Sayers D E, Theil E C, Rennick F J
J Biol Chem. 1983 Dec 10;258(23):14076-9.
Cell-specific variations in apoferritin structure correlate with variations in iron metabolism that suggest functional specificity of the protein shell. Using EPR spectroscopy, we previously showed that vanadyl binds to specific sites on apoferritin, and that VO2+ binding is reduced by Fe(II) and Fe(III) (the natural substrates) and by metals known to influence iron storage (Chasteen, N. D., and Theil, E. C. (1982) J. Biol. Chem. 257, 7672-7677). Such observations suggest that the metal-binding site is important to apoferritin function and may define a location where the influence of cell-specific structural features are exerted. To investigate the iron-protein complex further, we have used x-ray absorption spectroscopy and have characterized, for the first time to our knowledge, Fe(III) apparently attached to the protein, after analyzing the x-ray absorption spectrum of an Fe(III)-apoferritin complex (10 Fe/molecule) compared to that of ferritin (polynuclear Fe(III)OOH, about 2000/molecule). The environment of iron in the Fe(III)-protein complex was similar to that in an Fe(III)-oxalate (2:3) hexahydrate complex, both in near edge structure and extended x-ray absorption structure, confirming earlier predictions of carboxylates as protein ligands. The extended x-ray absorption fine structure data for both compounds was fit best by a model in which a second shell of low Z atoms (carbon) was close (0.53-0.55 A) to the first shell of coordinated oxygen. However, small differences between Fe(III)-apoferritin and Fe(III)-oxalate in the Fe-O environment suggest a distorted geometry in the Fe(III)-protein complex and/or the presence of a mixture of atoms, such as nitrogen and oxygen, coordinated to iron. Extension of this approach to other apoferritins and metals will be likely to clarify the role of cell-specific features of the apoprotein in the formation of the iron core.
脱铁铁蛋白结构中的细胞特异性变化与铁代谢的变化相关，这表明蛋白质外壳具有功能特异性。我们之前利用电子顺磁共振光谱表明，氧钒基结合到脱铁铁蛋白的特定位点，并且Fe(II)和Fe(III)（天然底物）以及已知影响铁储存的金属会降低VO2+的结合（查斯汀，N. D.，和泰尔，E. C.（1982年）《生物化学杂志》257卷，7672 - 7677页）。这些观察结果表明，金属结合位点对脱铁铁蛋白的功能很重要，并且可能定义了细胞特异性结构特征发挥作用的位置。为了进一步研究铁 - 蛋白质复合物，我们使用了X射线吸收光谱法，在分析了Fe(III) - 脱铁铁蛋白复合物（每分子10个铁原子）与铁蛋白（多核Fe(III)OOH，每分子约2000个铁原子）的X射线吸收光谱后，据我们所知首次对明显附着在蛋白质上的Fe(III)进行了表征。Fe(III) - 蛋白质复合物中铁的环境在近边结构和扩展X射线吸收结构方面与Fe(III) - 草酸盐（2:3）六水合物复合物中的相似，证实了早期关于羧酸盐作为蛋白质配体的预测。两种化合物的扩展X射线吸收精细结构数据通过一个模型拟合得最好，在该模型中，低Z原子（碳）的第二壳层靠近（0.53 - 0.55 Å）配位氧的第一壳层。然而，Fe(III) - 脱铁铁蛋白和Fe(III) - 草酸盐在Fe - O环境中的微小差异表明Fe(III) - 蛋白质复合物中存在扭曲的几何结构和/或存在与铁配位的氮和氧等原子的混合物。将这种方法扩展到其他脱铁铁蛋白和金属可能会阐明脱辅基蛋白的细胞特异性特征在铁核形成中的作用。
