The redox midpoint potential of the primary quinone of reaction centers in chromatophores of Rhodobacter sphaeroides is pH independent
The redox midpoint potential of the primary quinone of reaction centers in chromatophores of Rhodobacter sphaeroides is pH independent
复制标题
DOI：
10.1007/s00249-008-0301-4
复制
发表时间：
2008-09-01
影响因子：
2
通讯作者：
Wraight, Colin A.
中科院分区：
文献类型：
--
作者：
Maroti, Peter;Wraight, Colin A.
据报道，通过氧化还原电位测定法测量的天然膜（色素细胞）中细菌反应中心Q（A）的伯醌的氧化还原中点电位（E-m）是pH依赖性的（-60 mV/pH），直到高度独特的pK（a）（Rba中为9.8）。sphaeroides）用于还原状态。相反，在Rba的孤立RC中Q（A）的E（m）。尽管更易变，但通过氧化还原电位测定法和延迟荧光法两者已经发现，类球蛋白基本上与pH无关，延迟荧光法确定P（+）Q（A）（-）状态相对于P* 的自由能（Δ G（P*A））。本文用延迟荧光法测定了色素中P（+）Q（A）（-）的自由能。在色素的发射强度是两个数量级以上，从孤立的RC主要是由于熵效应的天线色素“画出”的RC的激发。Δ G（P*A）的pH依赖性与分离的RC的pH依赖性几乎相同，与Q（A）的电位氧化还原滴定形成鲜明对比。我们认为Q（A）可能通过Q（B）位点与QH（2）的反离子化而被还原，因此滴定实际上反映了醌池，给出了Q/QH（2）对预期的-60 mV/pH单位依赖性。然而，实现强pH依赖性所需的参数与Q（A）和Q（B）的预期性质并不完全一致。我们还考虑了电位滴定的时间尺度允许还原态（Q（A）（-））松弛到不同的构象，伴随着化学计量的H+结合的可能性。最后，我们讨论了从Rba的色素团的延迟荧光发射确定P+Q（A）（-）的自由能级所需的参数的选择。sphaeroides
The redox midpoint potential (E-m) of the primary quinone of bacterial reaction centers, Q(A), in native membranes (chromatophores) measured by redox potentiometry is reported to be pH dependent (-60 mV/pH) up to a highly distinctive pK(a) (9.8 in Rba. sphaeroides) for the reduced state. In contrast, the E (m) of Q(A) in isolated RCs of Rba. sphaeroides, although more variable, has been found to be essentially pH-independent by both redox potentiometry and by delayed fluorescence, which determines the free energy (Delta G(P*A)) of the P(+)Q(A)(-) state relative to P*. Delayed fluorescence was used here to determine the free energy of P(+)Q(A)(-) in chromatophores. The emission intensity in chromatophores is two orders of magnitude greater than from isolated RCs largely due to the entropic effect of antenna pigments "drawing out" the excitation from the RC. The pH dependence of Delta G(P*A) was almost identical to that of isolated RCs, in stark contrast with potentiometric redox titrations of Q(A). We considered that Q(A) might be reduced by disproportionation with QH(2) through the Q(B) site, so the titration actually reflects the quinone pool, giving the -60 mV/pH unit dependence expected for the Q/QH(2) couple. However, the parameters necessary to achieve a strong pH-dependence are not in good agreement with expected properties of Q(A) and Q(B). We also consider the possibility that the time scale of potentiometric titrations allows the reduced state (Q(A)(-) ) to relax to a different conformation that is accompanied by stoichiometric H+ binding. Finally, we discuss the choice of parameters necessary for determining the free energy level of P+Q(A)(-) from delayed fluorescence emission from chromatophores of Rba. sphaeroides.