Sun Chang, Taguchi Alexander T, Beal Nathan J, O'Malley Patrick J, Dikanov Sergei A, Wraight Colin A
Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
J Phys Chem Lett. 2015 Nov 19;6(22):4541-6. doi: 10.1021/acs.jpclett.5b01851. Epub 2015 Nov 4.
Unlike photosystem II (PSII) in higher plants, bacterial photosynthetic reaction centers (bRCs) from Proteobacteria have an additional peripheral membrane subunit "H". The H subunit is necessary for photosynthetic growth, but can be removed chemically in vitro. The remaining LM dimer retains its activity to perform light-induced charge separation. Here we investigate the influence of the H subunit on interactions between the primary semiquinone and the protein matrix, using a combination of site-specific isotope labeling, pulsed electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations. The data reveal substantially weaker binding interactions between the primary semiquinone and the LM dimer than observed for the intact bRC; the amount of electron spin transferred to the nitrogen hydrogen bond donors is significantly reduced, the methoxy groups are more free to rotate, and the spectra indicate a heterogeneous mixture of bound semiquinone states. These results are consistent with a loosening of the primary quinone binding pocket in the absence of the H subunit.
与高等植物中的光系统II（PSII）不同，变形菌门的细菌光合反应中心（bRCs）有一个额外的外周膜亚基“H”。H亚基对于光合生长是必需的，但在体外可以通过化学方法去除。剩余的LM二聚体保留其进行光诱导电荷分离的活性。在这里，我们结合位点特异性同位素标记、脉冲电子顺磁共振（EPR）和密度泛函理论（DFT）计算，研究H亚基对初级半醌与蛋白质基质之间相互作用的影响。数据显示，初级半醌与LM二聚体之间的结合相互作用比完整的bRC中观察到的要弱得多；转移到氮氢键供体的电子自旋量显著减少，甲氧基更自由地旋转，并且光谱表明结合的半醌状态是异质混合物。这些结果与在没有H亚基的情况下初级醌结合口袋的松弛一致。
