesis are the molecular mechanisms underlying light-harvesting, energy transfer and conversion with high efficiency. 

The energy conversion in photosynthesis is carried out by two large pigment-protein complexes: photosystem I (PSI) and photosystem II (PSII), among which, PSI is an extremely efficient solar energy converter. The light energy absorbed by the PSI light-harvesting pigments is transferred to chlorophylls in the reaction center, where charge separation occurs, and this process produces one electron for nearly every photon absorbed. The high chlorophyll concentration in plant PSI maximizes light harvesting. Furthermore, light-harvesting complex I (LHCI) contain several chlorophylls with red-shifted spectra, called red forms, which expand the light-harvesting capacity of plant PSI to the far-red region and allow energy transfer from high-energy to low-energy pigments. The excitation energy transfer in PSI is extremely fast, and the quantum efficiency is close to 100 percent, making PSI the most efficient energy transfer system. Therefore, the structure and function of PSI has been an important focus of photosynthesis research. Previously, the crystal structure of plant PSI-LHCI had been imaged at medium resolution. However, the structural basis for the high efficiency of light-harvesting, energy transfer and conversion within plant PSI remained to be elucidated.

The research team of KUANG Tingyun had been working on the structure and function of photosynthetic membrane proteins for a long time. With this recently published research, they have now resolved the crystal structure of plant PSI-LHCI super-complex at 2.8 ？ resolution. This study reveals the detailed structure of plant PSI-LHCI: It has a total molecular mass of 600 kDa, and includes 16 subunits (12 core subunits and four Lhcas), 155 Chls (143 Chls a and 12 Chls b), 35 carotenoids [26 β-carotenes (BCRs), five luteins (Luts) and four violaxanthins (Vios)], 10 lipids [six phosphatidylglycerols (PGs), three monogalactosyldiacylglycerols (MGDGs) and one digalactosyldiacylglycerol (DGDG)], three Fe4S4 clusters, two phylloquinones and several water molecules.

In this study, chlorophyll a and b were differentiated in the four Lhca subunits for the first time, and the location and geometrical arrangement of each pigment were resolved, yielding a new pigment network of LHCI. Furthermore, the structure and organization of special chlorophylls - red Chls, in LHCI - were revealed, and four plausible energy transfer pathways from LHCI to the PSI core complex were deduced. In addition, detailed differences among the four Lhca subunits and their interactions, and the interactions between Lhca subunits and PSI core subunits, were elucidated. 

This study provides a solid structural basis for our understanding of the highly efficient energy transfer within the PSI-LHCI super-complex and thus is a big step forward in understanding the mechanisms of photosynthesis. The principles revealed in natural photosynthesis may prove useful for developing highly efficient solar energy utilization systems, which is important for meeting increasing demand for energy as well as for solving the problems of food and the environment. 

Prof. Roberta Croce, an expert in the photosystem I research field, wrote a very positive commentary on this research in the same issue of Science, entitled “A close view of photosystem I” (Science, 348, 970-971). Media including Xinhuanet and the Washington Post reported on this research work immediately following its publication.

This work was supported by National Basic Research Program of China (Nos. 2011CBA00901, 2015CB150101), a Key Research Program of the Chinese Academy of Sciences (No. KGZD-EW-T05).