Bowes J M, Horton P, Bendall D S
Arch Biochem Biophys. 1983 Aug;225(1):353-9. doi: 10.1016/0003-9861(83)90040-1.
Chlorophyll a fluorescence has been used to monitor the redox state of the primary electron acceptor of photosystem II (PS II) in the blue-green alga Phormidium laminosum during equilibrium titrations. The shape of induction curves measured in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) have been analyzed. The induction curves were very similar in unfractionated thylakoid membranes and PS II particles. In both, the fast (alpha) phase was sigmoidal, and was followed by a slow (beta) exponential tail. Thus, the structural organization and complexity of the particles (J. M. Bowes and P. Horton, Biochim, Biophys. Acta 680, 127-133 (1982), as indicated by the occurrence of energy transfer between alpha centers and presence of beta centers, must preexist in the membranes. Redox titration of the initial level of fluorescence indicated the presence of a single quencher QH in the unfractionated thylakoids, midpoint potential: Em7.0 approximately -35 mV (n = 1). Thus, the occurrence of a single acceptor is characteristic for P. laminosum and the absence of a low potential acceptor in PS II particles (J.M. Bowes, P. Horton, and D.S. Bendall, FEBS Lett. 135, 261-264 (1981] was not the result of its removal during their preparation. The midpoint potential of Q varied by -60 mV/pH unit in PS II particles and membrane fragments, with a pK at pH greater than 8.5 (particles) and at pH 7.5 (fragments). In PS II particles, DCMU raised the pK by approximately 0.5 pH units. It is argued that the pH dependence of Q is conferred by protonation of a protein which accompanies its reduction rather than protonation of the semiquinone Q X itself.
在平衡滴定过程中，叶绿素a荧光已被用于监测蓝绿藻层状席藻中光系统II（PS II）初级电子受体的氧化还原状态。分析了在3-(3,4-二氯苯基)-1,1-二甲基脲（DCMU）存在下测得的诱导曲线形状。未分级的类囊体膜和PS II颗粒中的诱导曲线非常相似。在这两种情况下，快速（α）相呈S形，随后是缓慢（β）指数尾。因此，正如α中心之间能量转移的发生和β中心的存在所表明的，颗粒的结构组织和复杂性（J.M.鲍伊斯和P.霍顿，《生物化学与生物物理学报》680，127 - 133（1982））必定预先存在于膜中。初始荧光水平的氧化还原滴定表明，未分级的类囊体中存在单一淬灭剂QH，中点电位：Em7.0约为 - 35 mV（n = 1）。因此，存在单一受体是层状席藻的特征，而PS II颗粒中不存在低电位受体（J.M.鲍伊斯、P.霍顿和D.S.本德尔，《欧洲生物化学会联合会快报》135，261 - 264（1981））并非其制备过程中去除的结果。在PS II颗粒和膜片段中，Q的中点电位随pH单位变化 - 60 mV，在pH大于8.5（颗粒）和pH 7.5（片段）时具有pK值。在PS II颗粒中，DCMU使pK值提高了约0.5个pH单位。有人认为，Q对pH的依赖性是由其还原时伴随的蛋白质质子化赋予的，而不是半醌QX本身的质子化。
