高等植物光系统Ⅰ-捕光天线Ⅰ(PSⅠ-LHCⅠ)超分子复合物的晶体结构和能量传递途径

Crystal structure of plant PSⅠ-LHC Ⅰ supercomplex and its energy transfer mechanism

光合作用光能的吸收、传递和转化是由位于光合膜上具有特定的分子排列和空间构象的色素蛋白复合物光系统Ⅱ(PSⅡ)和光系统Ⅰ(PSⅠ)所推动的.其中PSⅠ是一个具有极高效率的太阳能转化系统,其量子转化效率几乎为100%,但其高效吸能、传能和转能的结构基础尚不清楚.从高等植物碗豆的叶片提取了高纯度的光系统Ⅰ-捕光天线Ⅰ(PSⅠ-LHCⅠ)色素蛋白超分子复合物,并制备和解析了其2.8?的晶体结构[1].PSⅠ-LHCⅠ超分子复合物由16个蛋白亚基组成,总分子量约600 k D.本结构全面解析了高等植物PSⅠ-LHCⅠ的精细结构,揭示了PSⅠ-LHCⅠ的4个不同的捕光天线(Lhca1,Lhca2,Lhca3,Lhca4)在与PSⅠ核心复合物结合状态下的结构和它们的异同,以及它们之间的相互关系;揭示了LHCⅠ全新的色素网络系统,辨别了叶绿素a和b的不同位置,并阐明了4对红叶绿素(red Chls)和13个类胡萝卜素的结合位点和结构.根据所解析的结构,提出了由LHCⅠ向PSⅠ核心复合物能量传递的4条重要的可能途径.这些结果为揭示高等植物PSⅠ高效吸能、传能和转能的机理奠定了坚实的结构基础.本文将介绍所解析的高等植物PSⅠ-LHCⅠ的精细结构,并讨论PSⅠ-LHCⅠ能量传递机制.

Photosynthesis uses light energy from the sun to convert CO2 and water into carbohydrates and oxygen, thus sustaining all aerobic life forms on Earth. Energy conversion in photosynthesis is carried out by two large membrane-protein complexes： photosystemⅠ（PSⅠ） and photosystemⅡ（PSⅡ）. In higher plants, the PSⅠcore is surrounded by a belt of 4 light-harvestingⅠsubunits（LHCⅠor Lhca）, forming a PSⅠ-LHCⅠsupercomplex. The PSⅠ-LHCⅠsupercomplex is an extremely efficient solar energy converter with a quantum efficiency close to 100%. In order to reveal the mechanism of energy harvesting and transfer within this large pigment-protein complex, it is essential to solve its crystal structure. The structure of the PSⅠ-LHCⅠsupercomplex has been analyzed at a resolution up to 3.3 ？ previously. However, this resolution was not enough to elucidate the detailed mechanism of light-harvesting and energy transfer in this complex. Recently we succeeded in analyzing the structure of the PSⅠ-LHCⅠsupercomplex from pea at 2.8 ？ resolution（1）. Our studies showed that the PSⅠ-LHCⅠsupercomplex contains 16 different subunits（including 12 core subunits Psa A-L and 4 LHCⅠsubunits Lhca1-4） and 205 cofactors（143 chlorophylls a, 12 chlorophylls b, 26 ？-carotenes, 5 luteins, 4 violaxanthin, 10 lipids）, with a total molecular mass of 600 k D. Our results identified chlorophyll a, chlorophyll b, and some carotenoids in the 4 LHCⅠsubunits for the first time, and revealed the differences in the structures of the 4 LHCⅠ subunits, their interactions, and the interactions between them and the PSⅠcore subunits. Comparison among the available six structures of the Lhc family members（Lhca1 to Lhca4, LHCⅡand CP29） revealed that, although all these Lhc proteins have a highly conserved second protein structure, notable differences were found in the two loop regions AC and BC as well as the N-terminal region. Most pigments are arranged at the same position except some distinct differences among different Lhc subunits in the position of several Chls bound at the interface between adjacent Lhca complexes, and between Lhca and PSⅠcore complex. Based on the structure resolved, 4 plausible energy transfer pathways（1Bs, 1Fl, 2Js, 3As/l） from LHCⅠto the PSⅠcore complex were deduced. Red forms of Chls were found to be involved in energy transfer from each Lhca to PSⅠcore. Our structure revealed that each Lhca binds a red chlorophyll dimer of Chl a3-Chl a9, which contribute to red-shifted spectra of Lhca complexes and have a pronounced effect on the energy transfer and trapping in the whole PSⅠ-LHCⅠcomplex. All the four red dimers locate at the interface between LHCⅠand PSⅠcore complex, which looks like four bridges connecting Lhca with the core. The Chl-Chl interactions between each Lhca and the core complex suggested that excitation energy from the LHCⅠbelt to the PSⅠcore would mainly flow via Lhca1 and Lhca3. In this review, we discuss the detailed structure of the PSⅠ-LHCⅠsupercomplex and the possible energy transfer mechanism within it. Taken together, this structure provides a solid structural basis for our understanding on energy transfer and photoprotection mechanisms within PSⅠ-LHCⅠsupercomplex, and thus will be a big step forward toward understanding the mechanisms of photosynthesis.