Niedzwiedzki Dariusz, Krupa Zbigniew, Gruszecki Wiesław I
Department of Biophysics, Institute of Physics, Maria Curie-Skłodowska University, Pl. M. Curie-Skłodowskiej 1, Lublin 20-031, Poland.
J Photochem Photobiol B. 2005 Feb 1;78(2):109-14. doi: 10.1016/j.jphotobiol.2004.09.012.
Three main xanthophyll pigments are bound to the major photosynthetic pigment-protein complex of Photosystem II (LHCII): lutein, neoxanthin and violaxanthin. Chromatographic analysis of the xanthophyll fraction of LHCII reveals that lutein appears mainly in the all-trans conformation, neoxanthin in the 9'-cis conformation and major fraction of violaxanthin in the all-trans conformation. Nevertheless, a small fraction of violaxanthin appears always in a cis conformation: 9-cis and 13-cis (approximately 4% and 2% in the darkness, respectively). Illumination of the isolated complex (5 min, 445 nm, 250 micromolm-2s-1) results in the substantial increase in the concentration of the cis steric conformers of violaxanthin: up to 6% of 9-cis and 4% of 13-cis. Similar effect can be obtained by dark incubation of the same preparation for 30 min at 60 degrees C. Heating-induced isomerization of the all-trans violaxanthin can also be obtained in the organic solvent system but the formation of the 9-cis stereoisomer has not been observed under such conditions. The fact that the appearance of the 9-cis form of violaxanthin is specific for the protein environment suggests that violaxanthin may replace neoxanthin in LHCII in the N1 xanthophyll binding pocket and that the protein stabilizes this particular conformation. The analysis of the electronic absorption spectra of LHCII and the FTIR spectra of the protein in the Amid I band spectral region indicates that violaxanthin isomerization is associated with the disaggregation of the complex. It is postulated that this reorganization of LHCII provides conditions for desorption of violaxanthin from the pigment protein complexes, its diffusion within the thylakoid membrane and therefore, availability to the enzymatic deepoxidation within the xanthophyll cycle. It is also possible that violaxanthin isomerization plays the role of a security valve, by consuming an energy of excessive excitations in the antenna pigment network (in particular, exchanged at the triplet state levels).
三种主要的叶黄素色素与光系统II（LHCII）的主要光合色素 - 蛋白质复合物结合：叶黄素、新黄质和紫黄质。对LHCII中叶黄素部分的色谱分析表明，叶黄素主要以全反式构象出现，新黄质以9'-顺式构象出现，紫黄质的主要部分以全反式构象出现。然而，一小部分紫黄质总是以顺式构象出现：9-顺式和13-顺式（在黑暗中分别约为4%和2%）。对分离出的复合物进行光照（5分钟，445纳米，250微摩尔光子·米-2·秒-1）会导致紫黄质顺式空间构象体的浓度大幅增加：9-顺式高达6%，13-顺式高达4%。将相同制剂在60摄氏度下暗孵育30分钟也可获得类似效果。在有机溶剂系统中也可通过加热诱导全反式紫黄质的异构化，但在这种条件下未观察到9-顺式立体异构体的形成。紫黄质9-顺式形式的出现对蛋白质环境具有特异性这一事实表明，紫黄质可能在LHCII的N1叶黄素结合口袋中取代新黄质，并且蛋白质稳定了这种特定构象。对LHCII的电子吸收光谱以及酰胺I带光谱区域中蛋白质的傅里叶变换红外光谱的分析表明，紫黄质异构化与复合物的解聚有关。据推测，LHCII的这种重组为紫黄质从色素 - 蛋白质复合物中解吸、在类囊体膜内扩散提供了条件，因此，使其可用于叶黄素循环中的酶促脱环氧化作用。紫黄质异构化也可能起到安全阀的作用，通过消耗天线色素网络中过量激发的能量（特别是在三重态水平交换的能量）。
