Photosynthetic [2Fe-2S] plant-type ferredoxins have a central role in electron transfer between the photosynthetic chain and various metabolic pathways. Several genes are coding for [2Fe2S] ferredoxins in cyanobacteria, with four in the thermophilic cyanobacterium Thermosynechococcus elongatus. The structure and functional properties of the major ferredoxin Fd1 are well known but data on the other ferredoxins are scarce. We report the structural and functional properties of a novel minor type ferredoxin, Fd2 of T. elongatus, homologous to Fed4 from Synechocystis sp. PCC 6803. Remarkably, the midpoint potential of Fd2, Em = -440 mV, is lower than that of Fd1, Em = -372 mV. However, while Fd2 can efficiently react with photosystem I or nitrite reductase, time-resolved spectroscopy shows that Fd2 has a very low capacity to reduce ferredoxin-NADP+ oxidoreductase (FNR). These unique Fd2 properties are discussed in relation with its structure, solved at 1.38 Å resolution. The Fd2 structure significantly differs from other known ferredoxins structures in loop 2, N-terminal region, hydrogen bonding networks and surface charge distributions. UV-Vis, EPR, and Mid- and Far-IR data also show that the electronic properties of the [2Fe2S] cluster of Fd2 and its interaction with the protein differ from those of Fd1 both in the oxidized and reduced states. The structural analysis allows to propose that valine in the motif Cys53ValAsnCys56 of Fd2 and the specific orientation of Phe72, explain the electron transfer properties of Fd2. Strikingly, the nature of these residues correlates with different phylogenetic groups of cyanobacterial Fds. With its low redox potential and its discrimination against FNR, Fd2 exhibits a unique capacity to direct efficiently photosynthetic electrons to metabolic pathways not dependent on FNR.

中文翻译：

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具有空前的结构和功能特性的替代植物状蓝藻铁氧还蛋白。

光合作用[2Fe-2S]植物型铁氧还蛋白在光合作用链和各种代谢途径之间的电子转移中起着核心作用。蓝藻中有几个基因编码[2Fe2S]铁氧还蛋白，嗜热蓝藻中有四个基因在延伸中。主要铁氧还蛋白Fd1的结构和功能特性是众所周知的，但有关其他铁氧还蛋白的数据却很少。我们报告的结构和功能特性的新型次要类型的铁氧还蛋白，T。伸长的Fd2，与Synechocystis sp的Fed4同源。PCC6803。值得注意的是，Fd2的中点电势Em = -440 mV，低于Fd1的中点电势Em = -372 mV。但是，尽管Fd2可以与光系统I或亚硝酸还原酶有效反应，时间分辨光谱表明，Fd2还原铁氧还蛋白-NADP +氧化还原酶（FNR）的能力非常低。讨论了这些独特的Fd2特性及其结构，并以1.38Å的分辨率进行了解析。Fd2结构在回路2，N端区域，氢键网络和表面电荷分布方面与其他已知的铁氧还蛋白结构显着不同。UV-Vis，EPR以及中红外和远红外数据也表明，Fd2的[2Fe2S]簇的电子性质及其与蛋白质的相互作用在氧化和还原状态下均不同于Fd1。结构分析允许提出，Fd2的基序Cys53ValAsnCys56中的缬氨酸和Phe72的特定方向可以解释Fd2的电子转移特性。惊人地 这些残基的性质与蓝细菌Fds的不同系统发育基团相关。Fd2具有低氧化还原电势和对FNR的辨别能力，具有将光合电子有效引导至不依赖于FNR的代谢途径的独特能力。