Plant-type ferredoxin (Fd) is an electron transfer protein in chloroplast. Redox-dependent structural change of Fd controls its association with and dissociation from Fd-dependent enzymes. Among many X-ray structures of oxidized Fd have been reported so far, very likely a given number of them was partially reduced by strong X-ray. To understand the precise structural change between reduced and oxidized Fd, it is important to know whether the crystals of oxidized Fd may or may not be reduced during the X-ray experiment. We prepared the thin plate-shaped Fd crystals from Chlamydomonas reinhardtii and monitored its absorption spectra during experiment. Absorption spectra of oxidized Fd crystals were clearly changed to that of reduced form in an X-ray dose-dependent manner. In another independent experiment, the X-ray diffraction images obtained from different parts of one single crystal were sorted and merged to form two datasets with low and high X-ray doses. An Fo–Fo map calculated from the two datasets showed that X-ray reduction causes a small displacement of the iron atoms in the [2Fe-2S] cluster. Both our spectroscopic and crystallographic studies confirm X-ray dose-dependent reduction of Fd, and suggest a structural basis for its initial reduction step especially in the core of the cluster.

中文翻译：

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X射线剂量依赖的衣藻衣藻[2Fe-2S]铁氧还蛋白的结构变化。

植物型铁氧还蛋白（Fd）是叶绿体中的电子转移蛋白。Fd的氧化还原依赖性结构变化控制其与Fd依赖性酶的缔合和解离。到目前为止，已经报道了许多氧化的Fd的X射线结构，很可能一部分给定数量的X射线被强X射线还原。为了了解还原的Fd和氧化的Fd之间的精确结构变化，重要的是要知道在X射线实验过程中氧化Fd的晶体是否可以还原。我们从莱茵衣藻（Chlamydomonas reinhardtii）准备了薄板状的Fd晶体并在实验过程中监测其吸收光谱。氧化的Fd晶体的吸收光谱以X射线剂量依赖性方式明显改变为还原形式。在另一个独立的实验中，对从一个单晶的不同部分获得的X射线衍射图像进行分类并合并，以形成两个X射线剂量低和高的数据集。从这两个数据集计算出的Fo-Fo图表明，X射线还原会导致[2Fe-2S]簇中铁原子的少量位移。我们的光谱学和晶体学研究均证实了X射线剂量依赖性的Fd还原，并提出了其初始还原步骤的结构基础，尤其是在团簇的核心。