Catecholate siderophore complexes

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. 

In contrast to the tris-catecholate siderophores, which form charged iron(III) complexes, the hydroxamate-based ferri-siderophore complexes are electrically neutral, which may influence their transport through biological membranes. 

The presence of siderophores in a medium may be shown by adding an iron(III) salt, as complex formation will be demonstrated by the colour of the Feni-siderophore complex, due to charge-transfer bands in the visible region. Chemical and spectroscopic tests allow ready classification into catecholate and hydroxamate types, for example the use of the Arnow and Czaky colorimetric reactions, respectively.1172... 

Catecholate Siderophores. Simple Catechol Complexes. As noted earlier, the common siderophore for enteric bacteria is the tricatechol, enterobactin (Figure 4). In order to perfect synthetic and... 

The visible and circular dichroism spectra of the chromic siderophore complexes are closely related to the corresponding spectra of simple model complexes of hydroxamate or catecholate ligands. This provides a spectroscopic probe for structure in assigning the geometries of the siderophore complexes. 

Hydroxamate- or catecholate-containing siderophores are strongly absorbing species with characteristic spectra (see Table 1) which can be utilized for spectrophotometric determination of the complex formation constant. Iron(III) hydroxamates absorb in the visible region, producing a broad absorption band in the 420-440 nm region. Iron(III) catecholates exhibit pH-dependent absorption maxima. Unfortunately, the overall Fe + ion complex formation constants cannot be determined directly at neutral pH, because the extremely high stability of siderophore complexes precludes direct measurements of the equilibrium of interest, which would yield the desired formation constant for a tris-bidentate siderophore complex, /3no (equation (2)). ... 

Figure 1 Representative siderophores of the hydroxamate and catecholate classes. The hydroxamates are synthesized from the amino acid ornithine that has been modified through hydroxylation and acetylation. Ferrichrome (a) is a prototypical example of the tri-hydroxamate class. Structurally, ferrichrome is a cyclic hexapeptide that consists of three modified ornithine residues (each of which has a hydroxamate side chain) and three glycines. Ferrichrome coordinates ferric iron through its three bidentate hydroxamate side chains. Triacetylfusarinine C (b) is also a cyclic tri-hydroxamate, but the three modified ornithine residues are joined by ester linkages rather than by peptide linkages. Ferrioxamine B (c) is a linear tri-hydroxamate consisting of three peptide-huked modified ornithine residues. Enterobactin (d) is a prototypical example of a catecholate siderophore. It consists of a tri-ester ring from which extend three side chains of chhydroxybenzoyl serine. Each of these siderophores binds ferric iron in a hexadentate manner, which results in full saturation of d orbitals and a very stable complex. Ferric forms are shown in (a) and (b). Desferri-forms are shown in (c) and (d)...

Figure 6.18. MacrcKyclic complex fonners. (a) Structure of a ferrichrome (desferri-ferrichrome), one of the strongest complex formers presently known for Fe(III). The iron-binding center is an octahedral arrangement of six oxygen donor atoms of trihy-droxamate. Such naturally occurring ferrichromes play an important role in the biosynthetic pathways involving iron. Complexing functionalities of some biogenic ligands (b) hydroxamate siderophores, (c) catechol siderophores, and (d) phytochelatines. For detailed structures see Neilands (1981).

The three catechol groups of enterobactin are carried on a cyclic serine triester structure. A variety of both cyclic and linear structures are found among other catechol siderophores. " For example, parabactin and agrobactin (Fig. 16-1) contain a backbone of spermidine (Chapter 24). After the Fe -enterobactin complex enters a bacterial cell the ester linkages of a siderophore are cleaved by an esterase. Because of the extremely high formation constant of M for... 

One probable mechanism for the release of iron from siderophores to the agents which are directly involved in cell metabolism is enzymatic reduction to the ferrous state. Due to the very low affinity of hydrdxamate and catecholate siderophores for Fe(II), the reduction converts the tightly bound ferric ion to the ferrous complex, which is unstable with respect to protonation and dissociation at neutral pH or below. Therefore comparison of siderophore complex redox potentials with those of physiological reductants can be very useful for the clarification of the mechanism of iron metabolism. Table IV shows the redox potentials [obtained by cyclic voltammetry (see Fig. 18)) of the siderophores tested so far. The values of all of the hydroxamates are within the... 

N, 6-N-di(2,3-dihydroxybenzoyl)-L-lysine (58) is a siderophore produced by Azotobacter vinelandii which has only two catechol groups. However, of the catecholate siderophores by far the best studied is enterobactin. A major difference between hydroxamate and catecholate siderophores occurs in their utilization as transport agents. For the former, the iron complex is taken up by the bacterial cell, the iron released, and the hydroxamate siderophore re-secreted for additional iron chelation. In contrast, enterobactin is destroyed by enzymatic hydrolysis within the cell and therefore the ligand is not recycled. This hydrolysis of the amide linkages of the iron(III) enterobactin lowers the redox potential of the chelate complex sufficiently to allow iron reduction — and thus uptake of iron into the cell metabolism (59, 60). 

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