Auracyanin (Ac) is a blue copper protein that mediates the electron transfer between Alternative Complex III (ACIII) and downstream electron acceptors in both fort chains of filamentous anoxygenic phototrophs. Here, we extracted and purified the air-oxidized RfxAc from the photoheterotrophically grown Roseiflexus castenholzii, and we illustrated the structural basis underlying its electron transferring features. Spectroscopic and enzymatic analyses demonstrated the reduction of air-oxidized RfxAc by the ACIII upon oxidation of menaquinol-4 and menaquinol-7. Crystal structures of the air-oxidized and Na-dithionite-reduced RfxAc at 2.2 and 2.0 Å resolutions, respectively, showed that the copper ions are coordinated by His77, His146, Cys141, and Met151 in minor different geometries. The Cu1-Sδ bond length increase of Met151, and the electron density Fourier differences at Cu1 and His77 demonstrated their essential roles in the dithionite-induced reduction. Structural comparisons further revealed that the RfxAc contains a Chloroflexus aurantiacus Ac-A-like copper binding pocket and a hydrophobic patch surrounding the exposed edge of His146 imidazole, as well as an Ac-B-like Ser- and Thr-rich polar patch located at a different site on the surface. These spectroscopic and structural features allow RfxAc to mediate electron transfers between the ACIII and redox partners different from those of Ac-A and Ac-B. These results provide a structural basis for further investigating the electron transfer and energy transformation mechanism of bacterial photosynthesis, and the diversity and evolution of electron transport chains.

中文翻译：

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来自光合细菌Rostiflexus castenholzii的蓝色铜蛋白金银花青素的电子转移特征的结构基础。

Auracyanin（Ac）是一种蓝色铜蛋白，可介导丝状产氧营养养分的两个堡垒链中的替代复合物III（ACIII）和下游电子受体之间的电子转移。在这里，我们从光异养生长的玫瑰蔷薇中提取并纯化了空气氧化的RfxAc，并阐明了其电子转移特征的结构基础。光谱和酶促分析表明，当甲萘醌4和甲萘醌7氧化后，ACIII可以减少空气氧化的RfxAc。分别在2.2和2.0Å分辨率下经空气氧化和亚硫连石还原的RfxAc的晶体结构表明，铜离子在较小的几何形状下由His77，His146，Cys141和Met151配位。Met151的Cu1-Sδ键长增加 Cu1和His77的电子密度傅立叶差异证明了它们在连二亚硫酸盐诱导的还原反应中的重要作用。结构比较进一步表明，RfxAc包含一个Chloroflexus aurantiacus Ac-A样铜结合袋和一个围绕His146咪唑暴露边缘的疏水斑块，以及一个位于Ac的类似Ac-B的富含Ser和Thr的极性斑块。表面上的其他位置。这些光谱和结构特征允许RfxAc介导ACIII和氧化还原配偶体之间的电子转移，不同于Ac-A和Ac-B。这些结果为进一步研究细菌光合作用的电子转移和能量转化机理以及电子传输链的多样性和进化提供了结构基础。