McDermott A E, Yachandra V K, Guiles R D, Cole J L, Dexheimer S L, Britt R D, Sauer K, Klein M P
Lawrence Berkeley Laboratory, Department of Chemistry, University of California, Berkeley 94720.
Biochemistry. 1988 May 31;27(11):4021-31. doi: 10.1021/bi00411a019.
The Mn donor complex in the S1 and S2 states and the iron-quinone acceptor complex (Fe2+-Q) in O2-evolving photosystem II (PS II) preparations from a thermophilic cyanobacterium, Synechococcus sp., have been studied with X-ray absorption spectroscopy and electron paramagnetic resonance (EPR). Illumination of these preparations at 220-240 K results in formation of a multiline EPR signal very similar to that assigned to a Mn S2 species observed in spinach PS II, together with g = 1.8 and 1.9 EPR signals similar to the Fe2+-QA- acceptor signals seen in spinach PS II. Illumination at 110-160 K does not produce the g = 1.8 or 1.9 EPR signals, nor the multiline or g = 4.1 EPR signals associated with the S2 state of PS II in spinach; however, a signal which peaks at g = 1.6 appears. The most probable assignment of this signal is an altered configuration of the Fe2+-QA- complex. In addition, no donor signal was seen upon warming the 140 K illuminated sample to 215 K. Following continuous illumination at temperatures between 140 and 215 K, the average X-ray absorption Mn K-edge inflection energy changes from 6550 eV for a dark-adapted (S1) sample to 6551 eV for the illuminated (S2) sample. The shift in edge inflection energy indicates an oxidation of Mn, and the absolute edge inflection energies indicate an average Mn oxidation state higher than Mn(II). Upon illumination a significant change was observed in the shape of the features associated with 1s to 3d transitions. The S1 spectrum resembles those of Mn(III) complexes, and the S2 spectrum resembles those of Mn(IV) complexes. The extended X-ray absorption fine structure (EXAFS) spectrum of the Mn complex is similar in the S1 and S2 states. Simulations indicate O or N ligands at 1.75 +/- 0.05 A, transition metal neighbor(s) at 2.73 +/- 0.05 A, which are assumed to be Mn, and terminal ligands which are probably N and O at a range of distances around 2.2 A. The Mn-O bond length of 1.75 A and the transition metal at 2.7 A indicate the presence of a di-mu-oxo-bridged Mn structure. Simulations indicate that a symmetric tetranuclear cluster is unlikely to be present, while binuclear, trinuclear, or highly distorted tetranuclear structures are possible. The striking similarity of these results to those from spinach PS II suggests that the structure of the Mn complex is largely conserved across evolutionarily diverse O2-evolving photosynthetic species.
利用X射线吸收光谱和电子顺磁共振（EPR）技术，对嗜热蓝细菌聚球藻属（Synechococcus sp.）中处于S1和S2状态的锰供体复合物以及放氧光合系统II（PS II）制剂中的铁 - 醌受体复合物（Fe2 + -Q）进行了研究。在220 - 240 K对这些制剂进行光照，会形成一个多线EPR信号，该信号与在菠菜PS II中观察到的归属于锰S2物种的信号非常相似，同时还会出现g = 1.8和1.9的EPR信号，这与菠菜PS II中观察到的Fe2 + -QA - 受体信号相似。在110 - 160 K进行光照，不会产生g = 1.8或1.9的EPR信号，也不会产生与菠菜PS II的S2状态相关的多线或g = 4.1的EPR信号；然而，会出现一个在g = 1.6处达到峰值的信号。该信号最可能的归属是Fe2 + -QA - 复合物的构型改变。此外，将140 K光照后的样品升温至215 K时，未观察到供体信号。在140至215 K之间持续光照后，平均X射线吸收锰K边拐点能量从暗适应（S1）样品的6550 eV变为光照（S2）样品的6551 eV。边拐点能量的变化表明锰发生了氧化，绝对边拐点能量表明平均锰氧化态高于Mn(II)。光照后，与1s到3d跃迁相关的特征形状发生了显著变化。S1光谱类似于Mn(III)复合物的光谱，S2光谱类似于Mn(IV)复合物的光谱。锰复合物的扩展X射线吸收精细结构（EXAFS）光谱在S1和S2状态下相似。模拟表明，在1.75 +/- 0.05 Å处有O或N配体，在2.73 +/- 0.05 Å处有过渡金属邻体（假定为锰），以及在2.2 Å左右一系列距离处可能为N和O的末端配体。1.75 Å的Mn - O键长和2.7 Å的过渡金属表明存在一个双μ - 氧桥连的锰结构。模拟表明不太可能存在对称四核簇，而双核、三核或高度扭曲的四核结构是可能的。这些结果与菠菜PS II的结果惊人相似，表明锰复合物的结构在进化上不同的放氧光合物种中基本保守。
