Enterobactin receptor

Neilands JB, Ericson TJ, Rastetter WH (1981) Stereospecifity of the Ferric Enterobactin Receptor of Escherichia coli K-12. J Biol Chem 256 3831... 

Since the 1,2-HOPO chelators form neutral complexes while the catecholates form charged complexes, it is reasonable to assume that charged species are essential for the enterobactin receptor recognition. The lack of recognition by the ferrichrome analog may well be attributed to the bulky substituents on the hydroxamate moiety in agreement with early observations by Emery and Emery and others ... 

Dean, C.R. Poole, K. Expression of the ferric enterobactin receptor (PfeA) of Pseudomonas aeruginosa involvement of a two-component regulatory system. Mol. Microbiol., 8, 1095-1103 (1993)... 

The existence of ferrichrome and ferric enterobactin receptors in the outer membrane of enteric bacteria confirms the discovery, first reported for vitamin Bi2 (34), for a genuine transport role for this segment of the cell envelope. The properties of the four analogous systems known at the present time are shown in Table IV. 

The synthesis of the colicin la receptor is clearly derepressed at low iron (73, 96), but a specific siderophore has not been assigned to this large polypeptide constituent, which is programmed by the cir gene at 43 min on the chromosome map. FeuB is the specific locus for the colicin B-ferric enterobactin receptor (66). 

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. 

Domain II) adjacent to the catechol-binding subunits of enterobactin and synthetic analogs are required for recognition by the ferric-enterobactin receptor. In contrast, when a methyl group was attached to the top of the rhodium ME-CAM complex, essentially no recognition occurred. 

Klug CS, Eaton SS, Eaton GR, Feix JB. 1998. Ligand-induced conformational change in the ferric enterobactin receptor FepA as studied by site-directed spin labeling and time-domain ESR. Biochemistry 37(25) 9016-9023. 

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 transport system of Bacillus subtilis accommodates the Fe " complexes of enterobactin (A-configured), enanfio-D-enterobactin and of corynebactin (bacilli-bactin) (both A). Since only A complexes can be bound to the receptor a configurational change from A to A is induced. Only the natural ferri-L-siderophores can be degraded enzymatically (399, 408). 

Once the siderophore-iron complexes are inside the bacteria, the iron is released and utilized for vital cell functions. The iron-free hydroxamate siderophores are commonly re-excreted to bring in an additional iron load (Enterobactin is at least partially degraded by a cytoplasmic esterase This cycle is repeated until specific intracellular ferric uptake regulation proteins (Fur proteins) bind iron, and signal that the intracellular iron level is satisfactory, at -which point ne-w siderophore and siderophore-receptor biosynthesis are halted and the iron-uptake process stops. This intricate feedback mechanism allows a meticulous control over iron(III) uptake and accumulation against an unfavorable concentration gradient so as to maintain the intracellular iron(III) level within the required narrow window. Several excellent reviews concerning siderophore-iron transport mechanisms have been recently published i.3,i6, is,40,45,60-62 ... 

Siderophores. If a suitably high content of iron (e.g., 50 pM or more for E. coli) is maintained in the external medium, bacteria and other microorganisms have little problem with uptake of iron. However, when the external iron concentration is low, special compounds called siderophores are utilized to render the iron more soluble.7 11 For example, at iron concentrations below 2 pM, E. coli and other enterobacteria secrete large amounts of enterobactin (Fig. 16-1). The stable Fe3+-enterobactin complex is taken up by a transport system that involves receptors on the outer bacterial membrane.9 12 13 Siderophores from many bacteria have in common with enterobactin the presence of catechol (orftzo-dihydroxybenzene) groups... 

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. 

The receptor for Fem-enterobactin retains its affinity for the complex and for colicin B in vitro after extraction. The receptor has a dissociation constant of about 10 nmol dm-3 for Fem-enterobactin. The gene for synthesis of the receptor is FepA... 

Two binding sites are commonly found catecholate, as in enterobactin, and hydroxamate, the motif in desferrioxamine B. The resulting complex is targeted by a membrane-bound receptor and captured by the organism. The complex is transported across the cell membrane where the iron is reduced to iron(II), which has a lower affinity for the siderophore, and subsequently decomplexed. 

Figure 1. Schematic of the two iron transport systems of microorganisms. The high affinity system is comprised of specific carriers of ferric ion (siderophores) and their cognate membrane hound receptors. Both components of the system are regulated by iron repression through a mechanism which is still poorly understood. The high affinity system is invoked only when the available iron supply is limiting otherwise iron enters the cell via a nonspecific, low affinity uptake system. Ferri-chrome apparently delivers its iron by simple reduction. In contrasty the tricatechol siderophore enterobactin may require both reduction and ligand hydrolysis for release...

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