Hydrogen bonding and spin density distribution in the Qb semiquinone of bacterial reaction centers and comparison with the Qa site.
Hydrogen bonding and spin density distribution in the Qb semiquinone of bacterial reaction centers and comparison with the Qa site.
复制标题
细菌反应中心Qb半醌的氢键和自旋密度分布以及与Qa位点的比较。
DOI：
10.1021/ja2001538
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发表时间：
2011
影响因子：
15
通讯作者：
Dikanov,SergeiA
中科院分区：
文献类型：
--
作者：
Martin,Erik;Samoilova,RimmaI;Narasimhulu,KupalaV;Lin,Tzu-Jen;O'Malley,PatrickJ;Wraight,ColinA;Dikanov,SergeiA
球形红细菌光合反应中心的初级电子受体（QA）和次级电子受体（QB）均为泛醌-10，但性质和功能有很大不同。为了研究赋予这些功能差异的蛋白质环境，我们应用X-波段HYSCORE，一种2D脉冲EPR技术，使用15 N标记的反应中心样品，用在1H 2 O和2 H2O溶剂中制备的天然高自旋Fe 2+交换为抗磁性Zn 2+，来表征QA和QB位点中半醌（SQ）周围的可交换质子。粉末HYSCORE方法首先由弗洛雷斯等人（Biophys. J.2007，92，671 - 682），对于两个具有各向异性超精细张量分量T的可交换质子，两者都在4.6 - 5.4 MHz范围内，具有良好的一致性。然后将HYSCORE应用于QBSQ，在那里我们发现了对应于T = 5.2，3.7 MHz和T = 1.9 MHz的质子线。基于密度泛函的量子力学/分子力学（QM/MM）计算，采用模型的QB网站，被用来分配所观察到的耦合到特定的氢键相互作用与QBSQ。这些计算使我们能够将T = 5.2MHz的质子归属于His-L190 NδH···O 4（羰基）氢键相互作用。T = 3.7 MHz的光谱特征很可能是由于O 1（羰基）与Gly-L225肽NH和Ser-L223羟基OH的氢键相互作用，其计算耦合非常接近该值。较小的1.9 MHz耦合被分配给Ile-L224的弱结合肽NH质子。与此结构模型的QB网站进行的计算显示不对称分布的不成对自旋密度在SQ比看到的QAsite，与现有的实验数据for 13 C和17 O羰基超精细耦合一致。这些相互作用的影响QB功能和比较QAsite进行了讨论。
In the photosynthetic reaction center fromRhodobacter sphaeroides, the primary (QA) and secondary (QB) electron acceptors are both ubiquinone-10, but with very different properties and functions. To investigate the protein environment that imparts these functional differences, we have applied X-band HYSCORE, a 2D pulsed EPR technique, to characterize the exchangeable protons around the semiquinone (SQ) in the QAand QBsites, using samples of15N-labeled reaction centers, with the native high spin Fe2+exchanged for diamagnetic Zn2+, prepared in1H2O and2H2O solvent. The powder HYSCORE method is first validated against the orientation-selected Q-band ENDOR study of the QASQ by Flores et al. (Biophys. J.2007,92, 671−682), with good agreement for two exchangeable protons with anisotropic hyperfine tensor components,T, both in the range 4.6−5.4 MHz. HYSCORE was then applied to the QBSQ where we found proton lines corresponding toT ≈5.2, 3.7 MHz andT≈ 1.9 MHz. Density functional-based quantum mechanics/molecular mechanics (QM/MM) calculations, employing a model of the QBsite, were used to assign the observed couplings to specific hydrogen bonding interactions with the QBSQ. These calculations allow us to assign theT= 5.2 MHz proton to the His-L190 NδH···O4(carbonyl) hydrogen bonding interaction. TheT= 3.7 MHz spectral feature most likely results from hydrogen bonding interactions of O1 (carbonyl) with both Gly-L225 peptide NH and Ser-L223 hydroxyl OH, which possess calculated couplings very close to this value. The smaller 1.9 MHz coupling is assigned to a weakly bound peptide NH proton of Ile-L224. The calculations performed with this structural model of the QBsite show less asymmetric distribution of unpaired spin density over the SQ than seen for the QAsite, consistent with available experimental data for13C and17O carbonyl hyperfine couplings. The implications of these interactions for QBfunction and comparisons with the QAsite are discussed.