Biological Ligands for Metal Ions - Biological Inorganic Chemistry - page 87

FIGURE 4.18 Chemical structures of selected siderophores to illustrate the four major structural classes (a) Enterobactin (b) Yersiniabactin
(c) Staphyloferrin (d) ferrichrome.

Enterobactin (pFe 35.5), the prototype of the catecholate siderophores, is the principal siderophore produced by

Eschericia coli ( Raymond et al., 2003 ) . It is a cyclic triester of dihydroxybenzoyl-serine. When enterobactin binds
iron, the six deprotonated hydroxyl groups of the dihydroxybenzoyl (or catecholate) functions wrap around the
metal ion in the centre of the molecule ( Fig. 4.19 ). Staphyloferrin A, the iron-transporting siderophore of
Staphylococci, contains a D-ornithine backbone to which two citric acid residues are linked, which are involved in
Fe(III) binding. Yersiniabactin is an example of a heterocyclic siderophore, from the highly pathological Yersinia
family. 15 In ferric-yersiniabactin, the iron atom is coordinated by the three nitrogens, and three negatively charged
oxygen atoms, arranged in a distorted octahedral arrangement ( Miller et al., 2003 ) .
The importance of iron for a bacteria like E. coli can be illustrated by the fact that 14 genes alone are required for
enterobactin-mediated iron uptake, including those for its synthesis, export, transport of the ferric-enterobactin back
into the cell and iron release. In total, E. coli has at least eight uptake systems for iron, encoded by some 50 genes.

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15. This family of charmers includes Y. pestis, the causative agent of the plague.