Siderophore

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A Siderophore (Greek for iron carrier) is an iron chelating compound secreted by microorganisms. Iron Fe³⁺ ions have a very low solubility at neutral pH and therefore cannot be utilized by organisms. Siderophores dissolve these ions as soluble Fe³⁺ complexes that can be taken up by active transport mechanisms. Many siderophores are nonribosomal peptides.

To put it another way: Under anoxic conditions, iron is generally in the +2 oxidation state (ferrous) and soluble. However, under oxic conditions, iron is generally in the +3 oxidation state (ferric) and forms various insoluble minerals. To obtain iron from such minerals, cells produce iron-binding siderophores that bind iron and transport it into the cell. One major group of siderophores consists of derivatives of hydroxamic acid, which chelate ferric iron very strongly.^[1]

Other strategies to enhance iron solubility and uptake are the acidification of the surrounding (e.g. used by plant roots) or the extracellular reduction of Fe³⁺ into the more soluble Fe²⁺ ions.

Examples of siderophores produced by various bacteria and fungi are ferrichrome (Ustilago sphaerogena), enterobactin (Escherichia coli), enterobactin and bacillibactin (Bacillus subtilis), ferrioxamine B (Streptomyces pilosus), fusarinine C (Fusarium roseum), yersiniabactin (Yersinia pestis), vibriobactin (Vibrio cholerae), azotobactin (Azotobacter vinelandii), pseudobactin (Pseudomonas B 10), erythrobactin (Saccharopolyspora erythraea) or ornibactin (Burkholderia cepacia).

1. ^ Biology of Microorganisms, Eleventh Edition, Pearson Education