Dismukes G C, Sauer K
Biochim Biophys Acta. 1978 Dec 7;504(3):431-45. doi: 10.1016/0005-2728(78)90065-8.
The orientation of membrane-bound radicals in spinach chloroplasts is examined by electron paramagnetic resonance (EPR) spectroscopy of chloroplasts oriented by magnetic fields. Several of the membrane-bound radicals which possess g-tensor anisotropy display EPR signals with a marked dependence on the orientation of the membranes relative to the applied EPR field. The fraction of oxidized and reduced plastocyanin, P-700, iron-sulfur proteins A and B, and the X center, an early acceptor of Photosystem I, can be controlled by the light intensity during steady-state illumination and can be trapped by cooling. The X center can be photoreduced and trapped in the absence of strong reductants and high pH, conditions previously found necessary for its detection. These results confirm its role as an early electron acceptor in P-700 photo-oxidation. X is oriented with its smallest principal g-tensor axis (gx) predominantly parallel to the normal to the thylakoid membrane, the same orientation as was found for an early electron acceptor based on time-resolved electron spin polarization studies. We propose that the X center is the first example of a high potential iron-sulfur protein which functions in electron transfer in its 'superreduced' state. We present evidence which suggests that iron-sulfur proteins A and B are 4Fe-4S clusters in an 8Fe-8S protein. Center B is oriented with gy predominantly normal to the membrane plane. The spectra of center A and plastocyanin do not show significant changes with sample orientation. In the case of plastocyanin, this may indicate a lack of molecular orientation. The absence of an orientation effect for reduced center A is reconcilable with a 4Fe-4S geometry, provided that the electron obtained upon reduction can be shared between any pair of Fe atoms in the center. Orientation of the 'Rieske' iron-sulfur protein is also observed. It has axial symmetry with g parallel close to the plane of the membrane. A model is proposed for the organization of these proteins in the thylakoid membrane. A new EPR signal was observed in oriented chloroplasts. This broad unresolved resonance displays a g value of 3.2 when the membrane normal is parallel to the field. It shifts to g = 1.9 when the membrane normal is perpendicular to the field. The signal is sensitive to illumination and to washing of the thylakoid membranes of broken chloroplasts. We suggest that there is a relation between this signal and the water-oxidizing enzyme system.
通过对磁场取向的菠菜叶绿体进行电子顺磁共振（EPR）光谱分析，研究了菠菜叶绿体中膜结合自由基的取向。几种具有g张量各向异性的膜结合自由基显示出EPR信号，该信号明显依赖于膜相对于所施加EPR场的取向。在稳态光照期间，氧化型和还原型质体蓝素、P - 700、铁硫蛋白A和B以及X中心（光系统I的早期受体）的比例可以通过光强度进行控制，并且可以通过冷却捕获。在没有强还原剂和高pH值（先前发现检测X中心所需的条件）的情况下，X中心可以被光还原并捕获。这些结果证实了其作为P - 700光氧化中早期电子受体的作用。X的最小主g张量轴（gx）主要平行于类囊体膜的法线方向，这与基于时间分辨电子自旋极化研究发现的早期电子受体的取向相同。我们提出X中心是高电位铁硫蛋白在其“超还原”状态下参与电子转移的首个例子。我们提供的证据表明铁硫蛋白A和B是8Fe - 8S蛋白中的4Fe - 4S簇。中心B的gy主要垂直于膜平面取向。中心A和质体蓝素的光谱在样品取向改变时未显示出显著变化。对于质体蓝素而言，这可能表明缺乏分子取向。如果还原时获得的电子可以在中心的任意一对铁原子之间共享，那么还原型中心A不存在取向效应就与4Fe - 4S的几何结构相符合。还观察到了“Rieske”铁硫蛋白的取向。它具有轴向对称性，g平行于膜平面且接近该平面。提出了这些蛋白质在类囊体膜中组织方式的模型。在取向的叶绿体中观察到一个新的EPR信号。当膜法线平行于磁场时，这个宽的未分辨共振显示g值为3.2。当膜法线垂直于磁场时，它移至g = 1.9。该信号对光照以及破碎叶绿体的类囊体膜洗涤敏感。我们认为这个信号与水氧化酶系统之间存在关联。
