Iron sequestering

The answer to the question why vicibactin is biosynthesized if vicibactin 7101 is as efficient in iron sequestering may be the greater stability of the acetylated compound (cf. fusarinines, Sect. 2.6). Vicibactin is identical with neurosporin produced by the fungus Neurospora crassa for which X-ray data of the Fe " complex are available. CD spectroscopy indicates a A-cis configuration both for crystals and for solution (108). 

To cope with their need for iron, microorganisms possess remarkably sophisticated mechanisms to scavenge iron from its plentiful, yet biologically inaccessible, environmental sources. Among the various mechanisms for iron uptake the most general is the use of iron sequestering agents. 

Iron Sequestering Agents Promoting Microbial Growth Siderophores... 

Chromium(III) complexes of a number of polyhydroxamic acids, microbial iron sequestering and transport agents (siderochromes) have been reported.797,798 The kinetic inertness of the chromium(III) complexes allows the facile separation of isomers for the model complex tris(iV-methyl-( - )-methoxyacetylhydroxamato)chromium(III), D-cis, L-cis and the l/d-trans isomers have been separated.798 The chromium complexes of desferrioxamine B (191) have been investigated the possible isomers are illustrated below (192-196). The cis isomer was isolated in relatively pure form.799 Thiohydroxamate800 and dihydroxamate (rhodotorulic acid) complexes have also been studied.801... 

Recently the naturally occurring powerful iron sequestering agent enterobactin has provided a model for the development of ligands which show high formation constants with Ga3+ and... 

The depletion in FeO may be understood in at least two ways. First, the crystalline grains may be equilibrium condensates from a hot solar nebular composition gas with iron sequestered to metals or sulfides (see e.g. Chapter 4). In this case the condensed grains either had to condense slowly to form crystal domains, or had been reheated and thermally annealed at a later epoch. The second, alternative explanation is that ferromagnesian amorphous silicate grains were thermally annealed in a reducing environment, e.g. in the presence of carbon. Heating such precursors leads to the formation of metallic spheroids embedded between the forsterite crystals, as the initial FeO component is reduced (see e.g. Fig. 8.3 and Connolly et al. 1994 Jones Danielson 1997 Leroux et al. 2003 Davoisne et al. 2006). Because carbon is ubiquitously present in primitive Solar System materials, this pathway offers a natural explanation to the observed FeO-poor silicate crystals. It is yet to be determined whether low-temperature crystallization processes, discussed in Section 8.1.1, would also lead to FeO depletion. 

Heme oxygenase (HO) is a microsomal enzyme catalysing the breakdown of b-type heme to equimolar amormts of biliverdin IXa, ferrous iron and carbon monoxide (CO) by cleaving the -meso carbon bridge. All of these products are biological active. Ferrous iron facilitates free radical formation but induces also ferritin an antioxidant iron sequestering protein. CO... 

Enterobactin (87) is a 12-membered ring macrotriolide initially isolated from Salmonella typhimurium [110]. It is involved in microbial transport and metabolism of iron. It is an iron-sequestering agent of the phenolate group and is overproduced by Escherichia coli and related enteric bacteria under low-iron stress. 

Table I. Relative ferric ion complexlng abilities at neutral pH for human transferrin and several microbial iron sequestering agents.

Relative kinetics of iron removal from human transferrin by several iron sequestering agents. 

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