Siderophores hydroxamate type

The FhuA receptor of E. coli transports the hydroxamate-type siderophore ferrichrome (see Figure 9), the structural similar antibiotic albomycin and the antibiotic rifamycin CGP 4832. Likewise, FepA is the receptor for the catechol-type siderophore enterobactin. As monomeric proteins, both receptors consist of a hollow, elliptical-shaped, channel-like 22-stranded, antiparallel (3-barrel, which is formed by the large C-terminal domain. A number of strands extend far beyond the lipid bilayer into the extracellular space. The strands are connected sequentially using short turns on the periplasmic side, and long loops on the extracellular side of the barrel. 

As mentioned above, transport of siderophores across the cytoplasmic membrane is less specific than the translocation through the outer membrane. In E. coli three different outer membrane proteins (among them FepA the receptor for enterobactin produced by most E. coli strains) recognise siderophores of the catechol type (enterobactin and structurally related compounds), while only one ABC system is needed for the passage into the cytosol. Likewise, OM receptors FhuA, FhuE, and Iut are needed to transport a number of different ferric hydroxamates, whereas the FhuBCD proteins accept a variety of hydroxamate type ligands such as albomycin, ferrichrome, coprogen, aerobactin, shizokinen, rhodotorulic acid, and ferrioxamine B [165,171], For the vast majority of systems, the substrate specificity has not been elucidated, but it can be assumed that many siderophore ABC permeases might be able to transport several different but structurally related substrates. 

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... 

Siderophores as chemical entities can generally be classed as either hydroxamates or catechols. Ferrichrome and enterobactin are prototypical members of the two classes, respectively. The tri-catechol siderophore, enterobactin, is of special interest since it has been demonstrated repeatedly that it can sv5>ply iron to bacteria in the presence of certain ferric tri-hydroxamate type siderophores (Kj> = 103 ) not utilized by the organisms (IjJ. 

Rhodotorulic acid, a hydroxamate type siderophore, binds 2/3 mole ferric ion to give an unchsurged, orange colored chelate which is fully formed at neutral pH (10). 

Since in each case the iron was completely transferred, we estimate that the Kf for the tri-catechols must be at least several orders of magnitude greater than those reported for the tri-hydroxamate type siderophore ligands. 

ABC transporters involved in the uptake of siderophores, haem, and vitamin B]2 are widely conserved in bacteria and Archaea (see Figure 10). Very few species lack representatives of the siderophore family transporters. These species are mainly intracellular parasites whose metabolism is closely coupled to the metabolism of their hosts (e.g. mycoplasma), or bacteria with no need for iron (e.g. lactobacilli). In many cases, several systems of this transporter family can be detected in a single species, thus allowing the use of structurally different chelators. Most systems were exclusively identified by sequence data analysis, some were biochemically characterised, and their substrate specificity was determined. However, only very few systems have been studied in detail. At present, the best-characterised ABC transporters of this type are the fhuBCD and the btuCDF systems of E. coli, which might serve as model systems of the siderophore family. Therefore, in the following sections, this report will mainly focus on the components that mediate ferric hydroxamate uptake (fhu) and vitamin B12 uptake (htu). 

A multiple-path mechanism has been elaborated for dissociation of the mono- and binuclear tris(hydroxamato)-iron(III) complexes with dihydroxamate ligands in aqueous solution. " Iron removal by edta from mono-, bi-, and trinuclear complexes with model desferrioxamine-related siderophores containing one, two, or three tris-hydroxamate units generally follows first-order kinetics though biphasic kinetics were reported for iron removal from one of the binuclear complexes. The kinetic results were interpreted in terms of discrete intrastrand ferrioxamine-type structures for the di-iron and tri-iron complexes of (288). " Reactivities for dissociation, by dissociative activation mechanisms, of a selection of bidentate and hexadentate hydroxamates have been compared with those of oxinates and salicylates. ... 

Hydroxamic acids and their amino acid derivatives formed a series of dioxomolybdenum(VI) complexes of the type cis-[Mo02(hdx)2] and [Mo02(hdxamc)2] (hdxH, (10) hdxamcH, (11)), which are attractive as models for siderophores and for the development of metal-chelating drugs. 

Ferrichrome was the first ligand of this type to be isolated (Figure 47). Ferrichrome is a cyclic peptide with three hydroxamic acid side-chains. It gives a neutral complex with Fe . A number of variations having substituents on the hexapeptide or on the acyl group are also found. The ferrichromes are synthesized by fungi, but they are also used by many bacteria as a source of iron, even though they do not synthesize the siderophore themselves. 

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