Conformational differences between the methoxy groups of QA and QB site ubisemiquinones in bacterial reaction centers: a key role for methoxy group orientation in modulating ubiquinone redox potential.
Conformational differences between the methoxy groups of QA and QB site ubisemiquinones in bacterial reaction centers: a key role for methoxy group orientation in modulating ubiquinone redox potential.
复制标题
细菌反应中心 QA 和 QB 位点泛半醌的甲氧基之间的构象差异：甲氧基方向在调节泛醌氧化还原电位中的关键作用。
DOI：
10.1021/bi400489b
复制
发表时间：
2013
期刊：
影响因子：
2.9
通讯作者：
Dikanov,SergeiA
中科院分区：
文献类型：
--
作者：
Taguchi,AlexanderT;O'Malley,PatrickJ;Wraight,ColinA;Dikanov,SergeiA
Ubiquinone is an almost universal, membrane-associated redox mediator. Its ability to accept either one or two electrons allows it to function in critical roles in biological electron transport. The redox properties of ubiquinonein vivoare determined by its environment in the binding sites of proteins and by the dihedral angle of each methoxy group relative to the ring plane. This is an attribute unique to ubiquinone among natural quinones and could account for its widespread function with many different redox complexes. In this work, we use the photosynthetic reaction center as a model system for understanding the role of methoxy conformations in determining the redox potential of the ubiquinone/semiquinone couple. Despite the abundance of X-ray crystal structures for the reaction center, quinone site resolution has thus far been too low to provide a reliable measure of the methoxy dihedral angles of the primary and secondary quinones, QAand QB. We performed 2D ESEEM (HYSCORE) on isolated reaction centers with ubiquinones13C-labeled at the headgroup methyl and methoxy substituents, and have measured the13C isotropic and anisotropic components of the hyperfine tensors. Hyperfine couplings were compared to those derived by DFT calculations as a function of methoxy torsional angle allowing estimation of the methoxy dihedral angles for the semiquinones in the QAand QBsites. Based on this analysis, the orientation of the 2-methoxy groups are distinct in the two sites, with QBmore out of plane by 20–25°. This corresponds to an ≈50 meV larger electron affinity for the QBquinone, indicating a substantial contribution to the experimental difference in redox potentials (60–75 mV) of the two quinones. The methods developed here can be readily extended to ubiquinone-binding sites in other protein complexes.
