Mansour A N, Thompson C, Theil E C, Chasteen N D, Sayers D E
J Biol Chem. 1985 Jul 5;260(13):7975-9.
Polynuclear iron complexes of Fe(III) and phosphate occur in seawater and soils and in cells where the iron core of ferritin, the iron storage protein, contains up to 4500 Fe atoms in a complex with an average composition of (FeO.OH)8FeO.OPO3H2. Although phosphate influences the size of the ferritin core and thus the availability of stored iron, little is known about the nature of the Fe(III)-phosphate interaction. In the present study, Fe-phosphate interactions were analyzed in stable complexes of Fe(III).ATP which, in the polynuclear iron form, had phosphate at interior sites. Such Fe(III).ATP complexes are important not only as models but also because they may play a role in intracellular iron transport and in iron toxicity; the complexes were studied by extended x-ray absorption fine structure, EPR, NMR spectroscopy, and measurement of proton release. Mononuclear iron complexes exhibiting a g' = 4.3 EPR signal were formed at Fe:ATP ratios less than or equal to 1:3, and polynuclear iron complexes (Fe greater than or equal to 250, EPR silent at g' = 4.3) were formed at an Fe:ATP ratio of 4:1. No NMR signals due to ATP were observed when Fe was in excess (Fe:ATP = 4:1). Extended x-ray absorption fine structure analysis of the polynuclear Fe(III).ATP complex was able to distinguish an Fe-P distance at 3.27 A in addition to the octahedral O at 1.95 A and 4-5 Fe atoms at 3.36 A. The Fe-O and Fe-Fe distances are the same as in ferritin, and the Fe-P distance is analogous to that in another metal-ATP complex. An observable Fe-P environment in such a large polynuclear iron cluster as the Fe(III).ATP (4:1) complex indicates that the phosphate is distributed throughout rather than merely on the surface, in contrast to earlier models of chelate-stabilized iron clusters. Complexes of Fe(III) and ATP similar to those described here may form in vivo either as normal components of intracellular iron metabolism or during iron excess where the consequent alteration of free nucleotide triphosphate pools could contribute to the observed toxicity of iron.
铁（III）与磷酸盐的多核铁配合物存在于海水、土壤以及细胞中，在细胞里，铁储存蛋白铁蛋白的铁芯含有多达4500个铁原子，它们与平均组成为(FeO.OH)8FeO.OPO3H2的复合物结合。尽管磷酸盐会影响铁蛋白核心的大小，进而影响储存铁的可用性，但关于铁（III）-磷酸盐相互作用的本质却知之甚少。在本研究中，对铁（III）·ATP的稳定配合物中的铁-磷酸盐相互作用进行了分析，这种配合物以多核铁形式存在，内部位点含有磷酸盐。此类铁（III）·ATP配合物不仅作为模型很重要，还因为它们可能在细胞内铁运输和铁毒性方面发挥作用；通过扩展X射线吸收精细结构、电子顺磁共振（EPR）、核磁共振（NMR）光谱以及质子释放测量对这些配合物进行了研究。当铁与ATP的比例小于或等于1:3时，会形成呈现g' = 4.3 EPR信号的单核铁配合物，而当铁与ATP的比例为4:1时，会形成多核铁配合物（铁含量大于或等于250，在g' = 4.3时EPR无信号）。当铁过量时（铁与ATP = 4:1），未观察到由ATP产生的NMR信号。对多核铁（III）·ATP配合物的扩展X射线吸收精细结构分析，除了能分辨出八面体氧在1.95 Å处以及4至5个铁原子在3.36 Å处外，还能分辨出铁-磷距离在3.27 Å处。铁-氧和铁-铁距离与铁蛋白中的相同，铁-磷距离与另一种金属-ATP配合物中的类似。在像铁（III）·ATP（4:1）配合物这样大的多核铁簇中可观察到的铁-磷环境表明，与早期螯合稳定铁簇模型相反，磷酸盐是分布在整个簇中而非仅仅在表面。与本文所述类似的铁（III）和ATP配合物可能在体内形成，要么作为细胞内铁代谢的正常成分，要么在铁过量时形成，此时随之而来的游离核苷酸三磷酸池的改变可能导致所观察到的铁毒性。
