Structure of cytochrome b6/f deduced from its function

细胞色素b6/f络合物接受被光系统II还原的质体喹啉的电子。复合物将电子转移到光系统I，并在类囊体膜上形成H^+梯度，同时形成ATP。在本研究项目中，我们从一种蓝藻的类囊体膜中纯化了该复合物，重点是开发简单的纯化工艺和高产率。我们还研究了配合物的底物（醌）特异性，并估计了醌结合袋的活性位点结构。该复合物是从一种嗜热蓝藻中纯化出来的。我们对现有的方法进行了以下两点改进：1)在NaBr处理过程中同时对类囊体膜进行超声处理以去除所有外源蛋白质；2)将脱氧胆酸盐与庚基硫糖苷一起加入以溶解配合物。通过改进，该配合物在较窄的硫酸铵浓度范围内得到了有效的沉淀，并可进一步提纯。四甲基对苯醌通常用于活度测量。三甲基对苯醌也被发现有效。二甲基-对苯醌或二氯-对苯醌（DCBQ）作为电子给体不活跃。从DCBQ是光系统II最有效的电子受体来看，b6/f配合物醌结合袋的结构与光系统II的Q_B位点有很大的不同。

Cytochrome b6/f complex accepts electrons from plastoquinol which was reduced by photosystem II.The complex transfers electrons to photosystem I and forms H^+ gradient across the thylakoid membranes to form ATP simultaneously. In this research project, we have purified the complex from thylakoid membranes of a cyanobacterium focussing on development of simple purification procedure and its high yield. We also have investigated the substrate (quinone) specificity of the complex and estimated the structure of active site of the quinone binding pocket.The complex was purified from a thermophilic cyanobacterium, Synechococcus vulcanus. We have improved on the existing procedure by the following two points : 1) thylakoid membranes were sonicated simultaneously during NaBr treatment to remove all the extrinsic proteins and 2) deoxycholate was added together with heptylthioglucoside to solubilize the complex. By this improvement, the complex was effectively precipitated in a narrow range of ammonium sulfate concentration and could be purified further.Tetramethyl-p-benzoquinone is normally used for activity measurement. Trimethyl-p-benzoquinone was found to be effective as well. Dimethyl-p-benzoquinones or dichloro-p-benzoquinones (DCBQ) were, however, not avtive as electron donor to the complex. Judging from the fact that DCBQ is the most effective electron acceptor for photosystem II,the structure of quinone binding pocket of b6/f complex is rather different from that of Q_B site of photosystem II.