Köster W, Braun V
Mikrobiologie II, Universität Tübingen, Federal Republic of Germany.
J Biol Chem. 1990 Dec 15;265(35):21407-10.
Due to its extreme insolubility, Fe3+ is not transported as a monoatomic ion. In microbes, iron is bound to low molecular weight carriers, designated siderophores. For uptake into cells of Escherichia coli Fe3+ siderophores have to be translocated across two membranes. Transport across the outer membrane is receptor-dependent and energy-coupled; transport across the cytoplasmic membrane seems to follow a periplasmic binding protein-dependent transport mechanism. In support of this notion we demonstrate specific binding of the Fe3+ hydroxamate compounds ferrichrome, aerobactin, and coprogen, which are transported via the Fhu system, to the periplasmic FhuD protein, and no binding of the transport inactive ferrichrome A, ferric citrate, and iron sulfate. About 10(4) ferrichrome molecules were bound to the FhuD protein of cells which overproduced plasmid-encoded FhuD. Binding depended on transport across the outer membrane mediated by the FhuA receptor and the TonB protein. Binding to FhuD was supported by the exclusive resistance of FhuD to proteinase K in the presence of the transport active hydroxamates. The overproduced precursor form of the FhuD protein was not protected by the Fe3+ hydroxamates indicating a conformation different to the mature form. The FhuD protein apparently serves as a periplasmic carrier for Fe3+ hydroxamates with widely different structures.
由于其极高的不溶性，Fe3+ 并非以单原子离子的形式运输。在微生物中，铁与低分子量载体结合，这些载体被称为铁载体。为了被大肠杆菌细胞摄取，Fe3+ 铁载体必须穿过两层膜。跨外膜的运输是受体依赖性且能量偶联的；跨细胞质膜的运输似乎遵循一种周质结合蛋白依赖性运输机制。为支持这一观点，我们证明了通过 Fhu 系统运输的 Fe3+ 异羟肟酸化合物铁色素、气杆菌素和粪生绿素与周质 FhuD 蛋白的特异性结合，而运输无活性的铁色素 A、柠檬酸铁和硫酸铁则无结合。约 10(4) 个铁色素分子与过量表达质粒编码的 FhuD 的细胞的 FhuD 蛋白结合。结合依赖于由 FhuA 受体和 TonB 蛋白介导的跨外膜运输。在运输活性异羟肟酸存在的情况下，FhuD 对蛋白酶 K 的独特抗性支持了其与 FhuD 的结合。过量表达的 FhuD 蛋白前体形式不受 Fe3+ 异羟肟酸的保护，表明其构象与成熟形式不同。FhuD 蛋白显然作为具有广泛不同结构的 Fe3+ 异羟肟酸的周质载体。
