Iron siderophore

Saccharomyces cerevisiae does not itself synthesize or secrete siderophores. However, somewhat like E. coli (Chapter 3), it can take up iron-siderophore... 

There are a variety of factors concerning siderophore architecture that can contribute to the stability of iron-siderophore complexes. The overall architecture for natural multidentate siderophores can be any one of a number of different types linear,... 

As mentioned previously, siderophores must selectively bind iron tightly in order to solubilize the metal ion and prevent hydrolysis, as well as effectively compete with other chelators in the system. The following discussion will address in more detail the effect of siderophore structure on the thermodynamics of iron binding, as well as different methods for measuring and comparing iron-siderophore complex stability. The redox potentials of the ferri-siderophore complexes will also be addressed, as ferri-siderophore reduction may be important in the iron uptake process in biological systems. 

Siderophore binding sites for iron(III) are for the most part negatively charged and therefore, in aqueous solution there is a competition between H+ and Fe3+ binding. Consequently, the equilibrium expression for the formation of the iron-siderophore complex must take into account proton participation in the reaction. 

Stability comparisons between siderophore complexes with different binding stoichiometries are complicated by the fact that the units for the concentration equilibrium constants are different. Also, since the Fe3+ binding moieties have different pKa values competition for binding with H+ differs, which will not be reflected in the pH-independent / mlh values. Therefore, it is important to have a scale for iron-siderophore complex... 

This variation in complex stability with change in pH is particularly important in the context of biological systems, as it can potentially play a role in the iron uptake mechanism of some organisms. In some cases, the iron-siderophore complex is taken... 

The effect of the amino acid spacer on iron(III) affinity was investigated using a series of enterobactin-mimic TRENCAM-based siderophores (82). While TRENCAM (17) has structural similarities to enterobactin, in that it is a tripodal tris-catechol iron-binding molecule, the addition of amino acid spacers to the TRENCAM frame (Fig. 10) increases the stability of the iron(III) complexes of the analogs in the order ofbAla (19)complex stability is attributed to the intramolecular interactions of the additional amino acid side chains that stabilize the iron-siderophore complex slightly. 

Another factor that can possibly affect the redox potential in biological systems is the presence of secondary chelating agents that can participate in coupled equilibria (3). When other chelators are present, coupled equilibria involving iron-siderophore redox occur and a secondary ligand will cause the siderophore complex effective redox potential to shift. The decrease in stability of the iron-siderophore complex upon reduction results in a more facile release of the iron. Upon release, the iron(II) is available for complexation by the secondary ligand, which results in a corresponding shift in the redox equilibrium toward production of iron(II). In cases where iron(II) is stabilized by the secondary chelators, there is a shift in the redox potential to more positive values, as shown in Eqs. (42)—(45). 

The kinetics and mechanism of iron-siderophore complex formation are influenced by the oxidation state and composition of the first coordination shell of iron. The iron sequestration... 

It has also been observed that the ionic strength of the solution or the presence of SDS micelles will affect the rate of dissociation of iron-siderophore complexes (22,181). 

Koster, W. (2001). ABC transporter-mediated uptake of iron, siderophores, heme and vitamin Bi2, Res. Microbiol., 152, 291-301. 

Demange P, Wendenbaum S, Bateman A, Dell A, Meyer JM, Abdallah MA (1986) Bacterial Siderophores Structure of Pyoverdins and Related Compounds. In Swinburne TR (ed) Iron, Siderophores, and Plant Diseases. Plenum, New York, p 131... 

Due to their capacity to specifically chelate ferric iron, siderophores have been used for chelation therapy to treat iron overload diseases . ... 

Figure 10.1. (a) Rhamnolipid from P. aeruginosa ATCC 9027 showing cadmium binding, (b) Structure of the iron-siderophore complex of enterobactin. 

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