Investigation of the calcium-binding site of the oxygen evolving complex of photosystem II using 87Sr ESEEM spectroscopy.
Investigation of the calcium-binding site of the oxygen evolving complex of photosystem II using 87Sr ESEEM spectroscopy.
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使用 87Sr ESEEM 光谱研究光系统 II 的放氧复合物的钙结合位点。
DOI：
10.1021/ja030614e
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发表时间：
2004
期刊：
影响因子：
--
通讯作者：
Britt,RDavid
中科院分区：
文献类型：
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作者：
Kim,SunHee;Gregor,Wolfgang;Peloquin,JeffreyM;Brynda,Marcin;Britt,RDavid
利用~（87）Sr三脉冲电子自旋回波包络调制（ESEEM）光谱研究了光系统II（PSII）释氧复合物（OEC）的钙/锶结合位点与顺磁性Mn团簇的接近性。Sr ~（2+）取代的Ca ~（2+）耗尽的PSII膜的CW-EPR谱显示如先前报道的修改的g = 2多线EPR信号。我们进行了三脉冲ESEEM对这个修改后的多线信号的Mn集群使用天然丰度Sr和87 Sr，分别。三脉冲ESEEM的天然丰度Sr样品表现出没有可检测的调制7%的丰度87 Sr。另一方面，87 Sr富集（93%）的样品清楚地揭示了调制所产生的I =9/287 Sr核弱磁耦合到Mn团簇。使用一个简单的点偶极近似的电子自旋，通过解析表达式分析87 Sr的ESEEM调制深度，表明Mn−Ca（Sr）距离为4.5 μ m。用数值矩阵对角化方法对三脉冲ESEEM进行了模拟，结果与分析结果吻合较好。一个更合适的锰团簇的四核磁性/结构模型将4.5 π点的偶极距离转换为3.8−5.0 π点的距离。DFT计算的43 Ca和87 Sr四极相互作用的Ca（和Sr取代）的结合位点在各种蛋白质中表明，缺乏核四极诱导分裂的ESEEM光谱of 87 Sr富集PSII样品是有关的一个非常高的对称度的配体周围的Sr 2+离子在取代的Ca网站。数值模拟表明，相对于零四极子的情况，适度的87 Sr四极子耦合降低了包络调制，因此我们认为在模拟中没有四极子耦合的情况下获得的3.8−5.0 μ m范围代表了实际锰钙距离的上限。这87锶脉冲EPR光谱提供了独立的直接证据，钙/锶结合位点是接近的锰簇在OEC的PSII。
The proximity of the calcium/strontium binding site of the oxygen evolving complex (OEC) of photosystem II (PSII) to the paramagnetic Mn cluster is explored with87Sr three-pulse electron spin−echo envelope modulation (ESEEM) spectroscopy. CW-EPR spectra of Sr2+-substituted Ca2+-depleted PSII membranes show the modifiedg= 2 multiline EPR signal as previously reported. We performed three-pulse ESEEM on this modified multiline signal of the Mn cluster using natural abundance Sr and87Sr, respectively. Three-pulse ESEEM of the natural abundance Sr sample exhibits no detectable modulation by the 7% abundance87Sr. On the other hand, that of the87Sr enriched (93%) sample clearly reveals modulation arising from theI=9/287Sr nucleus weakly magnetically coupled to the Mn cluster. Using a simple point dipole approximation for the electron spin, analysis of the87Sr ESEEM modulation depth via an analytic expression suggests a Mn−Ca (Sr) distance of 4.5 Å. Simulation of three-pulse ESEEM with a numerical matrix diagonalization procedure gave good agreement with this analytical result. A more appropriate tetranuclear magnetic/structural model for the Mn cluster converts the 4.5 Å point dipole distance to a 3.8−5.0 Å range of distances. DFT calculations of43Ca and87Sr quadrupolar interactions on Ca (and Sr substituted) binding sites in various proteins suggest that the lack of the nuclear quadrupole induced splitting in the ESEEM spectrum of87Sr enriched PSII samples is related to a very high degree of symmetry of the ligands surrounding the Sr2+ion in the substituted Ca site. Numerical simulations show that moderate87Sr quadrupolar couplings decrease the envelope modulation relative to the zero quadrupole case, and therefore we consider that the 3.8−5.0 Å range obtained without quadrupolar coupling included in the simulation represents an upper limit to the actual manganese−calcium distance. This87Sr pulsed EPR spectroscopy provides independent direct evidence that the calcium/strontium binding site is close to the Mn cluster in the OEC of PSII.