Enterobactins iron receptors

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. 

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...

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). 

While most workers report the outer membrane siderophore receptors to have molecular weights in the 75-95K range, some variation in the magnitude of these numbers may be attributed to the preparative and analytical methods as well as to the particular standards used. Since enterobactin will rapidly remove iron from ferrichrome, the transport of the latter must perforce be studied in mutants lacking the former. However, such mutants often display multiple lesions. Additionally, isogenic strains have seldom been used and variations in media and cultural conditions will further confound attempts to compare results reported from different laboratories. 

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. 

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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