Lakshmi K V, Eaton S S, Eaton G R, Brudvig G W
Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, and Department of Chemistry and Biochemistry, University of Denver, Denver, Colorado 80208, USA.
Biochemistry. 1999 Sep 28;38(39):12758-67. doi: 10.1021/bi990780s.
Inhibitory treatment by acetate, followed by illumination and rapid freezing, is known to trap the S(2)Y(Z)() state of the O(2)-evolving complex (OEC) in photosystem II (PS II). An EPR spectrum of this state exhibits broad split signals due to the interaction of the tyrosyl radical, Y(Z)(), with the S = 1/2 S(2) state of the Mn(4) cluster. We present a novel approach to analyze S(2)Y(Z)() spectra of one-dimensionally (1-D) oriented acetate-inhibited PS II membranes to determine the magnitude and relative orientation of the S(2)Y(Z)() dipolar vector within the membrane. Although there exists a vast body of EPR data on isolated spins in oriented membrane sheets, the present study is the first of its kind on dipolar-coupled electron spin pairs in such systems. We demonstrate the feasibility of the technique and establish a rigorous treatment to account for the disorder present in partially oriented 1-D membrane preparations. We find that (i) the point-dipole distance between Y(Z)(*) and the Mn(4) cluster is 7.9 +/- 0.2 A, (ii) the angle between the interspin vector and the thylakoid membrane normal is 75 degrees, (iii) the g(z)()-axis of the Mn(4) cluster is 70 degrees away from the membrane normal and 35 degrees away from the interspin vector, and (iv) the exchange interaction between the two spins is -275 x 10(-)(4) cm(-)(1), which is antiferromagnetic. Due to the sensitivity of EPR line shapes of oriented spin-coupled pairs to the interspin distance, the present study imposes a tighter constraint on the Y(Z)-Mn(4) point-dipole distance than obtained from randomly oriented samples. The geometric constraints obtained from the 1-D oriented sample are combined with published models of the structure of Mn-depleted PS II to propose a location of the Mn(4) cluster. A structure in which Y(Z) is hydrogen bonded to a manganese-bound hydroxide ligand is consistent with available data and favors maximal orbital overlap between the two redox center that would facilitate direct electron- and proton-transfer steps.
已知通过乙酸盐进行抑制处理，随后光照并快速冷冻，可捕获光系统II（PS II）中放氧复合体（OEC）的S(2)Y(Z)()状态。该状态的电子顺磁共振（EPR）谱由于酪氨酸自由基Y(Z)()与Mn(4)簇的S = 1/2 S(2)状态相互作用而呈现出宽分裂信号。我们提出了一种新方法，用于分析一维（1-D）取向的乙酸盐抑制的PS II膜的S(2)Y(Z)()谱，以确定膜内S(2)Y(Z)()偶极矢量的大小和相对取向。尽管在取向膜片中存在大量关于孤立自旋的EPR数据，但本研究是此类系统中关于偶极耦合电子自旋对的首次研究。我们证明了该技术的可行性，并建立了一种严格的处理方法来考虑部分取向的1-D膜制剂中存在的无序性。我们发现：（i）Y(Z)(*)与Mn(4)簇之间的点偶极距离为7.9±0.2 Å；（ii）自旋间矢量与类囊体膜法线之间的夹角为75度；（iii）Mn(4)簇的g(z)轴与膜法线成7０度角，与自旋间矢量成35度角；（iv）两个自旋之间的交换相互作用为-275×10(-4) cm(-1)，这是反铁磁性的。由于取向自旋耦合对的EPR线形对自旋间距离敏感，本研究对Y(Z)-Mn(4)点偶极距离的约束比从随机取向样品中获得的更严格。从1-D取向样品获得的几何约束与已发表的贫锰PS II结构模型相结合，提出了Mn(4)簇的位置。Y(Z)与锰结合的氢氧化物配体形成氢键的结构与现有数据一致，并且有利于两个氧化还原中心之间的最大轨道重叠，这将促进直接的电子和质子转移步骤。
