Oxidation of cytochromes c and c2 by bacterial photosynthetic reaction centers in phospholipid vesicles. 1. Studies with neutral membranes.
Oxidation of cytochromes c and c2 by bacterial photosynthetic reaction centers in phospholipid vesicles. 1. Studies with neutral membranes.
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磷脂囊泡中细菌光合反应中心对细胞色素 c 和 c2 的氧化。
DOI：
10.1021/bi00555a035
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发表时间：
1980
期刊：
影响因子：
2.9
通讯作者：
C. Wraight
中科院分区：
文献类型：
--
作者：
R. E. Overfield;C. Wraight
从球形红假单胞菌分离的光合作用反应中心对细胞色素c2的氧化被发现与先前在低洗涤剂溶液中观察到的反应中心对细胞色素c2的氧化动力学相似[Overfield，R.E.，Wraight，C.A.，&Devum，D.(1979)FEBS Lett]。105、137-142]。当离子强度较低时，动力学为两相动力学。快相表明形成了细胞色素-反应中心络合物，其表观结合常数Kb约为10(5)M-1。但Kb随盐浓度的增加而显著降低，在0.1M的氯化钠溶液中未检测到快速氧化现象。缓慢的细胞色素氧化在细胞色素和反应中心都是一级的，因此总体上是二级的。当初级光产物的一级反向反应速率与细胞色素氧化速率相比明显时，观察到偏离理论二级行为。这可能导致对二阶速率常数的严重高估。磷脂酰胆碱囊泡中反应中心对细胞色素c2的缓慢氧化相遇频率比与溶解反应中心的相遇频率低40%。这归因于反应中心在囊泡膜中的扩散系数远低于在溶液中的扩散系数。中性囊泡未检测到维度降低的影响。在溶液和几种不同的磷脂酰胆碱，包括在其相变温度上下的磷脂酰胆碱的反应中心中，测定了细胞色素c2氧化慢相的活化能为8.0+/-0.4kcal x mol-1。因此，脂质的物理状态似乎不会影响任何导致细胞色素氧化的限速步骤。细胞色素c和c2的缓慢氧化动力学的离子强度依赖性证实了细胞色素-反应中心相互作用的静电性质，而pH依赖性表明在pH 9.5时对这种相互作用重要的一个或多个基团的滴定。
The oxidation of cytochrome c2 by photosynthetic reaction center isolated from Rhodopseudomonas sphaeroides and incorporated into unilamellar phosphatidylcholine vesicles was found to be kinetically similar to that observed earlier for reaction centers in low detergent solution [Overfield, R.E., Wraight, C.A., & DeVault, D. (1979) FEBS Lett. 105, 137-142]. At low ionic strength the kinetics were biphasic. The fast phase indicated the formation of a cytochrome-reaction center complex with an apparent binding constant, KB, of about 10(5) M-1. However, KB decreased dramatically with increasing salt concentration, and no fast oxidation was detectable in 0.1 M NaCl. The slow cytochrome oxidation was first order in both cytochrome and reaction centers and, thus, second order overall. Deviations from theoretical second-order behavior were observed when the rate of the first-order back reaction of the primary photoproducts was significant compared to the cytochrome oxidation. This can cause serious overestimation of the second-order rate constant. The slow oxidation of cytochrome c2 by reaction centers in phosphatidylcholine vesicles exhibited a 40% lower encounter frequency than with the solubilized reaction center. This was attributed to the much lower diffusion coefficient of the reaction center in the vesicle membrane than in solution. No effects of diminished dimensionality were detected with neutral vesicles. An activation energy of 8.0 +/- 0.4 kcal x mol-1 was determined for the slow phase of cytochrome c2 oxidation by reaction centers in solution and in vesicles of several different phosphatidylcholines, including dimyristoylphosphatidylcholine above and below its phase transition temperature. Thus, the physical state of the lipid did not appear to affect any rate-limiting steps leading to cytochrome oxidation. The ionic strength dependence of the slow kinetics of oxidation of cytochromes c and c2 confirmed the electrostatic nature of the cytochrome-reaction center interaction, and the pH dependence indicated the titration of a group or groups, important to this interaction, at pH 9.5.