Mukerji I, Andrews J C, DeRose V J, Latimer M J, Yachandra V K, Sauer K, Klein M P
Department of Chemistry, Lawrence Berkeley Laboratory, University of California, Berkeley 94720.
Biochemistry. 1994 Aug 16;33(32):9712-21. doi: 10.1021/bi00198a042.
X-ray absorption spectroscopy has been performed on oriented photosystem II membrane particles isolated from spinach. Structural features of the tetranuclear Mn cluster and the orientation of the cluster with respect to the lipid bilayer were determined in both the S1 and S2 states of the Kok cycle. Variation of the sample orientation with respect to the X-ray e-vector yields highly dichroic K-edge and extended X-ray absorption fine structure spectra (EXAFS), indicative of an asymmetric tetranuclear cluster. Mn-Mn vectors at 2.72 and 3.38 A can be resolved from these measurements using quantitative analysis. The 2.72-A vector, consisting of at least two component vectors, is oriented at an average angle of 60 degrees +/- 7 degrees to the membrane normal, with an average of 1.1 +/- 0.1 interactions per Mn atom. The 3.38-A vector, most probably an average of two vectors, makes an angle of 43 degrees +/- 10 degrees with respect to the membrane normal, with an average of 0.45 +/- 0.07 backscatterer per Mn atom. Upon advance to the S2 state, the orientation of these vectors and the average numbers of backscatterers are approximately invariant. Analysis of more subtle features of the EXAFS reveals changes accompanying this S-state advance that are consistent with the oxidation of Mn during this transition. However, the dominant structural features of the oxygen-evolving complex remain constant in the S1 and S2 states. The structure of the Mn complex and the orientation of the complex in the membrane within the context of dichroism of the X-ray absorption data are discussed.
已对从菠菜中分离出的定向光系统II膜颗粒进行了X射线吸收光谱分析。在Kok循环的S1和S2状态下，确定了四核锰簇的结构特征以及该簇相对于脂质双层的取向。样品相对于X射线电子矢量的取向变化产生了高度二色性的K边和扩展X射线吸收精细结构光谱（EXAFS），表明存在不对称四核簇。使用定量分析可以从这些测量中分辨出2.72 Å和3.38 Å处的锰-锰矢量。2.72 Å的矢量由至少两个分量矢量组成，与膜法线的平均夹角为60度±7度，每个锰原子平均有1.1±0.1个相互作用。3.38 Å的矢量很可能是两个矢量的平均值，相对于膜法线的夹角为43度±10度，每个锰原子平均有0.45±0.07个背散射体。进入S2状态后，这些矢量的取向和背散射体的平均数大致不变。对EXAFS更细微特征的分析揭示了伴随这种S态进展的变化，这些变化与该转变过程中锰的氧化一致。然而，在S1和S2状态下，析氧复合物的主要结构特征保持不变。本文讨论了在X射线吸收数据的二色性背景下，锰复合物的结构及其在膜中的取向。
