Mixed siderophores

Pyoverdins and Azotobactin are known examples of natural fluorescent siderophores that have been isolated from Pseudomonas aeruginosa and Azotobacter vinelandii, res-pectively ° ° . Pyoverdin Pa (171) ° and pyoverdin Pa Til (172) " are members of the pyoverdin family, which are peptide based hexadentate mixed siderophores that consist of a fluorescent chromophore dihydroxyquinoline, which provides a catecholate ligand for Fe(III) coordination in addition to two bidentate hydroxamate ligating groups. 

Azotobactin is a highly fluorescent hexadentate mixed siderophore containing a pyoverdin-like bidentate ligating chromophore, derived from 2,3-diamino-6,7-dihydroxyquinoline, a hydroxamate and a a-hydroxycarboxylic acid for Fe(in) coordination. ... 

Another study of a synthetic siderophore analog of mixed cathechol and hydroxamate donor groups exhibited similar spectral shifts with pH as observed for tris-catecholate side-rophores, consistent with a salicylate-binding mode shift (102). While this synthetic siderophore mimic supports the growth of many species of bacteria, it also exhibits a low iron-binding affinity relative to other hexadentate siderophores, with a pFe value of 18.3 (see L(cat2hydroxamate) Table HE). 

Siderophores can be classified by different criteria. In this review related structural types will be grouped together. Some arbitrariness cannot be avoided due to the occurrence of mixed types . Cross-references will then be given. Trivial names... 

Figure 2 Structures of representative catecholate and mixed-ligand siderophores (a) enterobactin (b) parabactin (c) mycobacins (R = various alkyl chains, = R = R = MeorH.R = alkyl chains or H ) (d) fluorescent chromophore of pseudobactins and pyoverdins...

Siderophores with mixed ligands and heterocyclic chelating groups... 

Figure 23 Enterobactin, a catechol siderophore with a dialectical (CO, NO ) mixed donor ligand...

GledhiU, M., McCormack, P., Ussher, S., Achterberg, E. P., Mantoura, R. F. C., and Worsfold, P. J. (2004). Production of siderophore type chelates by mixed bacterioplankton populations in nutrient enriched seawater incubations. Mar. Chem. 88, 75—83. 

The ferrichromes, fusarinines, and ferrioxamines are typical trihydroxamate siderophores, while enterobactin is a cyclic tricatecholate siderophore. However, there are several exceptions which employ mixed forms of coordination. For example, aero-bactin, schizokinen and arthrobactin contain, in addition to two hydroxamate groups, a a-hydroxy-carboxylate unit which completes the hexadentate ligand structure 30). The recently isolated ferrioxamine H, with two hydroxamate and one carboxylate, is only pentadentate — the sixth coordination site presumably occupied by water 31). 

Fig. 2. Structures of catechol-containing and mixed ligand siderophores...

For a mixed hydroxamate siderophore, mycobactin P, X-ray crystallographic data show that the Fe(III) complex crystallizes in the A-cis configuration. The absolute configuration of pseudobactin and pseudobactin A is A as could be confirmed by crystallographic data as well as by the CD spectra of the iron and the chromium... 

All outer-membrane transporters (OMTs) involved in iron uptake are made up of a 22-stranded p—barrel, which is occluded by an independently folded mixed a P globular cork domain of around 160 amino acid residues. This is illustrated for a vitamin B12 receptor and the ferric siderophore receptors for citrate, enterobactin, ferrichrome, pyochelin, and pyoverdin from E. coli and P. aeruginosa in Fig. 7.6. The ferric siderophore sits on top of the cork domain, as can be seen in Fig. 7.7 for FecA. The binding of the ligand induces a conformational... 

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