Modulation of the midpoint potential of the [2Fe-2S] Rieske iron sulfur center by Qo occupants in the bc1 complex.
Modulation of the midpoint potential of the [2Fe-2S] Rieske iron sulfur center by Qo occupants in the bc1 complex.
复制标题
bc1 复合体中 Qo 占据者对 [2Fe-2S] Rieske 铁硫中心中点电位的调节。
DOI：
10.1021/bi026198c
复制
发表时间：
2002
期刊：
影响因子：
2.9
通讯作者：
Crofts,AntonyR
中科院分区：
文献类型：
--
作者：
Shinkarev,VladimirP;Kolling,DerrickRJ;Miller,TimJ;Crofts,AntonyR
Following addition of myxothiazol to antimycin-treated chromatophores fromRhodobacter sphaeroidespoised at an ambient redox potential (Eh) of ∼300 mV, the amplitude of the flash-induced cytochromec1oxidation in the ms range increased, indicating a decrease in the availability of electrons from the immediate donor toc1, the Rieske iron−sulfur protein (ISP). Because the effect was seen only over the limitedEhrange, we conclude that it is due to a decrease in the apparent midpoint redox potential (Em) of the ISP by about 40 mV on addition of myxothiazol. This is in line with the change inEmpreviously seen in direct redox titrations. Our results show that the reduced ISP binds with quinone at the Qosite with a higher affinity than does the oxidized ISP. The displacement of ubiquinone by myxothiazol leads to elimination of this preferential binding of the ISP reduced form and results in a shift in the midpoint potential of ISP to a more negative value. A simple hypothesis to explain this effect is that myxothiazol prevents formation of hydrogen bond of ubiquinone with the reduced ISP. We conclude that all Qosite occupants (ubiquinone, UHDBT, stigmatellin) that form hydrogen bonds with the reduced ISP shift the apparentEmof the ISP in the same direction to more positive values. Inhibitors that bind in the domain of the Qosite proximal to hemebL(myxothiazol, MOA−stilbene) and displace ubiquinone from the site cause a decrease inEmof ISP. We present a new formalism for treatment of the relation betweenEmchange and the binding constants involved, which simplifies analysis. Using this formalism, we estimated that binding free energies for hydrogen bond formation with the Qosite occupant, range from the largest value of ∼23 kJ mol-1in the presence of stigmatellin (appropriate for the buried hydrogen bond shown by structures), to a value of ∼3.5 kJ mol-1in the native complex. We discuss this range of values in the context of a model in which the native structure constrains the interaction of ISP with the Qosite occupant so as to favor dissociation and the faster kinetics of unbinding necessary for rapid turnover.
