The nature of a chlorophyll ligand in Lhca proteins determines the far red fluorescence emission typical of photosystem I
The nature of a chlorophyll ligand in Lhca proteins determines the far red fluorescence emission typical of photosystem I
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DOI：
10.1074/jbc.m309203200
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发表时间：
2003-12-05
影响因子：
4.8
通讯作者：
Croce, R
中科院分区：
文献类型：
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作者：
Morosinotto, T;Breton, J;Croce, R
高等植物的光系统I的特征在于与其捕光复合物I部分相关的典型长波长荧光发射。这些低能量叶绿素光谱形式的起源进行了研究，通过使用Lhca 1 - 4基因的定点突变和体外重组成色素蛋白复合物。我们发现，红移吸收起源于叶绿素-叶绿素（Chl）激子相互作用，涉及Chl A5在每个四个Lhca天线复合物。的红移吸收/荧光光谱形式的存在下的一个基本要求是存在的天冬酰胺作为一个配体的叶绿素a生色团在Lhca复合物的结合位点A5。在Lhca 3和Lhca 4中，表现出最红移的红色形式，其被组氨酸取代保持了色素结合，然而，红色光谱形式被废除。相反，在Lhca 1中，具有非常低的红色形式的幅度，Asn取代His产生荧光发射的红移，从而证实Chl A5配体的性质决定了发色团的正确组织，导致负责最红色形式的激子相互作用。在730 nm处的红移荧光发射在此被提出源自于类似于700 nm处的吸收带，其代表具有683 nm处的高能带的激子相互作用的低能量贡献。由于突变不影响叶绿素A5的方向，我们建议，协调的Asn的叶绿素A5保持在正确的距离与叶绿素B5。
Photosystem I of higher plants is characterized by a typically long wavelength fluorescence emission associated to its light-harvesting complex I moiety. The origin of these low energy chlorophyll spectral forms was investigated by using site-directed mutagenesis of Lhca1-4 genes and in vitro reconstitution into recombinant pigment-protein complexes. We showed that the red-shifted absorption originates from chlorophyll-chlorophyll (Chl) excitonic interactions involving Chl A5 in each of the four Lhca antenna complexes. An essential requirement for the presence of the red-shifted absorption/ fluorescence spectral forms was the presence of asparagine as a ligand for the Chl a chromophore in the binding site A5 of Lhca complexes. In Lhca3 and Lhca4, which exhibit the most red-shifted red forms, its substitution by histidine maintains the pigment binding and, yet, the red spectral forms are abolished. Conversely, in Lhca1, having very low amplitude of red forms, the substitution of Asn for His produces a red shift of the fluorescence emission, thus confirming that the nature of the Chl A5 ligand determines the correct organization of chromophores leading to the excitonic interaction responsible for the red-most forms. The red-shifted fluorescence emission at 730 nm is here proposed to originate from an absorption band at similar to 700 nm, which represents the low energy contribution of an excitonic interaction having the high energy band at 683 nm. Because the mutation does not affect Chl A5 orientation, we suggest that coordination by Asn of Chl A5 holds it at the correct distance with Chl B5.