Cook Jeremy D, Bencze Krisztina Z, Jankovic Ana D, Crater Anna K, Busch Courtney N, Bradley Patrick B, Stemmler Ann J, Spaller Mark R, Stemmler Timothy L
Department of Biochemistry and Molecular Biology, Wayne State University, School of Medicine, 540 East Canfield Avenue, Detroit, Michigan 48201, USA.
Biochemistry. 2006 Jun 27;45(25):7767-77. doi: 10.1021/bi060424r.
Friedreich's ataxia, an autosomal cardio- and neurodegenerative disorder that affects 1 in 50,000 humans, is caused by decreased levels of the protein frataxin. Although frataxin is nuclear-encoded, it is targeted to the mitochondrial matrix and necessary for proper regulation of cellular iron homeostasis. Frataxin is required for the cellular production of both heme and iron-sulfur (Fe-S) clusters. Monomeric frataxin binds with high affinity to ferrochelatase, the enzyme involved in iron insertion into porphyrin during heme production. Monomeric frataxin also binds to Isu, the scaffold protein required for assembly of Fe-S cluster intermediates. These processes (heme and Fe-S cluster assembly) share requirements for iron, suggesting that monomeric frataxin might function as the common iron donor. To provide a molecular basis to better understand frataxin's function, we have characterized the binding properties and metal-site structure of ferrous iron bound to monomeric yeast frataxin. Yeast frataxin is stable as an iron-loaded monomer, and the protein can bind two ferrous iron atoms with micromolar binding affinity. Frataxin amino acids affected by the presence of iron are localized within conserved acidic patches located on the surfaces of both helix-1 and strand-1. Under anaerobic conditions, bound metal is stable in the high-spin ferrous state. The metal-ligand coordination geometry of both metal-binding sites is consistent with a six-coordinate iron-(oxygen/nitrogen) based ligand geometry, surely constructed in part from carboxylate and possibly imidazole side chains coming from residues within these conserved acidic patches on the protein. On the basis of our results, we have developed a model for how we believe yeast frataxin interacts with iron.
弗里德赖希共济失调症是一种常染色体显性遗传的心脏和神经退行性疾病，每50000人中就有1人受其影响，它是由铁调素水平降低引起的。尽管铁调素是由细胞核编码的，但它被靶向运输到线粒体基质中，并且对于细胞铁稳态的正常调节是必需的。细胞内血红素和铁硫（Fe-S）簇的产生都需要铁调素。单体铁调素与亚铁螯合酶具有高亲和力结合，亚铁螯合酶是一种在血红素生成过程中将铁插入卟啉的酶。单体铁调素还与Isu结合，Isu是组装Fe-S簇中间体所需的支架蛋白。这些过程（血红素和Fe-S簇组装）对铁有共同需求，这表明单体铁调素可能作为常见的铁供体发挥作用。为了提供一个分子基础以更好地理解铁调素的功能，我们已经对与单体酵母铁调素结合的亚铁的结合特性和金属位点结构进行了表征。酵母铁调素作为负载铁的单体是稳定的，并且该蛋白可以以微摩尔结合亲和力结合两个亚铁原子。受铁存在影响的铁调素氨基酸位于螺旋-1和链-1表面的保守酸性区域内。在厌氧条件下，结合的金属在高自旋亚铁状态下是稳定的。两个金属结合位点的金属-配体配位几何结构与基于六配位铁-（氧/氮）的配体几何结构一致，肯定部分是由来自蛋白质上这些保守酸性区域内残基的羧酸盐和可能的咪唑侧链构成的。基于我们的研究结果，我们已经建立了一个关于酵母铁调素如何与铁相互作用的模型。
