Analysis of the Electron Transfer Pathway in the Green Sulfur Bacterial-Type Photochemical Reaction Center by a Conversion into the Pseudo Heterodimeric Structure

为了深入理解高等植物和蓝藻光系统I反应中心的分子构建原理，我们对绿色硫细菌和太阳细菌的光系统I反应中心的结构和功能关系进行了研究。现将这两年的研究进展综述如下。1.次级电子受体A_1（醌）的寻找：在Modesticaldum的反应中心核心复合物中成功地检测到了由还原醌产生的ESR信号。目前，我们正试图通过HPLC和质谱分析来确定其化学物种。同时，我们试图获得一个缺失menG的突变体，该突变体编码醌生物合成途径中依赖SAM的甲基转移酶。突变体中氧化的P800^+的逆反应预计在闪光激发后会表现出某种不同的动力学。2、硫氧化途径分析：绿色硫细菌利用还原态硫化合物作为电子源。然而，有关硫氧化和光合电子传递途径之间的联系的信息一直很少。构建了三个突变体，即ΔcycA（细胞色素c-554 CycA的缺失突变体）、ΔsoxB（SoxB缺失突变体，是S_2O_3氧化的多酶系统中的一个亚基<2->）和ΔcycA的双突变体ΔsoxB，研究了硫氧化与光合电子传递途径的关系。虽然CycA确实是S_2O_3 ~+氧化途径的末端电子受体<2->，但另一个组分可能参与了S_2O_3 ~+氧化途径。此外，S_2<2->和S_2O_3氧化<2->反应释放的每个电子将通过一个离散的电子转移途径转移到P840反应中心。

We studied on the structural and functional relationship of the type 1 reaction center in green sulfur bacteria and heliobacteria in order to understand profoundly the principle for the molecular construction of photosystem I reaction center in higher plants and cyanobacteria. The research progress in these two years is summarized as follows. 1. Searching for the Secondary Electron Acceptor A_1 (Quinone): The ESR signal derived from reduced quinone was successfully detected in the reaction center core complex in Heliobacterium modesticaldum. At present, we are now trying to identify its chemical species with HPLC and mass spectroscopic analyses. Meanwhile, we tried to obtain a mutant deleted of menG which encode a SAM-dependent methyl transferase in the quinone biosynthetic pathway. The back reaction of the oxidized P800^+ in the mutant had been expected to show some kind of different kinetics after a flash excitation. But we failed to obtain it. 2: Analyses of Sulfur Oxidation Pathway: Green sulfur bacteria use reduced sulfur compounds as an electron source. However, information concerning the linkage between sulfur oxidation and photosynthetic electron transfer pathways has long been scarce. Three mutants, that is, ΔcycA (a deletion mutant of CycA, cytochrome c-554), ΔsoxB (the SoxB-deleted one, which is one of subunits forming a multi-enzyme system involved in S_2O_3^<2-> oxidation) and a double mutant of ΔcycAΔsoxB, were constructed and we studied on the relationship between sulfur oxidation and photosynthetic electron transfer pathways. Although CycA surely serves as a terminal electron acceptor in the S_2O_3^<2-> oxidation pathway, another component should be involved in it as a probable bypath. Furthermore, each electron liberated from the S^<2-> and S_2O_3^<2-> oxidations, respectively, would be transferred to the P840 reaction center by a discrete electron transfer pathway.