Receptor ferric enterobactin

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. 

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

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. 

As seen in Table 7, colicins V and la require the tonB gene, but the receptors for these agents have not been correlated with any specific siderophore or other nutrient substance. The binding of ferric enterobactin has been defined as the biochemical function of the coli-cin B receptor (53,). Iron supply to the cell interferes with adsorption of colicin la, thus suggesting this receptor is designed for a siderophore, the specific nature of which is still unkown (73). 

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. 

The largest major outer membrane protein, designated protein a, is present in appreciable quantity in E. coli K12 only when the organism is grown at 37°C or above. Protein a is able to act as a protease catalysing the in vivo cleavage of the outer membrane receptor for ferric enterobactin (p. 94). An additional role for protein a in the regulation of capsular polysaccharide formation has also been proposed. 

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. 

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 ... 

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...

After Fe + siderophores have docked at the binding pocket of the outer-membrane receptors of E. colt, translocation into the periplasmic space is mediated by the TonB complex. Once released into the periplasm, siderophores are rapidly bound by the specific periplasmic binding proteins FhuD (hydroxamate siderophores), FepB (enterobactin), and FecB (ferric dicitrate).FhuD, for example, exhibits a broad substrate specificity for a variety of hydroxamate siderophores including ferrichrome, coprogen, aerobactin, ferrioxamine B, shizokinen, rhodotorulic acid, and the antibiotic albomycin. The dissociation constants of FhuD with these siderophores range from 0.3 to 5.4 X-ray structures of FhuD... 

All outer-membrane transporters (OMTs) involved in iron uptake are made up of a 22-stranded p—barrel, which is occluded by an independently folded mixed a P globular cork domain of around 160 amino acid residues. This is illustrated for a vitamin B12 receptor and the ferric siderophore receptors for citrate, enterobactin, ferrichrome, pyochelin, and pyoverdin from E. coli and P. aeruginosa in Fig. 7.6. The ferric siderophore sits on top of the cork domain, as can be seen in Fig. 7.7 for FecA. The binding of the ligand induces a conformational... 

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