The advantages of using bacterial systems to study the mechanism and function of cytochrome bc (1) complexes do not extend readily to their structural investigations. High quality crystals of bacterial complexes have been difficult to obtain despite the enzymes' smaller sizes and simpler subunit compositions compared to their mitochondrial counterparts. In the course of the structure determination of the bc (1) complex from R. sphaeroides, we observed that the growth of only low quality crystals correlated with low activity and stability of the purified complex, which was mitigated in part by introducing a double mutations to the enzyme. The S287R(cyt b)/V135S(ISP) mutant shows 40% increase in electron transfer activity and displays a 4.3 degrees C increase in thermal stability over wild-type enzyme. The amino acid histidine was found important in maintaining structural integrity of the bacterial complex, while the respiratory inhibitors such as stigmatellin are required for immobilization of the iron-sulfur protein extrinsic domain. Crystal quality of the R. sphaeroides bc (1) complex can be improved further by the presence of strontium ions yielding crystals that diffracted X-rays to better than 2.3 A resolution. The improved crystal quality can be understood in terms of participation of strontium ions in molecular packing arrangement in crystal.

中文翻译：

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来自厌氧，光合细菌球形红球菌的细胞色素bc1复合物晶体结构之路。

使用细菌系统研究细胞色素bc（1）复合物的机制和功能的优势并不容易扩展到其结构研究。尽管与线粒体对应物相比，酶的尺寸更小且亚基组成更简单，但仍难以获得高质量的细菌复合物晶体。在球形红球菌bc（1）配合物的结构确定过程中，我们观察到，仅劣质晶体的生长与纯化的配合物的低活性和稳定性相关，这通过引入双突变而得到部分缓解对酶。与野生型酶相比，S287R（cyt b）/ V135S（ISP）突变体的电子传递活性提高了40％，热稳定性提高了4.3摄氏度。发现氨基酸组氨酸对维持细菌复合物的结构完整性很重要，而固定铁硫蛋白外源结构域则需要呼吸抑制剂如柱头蛋白。锶离子球菌bc（1）配合物的晶体质量可通过锶离子的存在而得到进一步改善，产生的晶体将X射线衍射到优于2.3 A的分辨率。可以通过锶离子参与晶体中的分子堆积排列来理解改善的晶体质量。sphaeroides bc（1）配合物可以通过锶离子的存在而得到进一步改善，产生的晶体将X射线衍射到优于2.3 A的分辨率。可以通过锶离子参与晶体中的分子堆积排列来理解改善的晶体质量。sphaeroides bc（1）配合物可以通过锶离子的存在而得到进一步改善，产生的晶体将X射线衍射到优于2.3 A的分辨率。可以通过锶离子参与晶体中的分子堆积排列来理解改善的晶体质量。