Siderophores catecholate 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. 

Chakraborty RN, Patel HN, Desai SB (1990) Isolation and Partial Characterization of Catechol-Type Siderophore from Pseudomonas stutzeri. Curr Microbiol 20 283 Challis GL (2005) A Widely Distributed Bacterial Pathway for Siderophore Biosynthesis Independent of Nonribosomal Peptide Synthetases. ChemBioChem 6 601 Chambers CE, McIntyre DD, Mouck M, Sokol PA (1996) Physical and Structural Characterization of Yersiniophore, a Siderophore Produced by Clinical Isolates of Yersinia entero-colitica. BioMetals 9 157... 

Shah S, Rao KK, Desai A (1993) Production of Catecholate Type of Siderophores by Azospirillum lipoferum M. Indian J Exp Biol 31 41... 

The NRPS-based biosynthesis of the myxochelins - catecholate-type siderophores produced by a number of myxobacterial strains - has also been investigated. The biosynthetic pathway, the only known myxobac-terial secondary metabolite gene cluster involved in iron acquisition, has been identified in S. aurantiaca Sg al5, S. cellulosum So ce56, and M. xanthus DK1622 (unpublished data), and six genes are found to be involved... 

QzHsvFeNijO,, Mr 1041.83, a water-soluble and fluorescing siderophore of the hydroxamate/catechol type R is produced by cultures of Pseudomonas fluo-rescens under iron deficiency conditions. P. is a plant growth substance but also has antifungal activity against Fusarium sp. 

A. Preparation of Siderophores. Published methods were used for the Isolation of agrobactin ( 6), agrobactin A (6), psn abactin ( 8), enterobactin 2.) rhodotorulic acid (10) ferrichrome (ll) and ferrichrome A (l ). Parabactin A was obtained by a synthetic proced ure, to be reported separately. The catechol type siderophores were checked for purity by thin layer chromatography on silica gel in 1 1 chloroform methanol and by measurement of the absorption intensity of the band in the near ultraviolet. 

Titrations were performed as above except that 2 ymoles of ferric chloride and 3 Pmoles of rhodotorulic acid were mixed and neutralized prior to introduction of 2 ymoles of catechol type siderophore dissolved in ethanol. The eq.tiivalents of standard alkali required to neutralize the solution were then noted. 

A titration vial was charged with 0.5 nil ethanol, 2 pmoles of catechol type siderophore in 0.2 ml ethanol, 2 ymoles of FeCl, in 0.1 ml methanol and 0.5 ml of water. The pH was brought to about 7 with standard 0.1 N NaOH and 0.2 ml containing 2 ymoles of competing catechol type siderophore was then added. If necessary, the pH was readjusted to 7. 

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... 

These new siderophores are identified by acronyms based upon CAM derived from catecholamide which results from substituting 2,3-dihydroxylbenzoyl or its derivatives (29) on a series of polyamines. The introduction of a carboxylic moiety at C-4 or a sulpho moiety at C-5 in the benzene ring extends the acronym to CAMC (29, R = -COOH, Ri = H) or CAMS (29, R = H, Ri = -S03H). The introduction of these two substituents leads to increased oxidation resistance of the catechol functions as well as increasing the hydrophilicity257. The participation of the hydroxy group at C-3 in chelation was established by preparation of an equivalent salicylamide siderophore227. Around 30 CAM-type siderophores have now been reported. 

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