Siderophore

Siderophores (from sidero = iron and phoros = carrying) are a group of small molecules that chelate iron with a very high and specific affinity and are formed by aerobic bacteria (fungi and plants are also able to form siderophores). Their formation is coupled to the iron concentration within the cell via a special control circuit.

In principle, microorganisms require iron for their growth. To meet their needs, they form small molecules (mostly oligopeptides) with a high affinity for iron. These are released into their environment, where they complex very selectively with iron(III) ions and are subsequently taken up again by the microorganisms as iron-loaded siderophore-iron complexes via specific transport systems.

Since the complex binding constant of the siderophores for Fe(II) ions is much lower than for trivalent iron, the complexed metal ion is again released from the complex in the microorganisms after reduction to iron(II).

Further importance is attached to the siderophores as a storage system for iron; i.e., the siderophores loaded with iron can be stored within the cells.

The iron transport system of bacteria is already being used in experimental approaches to introduce antibacterial agents and labels into a bacterial cell by means of special molecular probes. Thus, these "molecular Trojans" can be used to make infections in smaller animals visible and treatable by means of fluorescent labels and corresponding imaging methods.

Known siderophores are the enterobactins (enterobacteria), ferrioxamines (actinobacteria), pyoverdins (pseudomonas), mycobactins and exochelins(mycobacteria). Enterobactins and other bacterial siderophores form much more stable complexes with iron(III) than human siderophores. They are therefore able to extract iron from haemoglobin (after haemolysis) and transferrin from the human body.

1. Miethke M et al.(2007) Siderophore-based iron acquisition and pathogen control. Microbiol Mol Biol Rev 71: 413-451.
2. Crosa JH et al. (2002) Genetics and assembly line enzymology of siderophore biosynthesis in bacteria. Microbiol Mol Biol Rev 66: 223-249.