Reactions of isocytochrome c2 in the photosynthetic electron transfer chain of Rhodobacter sphaeroides.
Reactions of isocytochrome c2 in the photosynthetic electron transfer chain of Rhodobacter sphaeroides.
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球红杆菌光合电子传递链中异细胞色素 c2 的反应。
DOI：
10.1021/bi961648k
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发表时间：
1997
期刊：
影响因子：
--
通讯作者：
Donohue,TJ
中科院分区：
文献类型：
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作者：
WitthuhnJr,VC;Gao,J;Hong,S;Halls,S;Rott,MA;Wraight,CA;Crofts,AR;Donohue,TJ
在光氧化反应中心(RC)复合体中，细胞色素2(Cytc2)是P870+的正常电子供体，缺乏细胞色素2(Cytc2)的类红杆菌菌株不能进行光合作用生长。然而，抑制cytc2突变体的光合作用缺陷的spd突变会提高cytc2异构体的水平。我们监测了整个细胞、发色团和纯化组分的光合作用电子传递，以确定等胞质c2是否以及如何减少光氧化的RC复合体。这些研究表明，在使用同细胞C2和野生型细胞的菌株中，光合作用电子传递的几个基本方面是相似的。例如，P870+还原伴随细胞色素氧化。此外，众所周知的Cytbc1复合体抑制剂安替米星和霉唑也阻止了光合作用的电子传递。然而，即使在这些菌株中存在的等细胞C2水平增加(∼是野生型细胞中细胞C2水平的40%)，也几乎没有(如果有的话)快速的(<；5μS)电子转移到P870+，这是在闪光时细胞色素与RC复合体结合的特征。因此，等细胞C2功能似乎限制了P870+还原的活性。事实上，在体外低离子强度下，等胞体C2与RC络合物(KD∼40μM)的表观亲和力明显低于CytC2(KD∼1.0μM)。这种亲和力的降低似乎并不是由于等细胞C2与RC结合模式的改变造成的，因为静电相互作用对CytC2和等细胞C2与这个膜结合的氧化还原伙伴的结合有相似的总体贡献。因此，cytc2和iscytc2之间的序列、结构或局部构象差异显著改变了它们对这一生理上相关的氧化还原伙伴的表观亲和力。
Rhodobacter sphaeroidesstrains lacking cytochromec2(cytc2), the normal electron donor to P870+in light-oxidized reaction center (RC) complexes, are unable to grow photosynthetically. However,spdmutations thatsuppress thephotosyntheticdeficiency of cytc2mutants elevate levels of the cytc2isoform, isocytc2. We monitored photosynthetic electron transfer in whole cells, in chromatophores, and with purified components to ascertain if and how isocytc2reduced light-oxidized RC complexes. These studies revealed that several fundamental aspects of photosynthetic electron transfer were similar in strains that use isocytc2and wild-type cells. For example, P870+reduction accompanied cytochromecoxidation. In addition, photosynthetic electron transfer was blocked by the well-known cytbc1complex inhibitors antimycin and myxothiazol. However, even at the increased isocytc2levels present in these strains (∼40% that of cytc2in wild-type cells), there was little, if any, of the rapid (<5 μs) electron transfer to P870+that is characteristic of cytochromes bound to RC complexes at the time of the light flash. Thus, it appears that isocytc2function limits thein vivorate of P870+reduction. Indeed, at low ionic strengthin vitro, the apparent affinity of isocytc2for RC complexes (KD∼ 40 μM) is significantly lower than that of cytc2(KD∼ 1.0 μM). This reduced affinity does not appear to result from an altered mode of RC binding by isocytc2since electrostatic interactions make similar overall contributions to the binding of both cytc2and isocytc2to this membrane-bound redox partner. Thus, sequence, structural, or local conformational differences between cytc2and isocytc2significantly alter their apparent affinities for this physiologically relevant redox partner.