Hartmann A, Braun V
J Bacteriol. 1980 Jul;143(1):246-55. doi: 10.1128/jb.143.1.246-255.1980.
During the transport of iron as ferrichrome complex into cells of Escherichia coli K-12, the ligand was modified and excreted into the medium. The rate of the formation of the modified product corresponded with the rate of iron transport. The modified product showed a decreased affinity for ferric iron and did not serve as an effective iron ionophore. After all of the ferrichrome had been converted, the modified product was taken up into the cell in an iron-free form. The uptake of ferrichrome and of the modified product depended on the transport system specified by the tonA and tonB genes. The modified product could be converted back into ferrichrome by mild acid or alkaline hydrolysis. One mole of acetate was released per mole of ferrichrome. It is proposed that one N-hydroxyl group of ferrichrome is acetylated to explain the low affinity for iron as the N-hydroxyl groups form the ligands for iron (III). A weak ester linkage by which the acetyl group is covalently bonded would account for the easy hydrolysis. The iron-free form of ferrichrome, deferri-ferrichrome, was also rapidly converted when incubated with cells with a functional transport system. It is therefore likely that iron is released from ferrichrome by reduction before modification takes place. The conversion of the ligand could be a mechanism by which cells rid themselves of a potentially deleterious ligand for iron in the cytoplasm. A possible role in ferrichrome transport is discussed.
在作为高铁色素复合物的铁转运到大肠杆菌K-12细胞内的过程中，配体发生修饰并分泌到培养基中。修饰产物的形成速率与铁的转运速率相对应。修饰产物对三价铁的亲和力降低，并且不能作为有效的铁离子载体。当所有高铁色素都被转化后，修饰产物以无铁形式被细胞摄取。高铁色素和修饰产物的摄取取决于由tonA和tonB基因指定的转运系统。修饰产物可以通过温和的酸或碱水解转化回高铁色素。每摩尔高铁色素释放出一摩尔乙酸盐。有人提出高铁色素的一个N-羟基被乙酰化，以解释其对铁的低亲和力，因为N-羟基形成了铁（III）的配体。乙酰基通过一个弱酯键共价结合可以解释其易于水解的原因。无铁形式的高铁色素，去铁高铁色素，在与具有功能性转运系统的细胞一起孵育时也会迅速转化。因此，很可能在修饰发生之前，铁通过还原作用从高铁色素中释放出来。配体的转化可能是细胞清除细胞质中潜在有害的铁配体的一种机制。文中还讨论了其在高铁色素转运中的可能作用。
