SINGLE-CRYSTAL EXAFS OF NITROGENASE
SINGLE-CRYSTAL EXAFS OF NITROGENASE
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DOI：
10.1021/ja00265a034
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发表时间：
1986-03-05
影响因子：
15
通讯作者：
CRAMER, SP
中科院分区：
文献类型：
--
作者：
FLANK, AM;WEININGER, M;CRAMER, SP
用偏振x射线吸收光谱法研究了氮酶Mo-Fe蛋白的单晶结构。在不同的取向下，Mo- fe的振幅变化为2.5倍，而Mo- s的振幅变化仅为。+-。15%。EXAFS光谱的取向依赖性被用于研究Mo-Fe蛋白中Mo， Fe， S簇的几何形状和取向。这是单晶EXAFS首次应用于晶体结构未知的酶。还研究了模型化合物（Ph4P）2[Cl2FeS2MoS2FeCl2]和（Et4N）3[Fe6Mo2S8(SET)9]的单晶取向依赖性，以量化该技术的实验精度。该分析方法利用x射线衍射证明了晶体2重轴和分子2重轴，克服了每个细胞四个钼位点的困难。假设Mo， Fe， S簇的对称性，以及一个簇相对于晶体轴的取向，就有可能计算出预期的EXAFS取向依赖关系。并与实验光谱进行了比较，得到了不同方向的线性对称、弯曲对称、四面体对称和方锥对称。结果表明，Fe-Mo-Fe原子线性排列的簇不能很好地模拟实验数据，而Fe-Mo-Fe角在50度之间的三核簇则不能很好地模拟实验数据。和130.度。给予满意的同意。四面体MoFe3和方锥体MoFe4簇对称也给出了令人满意的方向依赖性模拟。假设Mo-Fe为四面体，其中一个Mo-Fe簇的三重轴的最佳取向为75 +-角。10.学位。从结晶学的a轴和215 +-。10或285 +-。10.学位。从b轴开始。
Single crystals of the nitrogenase Mo-Fe protein have been examined by polarized X-ray absorption spectroscopy. For different orientations the Mo-Fe amplitude of the Mo K-edge EXAFS was found to change by a factor of 2.5, whereas the Mo-S component varied by only .+-. 15%. The orientation dependence of the EXAFS spectra has been used to investigate the geometry and orientation of the Mo, Fe, S clusters within the Mo-Fe protein. This represents the first application of single-crystal EXAFS to an enzyme of unknown crystal structure. The orientation dependence for single crystals of the model compounds (Ph4P)2[Cl2FeS2MoS2FeCl2] and (Et4N)3[Fe6Mo2S8(SET)9] was also examined to quantify the experimental precision of this technique. The analysis procedures overcame the difficulty of four molybdenum sites per unit cell by using the X-ray diffraction evidence for a crystallographic 2-fold axis and a molecular 2-fold axis. Given initial assumptions about the symmetry of the Mo, Fe, S clusters, as well as the orientation of one cluster with respect to the crystallographic axes, it was possible to calculate the expected EXAFS orientation dependence. The patterns for linear, bent, tetrahedral, and square-pyramidal symmetrics in various orientations were then compared with the experimental spectra. It was found that the experimental data were not well simulated by clusters with a linear arrangement of Fe-Mo-Fe, atoms, whereas trinuclear clusters with a Fe-Mo-Fe angle between 50.degree. and 130.degree. gave satisfactory agreement. Tetrahedral MoFe3 and square-pyramidal MoFe4 cluster symmetrics also gave satisfactory simulations of the orientation dependence. Assuming a tetrahedral Mo-Fe geometry, the preferred orientation of the 3-fold axis of one of the Mo-Fe clusters was found to lie at an angle of 75 .+-. 10.degree. from the crystallographic a axis and 215 .+-. 10 or 285 .+-. 10.degree. from the b axis.