Iron-siderophore complex transport

Some metals may need to be mobilized from the environment to make them bioavailable. Iron in particular must be rendered more soluble to be accessible for uptake. Microorganisms and some plants have evolved with secreted ligands known as siderophores (or phytosiderophores). These ligands bind Fe + with extraordinary affinity. For example, a complex of the siderophore enterobactin with ferric iron has a formal stability constant of 10 (19). Once siderophores compete with other environmental ligands for iron, the ferric iron-siderophore complex then binds to specific transport proteins at the microbial... 

How sensitive is the transport process to the exact shape or geometry of the iron-siderophore complex ... 

Fig. 2 Iron(III) transport system in E. coli and other enterobactriaceae. The donation of the iron-siderophore complex to the periplasmic binding protein is facilitated by the cytoplasmic protein TonB. The cytoplasmic iron-siderophore transporters function via a typical ABC-type mechanism, which is driven by ATP hydrolysis.

In contrast to the tris-catecholate siderophores, which form charged iron(III) complexes, the hydroxamate-based ferri-siderophore complexes are electrically neutral, which may influence their transport through biological membranes. 

Escherichia coli has at least five independent transport systems, one of which is the low affinity pathway described above. In addition, it synthesizes enterobactin as a siderophore it can take up the iron(III) complex of ferrichrome, a siderophore synthesized by certain fungi there is a citrate-induced system, and a less common process involving aerobactin. 

The transport or release of iron has been much discussed. One view is that the iron is transferred from the siderophore complex at the outer membrane to another membrane-bound protein, for example in the uptake of iron by rhodotorulic acid in Rhodotorula. The other view is that the intact Fein-siderophore complex is taken up into the cell. This is supported by studies with inert chromium(III) complexes in E. coli and also by labelling studies. 

Most bacteria have the ability to produce and secrete molecules—called siderophores—to fulfill their iron requirements. Siderophores are special iron-chelating agents that facilitate iron solubilization and uptake. They are water-soluble, low-molecular weight molecules that bind ferric ions strongly. The ability of bacteria to utilize siderophores is associated with the presence of transport systems that can recognize and mediate uptake of the ferric-siderophore complexes into the cell. These iron-acquisition systems are regulated in response to iron availability, and their action thus increases under iron limitation conditions. 

In times of iron deficiency, many bacteria and fungi release low molecular weight chelators called siderophores (see Iron Transport Siderophores). These molecules bind ferric iron tightly and the ferric-siderophore complexes are then transported into the cell by a system of uptake proteins. The first stage in the uptake process involves an outer membrane receptor specific to each siderophore. One of the best characterized of these receptors is FhuA, the ferrichrome uptake receptor of E. coli, and we will describe this in detail. However, though other ferric-siderophore complexes are taken up by cells, and their iron released by systems similar to those of ferrichrome, their mechanisms may vary from those of ferrichrome in some respects. FepA and FecA" are two of the outer membrane ferric-siderophore receptors that have recently been structurally characterized. 

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