Enterobactin structure

Figure 3.3 Comparison of the FepA and FhuA crystal structures. A portion of the 13-barrel (in violet is removed to show the globular cork domain (in yellow) that inserts from the periplasm into the channel of the 11-barrel. FhuA is loaded with ferrichrome (iron is shown as a green ball) (Ferguson et ah, 1998 Locher et ah, 1998). The FepA crystal structure does not reveal Fe3+-enterobactin, but the FepA structure shown might be partially occupied by enterobactin (Buchanan et ah, 1999). 

The crystal structure of FepA, the outer membrane transporter of Fe3+-enterobactin, has also been determined (Figure 3.3), and has a structure similar to the FhuA structure (Buchanan et al, 1999). In the FepA structure, the TonB box (residues 12-18) can be clearly seen located inside the barrel and extending into the periplasm, as one would expect from its interaction with the periplasmic portion of TonB. 

Rings with rigid pendant arms . The interesting ligand tris(2,3-dihy-droxybenzoyl)l,5,9-triazacyclotridecane (102), was synthesized specifically to act as a reagent for Fe(in) (Weitl Raymond, 1979). It is structurally quite closely related to enterobactin (103), the natural molecule used by E. coli to transport Fe(m) through its cell walls. The protonation and complexation equilibria of Fe(m) with (102) have been... 

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. 

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. 

Figure 7.3 Structures of the siderophores enterobactin and ferrichrome. (From Andrews et al., 2003. Reproduced with permission from Blackwell Publishing Ltd.)...

The NMR structure of the 37 kDa PCP-TE didomain construct from the E. coli enterobactin NRPS synthetase provides a detailed picture of the functional interactions between these two domains. As described in Section 5.19.5, the enterobactin TE works with the adjacent PCP domain to catalyze a trimerization/cyclization of 2,3-dihydroxybenzyl serine to generate the siderophore natural product (see Figure 4(b)). NMR analysis... 

Bister B, Bischoff D, Nicholson GJ, Valdebenito M, Schneider K, Winkelmann G, Hantke K, Siissmuth RD (2004) The Structure of Salmochelins C-Glucosylated Enterobactins of... 

A series of enterobactin analogs has been prepared as structural analogs to Fe(III) complexes. These species have facile oxidation processes (Ef = —0.53 to —l.OOV versus Cp2Fe/DMF) and are based on three catecholato ligands joined to a central aromatic ring (Fig. 7) or nitrogen atom by amide bridges [52]. The parent [V(cat)3] (cat = 1,2-catecholato)... 

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

Figure 16-1 Structures of several siderophores and of their metal complexes. (A) Enterobactin of E. coli and other enteric bacteria 12 (B) parabactin (R = H) from Paracoccus denitrijkans and agrobactin (R = OH) from Agrobacterium tumefaciens 20 (C) ferrichrome 9 (D) pyochelin from Pseudomonas aeruginosa.21...

The best known example is enterobactin (otherwise called enterochelin), which is produced apparently by all enteric bacteria. It has three 2,3-dihydroxybenzoyl groups attached to a macrocyc-lic lactone derived from three residues of L-serine condensed head-to-tail. The structures of enterobactin and its iron complex are shown in Figure 45, which shows that the iron is bound by six phenolate oxygen atoms in an octahedral environment. Enterobactin has the highest known affinity for Fem, with log K = 52 at pH 7.4.1182 The iron(III) complex can exist as isomeric forms, which may be associated with selectivity in binding to the receptor site. 

In biochemical systems [Fe(enterobactin)]3- enters the bacterial cell through an outer membrane protein, the structure of the E. coll version of which has recently been determined.74 The catechetol functionalities are crucial to this transport but the trilactone ring can be replaced with many other... 

The siderophore enterobactin (enterochelin) (64) is a cyclic lactone of three N-(2,3-dihydroxybenzoyl) L-serine moieties produced by E. coli under iron stress. Enterobactin (64) was first isolated from iron-limited cultures of Salmonella typhimur-ium [83], E. coli [84], and Aerobacter aerogenes [84]. Structural analysis has confirmed that 64 chelates iron as a hexadentate ligand via the two hydroxyl groups on each catechol moiety (see Fig. 13) [85]. Of all the siderophores characterized to date, 64 has been shown to have the highest affinity for ferric iron, with a stability constant of 1052 M 1 [86, 87], which is remarkable, considering the affinity of EDTA for iron is 27 orders of magnitude lower. In mammals, serum albumin [88] and siderocalin [89, 90] bind the hydrophobic 64 which impedes siderophore-mediated transfer of iron to bacteria. Consequently, bacteria such as E. coli and... 

A comparison of siderophores that have been both totally biosynthesized and chemically synthesized reveals that the more structurally simple compounds (pyo-chelin, myxochelin) can be synthesized following a biomimetic route, whereas more complex molecules (yersiniabactin, enterobactin, vibriobactin) often tend to... 

It can be seen from molecular models that two diastereoisomers are possible for the ferric enterobactin complex, A-cis and A-cis. These are not mirror images because of the optical activity of the ligand. The similarity of the roles played by the ferrichromes and enterobactin lent additional speculative interest to the preferred absolute configuration of the iron complex (20). The structural studies of the tris catechol complexes (vide infra) and the spectroscopic properties of the chromic... 

Thus the vis-uv spectra of [Cr(cat)3]3" and [Cr( enterobactin)]3 are too dissimilar to allow detailed comparisons and confident prediction of structure based on such comparisons. However, there is a dramatically different situation found in comparing the CD spectra of [Cr(cat)3]3" and [Cr(enterobactin)]3, which are found to be substantially identical (Figure 10). This is because the interfering charge transfer band is not associated with the chiral center and hence does not contribute to the optical activity. 

The molecular structure of enterobactin has not as yet been established by diffraction techniques and, although coordination of ferric ion... 

The largest /f-bar re Is have been observed with the monomeric iron transporter proteins FhuA and FepA. The structure of FhuA was established independently by two groups (Locher et al., 1998 Ferguson et al., 1998). It is known with and without a ligated siderophore. The structure of the ferric enterobactin receptor FepA is homologous to that of FhuA showing identical topology and a similar transport mechanism (Buchanan etal., 1999). In both cases there are more than 700 residues assembled in two domains an N-terminal 150-residue domain is located inside a C-terminal 22-stranded (6-barrel with a shear number S = 24. 

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