Enterobactin synthesis

The exbB mutants (47) are derepressed in enterobactin synthesis and produce this siderophore in iron-containing media. The means whereby iron represses enterobactin synthesis is still obscure. Several years ago it was noted that growth of E. coli on low iron media led to changes in various fRNAs (77). In E. coli K-12 aromatic amino-acid synthesizing enzymes are also derepressed in low iron media, possibly because of the diversion of the chorismate pool to enterobactin (78). 

Weitl, F. L., Raymond, K. N. ibid. 101, 2728 (1979). Note in particular references for DCC mediated condensations of amino acids with unprotected catechols. According to this paper enterobactin synthesis (Ref. 98) was performed with protected catechols under the reaction conditions described... 

The first synthesis of enterobactin, a microbial chelator and transporter of environmental iron, was accomplished by the coupling of three protected L-serine units and macrocyclization by the double activation method. 

The ent-fes-fep gene cluster is necessary for the synthesis of enterobactin and transport of the iron loaded siderophore. The fes gene product was shown to be necessary for utilization of the siderophore-bound iron inside the cell. The protein has an esterase activity which cleaves the ester bonds of the cyclic 2,3-dihydroxybenzoylserine ester in enterobactin. However, the esterase activity of Fes does not seem to be important for iron mobilization since Fes is also necessary for the utilization of iron from enterobactin analogues which do not have ester bonds (Heidinger et ah, 1983). No reductase activity has been found in Fes (Brickman and McIntosh, 1992) or in any other protein encoded in the ent-fes-fep gene cluster. 

The importance of iron for a bacteria-like E. coli can be illustrated by fact that 14 genes alone are required for enterobactin-mediated iron uptake, including those for its synthesis, export, transport of the ferric-enterobactin back into the cell and iron release (Figure 3.16). In total, E. coli has at least 8 uptake systems for iron, encoded by some 50 genes. 

Similarly, iterative NRPSs operate in a linear fashion but utilize at least one domain or module multiple times for the synthesis of a single NRP product. Thus, peptides assembled by iterative synthetases contain short, repeating units of peptide building blocks. In such systems, the terminal PCP-TE (or infrequendy PCP-C) didomain is responsible for both condensation of the repeating peptide units and chain release from the assembly line. NRPs biosynthesized in this manner include enniatin, enterobactin, bacillibactin, " gramicidin and the depsi-peptides valinomycin and cereulide. Of these examples, condensation of the precursor peptides for both enterobactin and gramicidin S has been extensively studied and will be discussed in detail. 

Corey EJ, Bhattacharyya S (1977) Total Synthesis of Enterobactin, a Macrocyclic Iron Transporting Agent of Bacteria. Tetrahedron Lett 3919... 

Shanzer A, Libman J (1983) Total Synthesis of Enterobactin via an Organotin Template. J Qiem Soc Qiem Commun 846... 

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... 

The high affinity for oxidized iron makes the siderophores ideal candidates for chelation therapy where the body is becoming overwhelmed by iron(III) either through acute poisoning or conditions like haemochromatosis that can occur when patients receive frequent blood transfusions. While enterobactin would seem to be the primary choice it has two major drawbacks its synthesis is complicated and, although both isomers bind iron(III) to the same extent, only the L-isomer has activity in vivo. Consequently desferrioxamine B is the agent of choice. 

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). 

This cyclization has been used for a synthesis of enterobactin (5), a natural iron carrier (previous synthesis, 8, 215-216) by cyclization of a linear trimer of serine. Thus the lactone 3, derived from i.-serinc, cyclizes in the presence of 1 to the tricyclic lactitnc 4 in 23% yield. This product was converted into 5 by dctritylation and acylation. ... 

The synthesis of enantiomers of natural products is a field of increasing interest because such compounds are used in structural studies and as probes for the elucidation of biological processes, as e.g. exemplified by the synthesis of e f-enterobactin [134]. We have reported on the synthesis of (2iS)-2-P-L-glucopyranosyloxy-4-hydroxy-7-methoxy-2//-l,4-benzoxazin-3(4//)-one (e f-GDIMBOA) starting from L-glucose and DIMBOA [135] following the method described in Fig. (18) on principle. 

The total synthesis of protected enterobactin 10, developed by Shanzer and Libman [14 a], involves the use of organotin species as templates in the cyclooligomerization of ff-lactones to macrocyclic polylactones (Scheme 2) [14 b]. The [Bu2Sn(OCH2CH20)]2 acts like a homogeneous catalyst in this reaction. 

Scheme 2. The template-controlled total synthesis of the protected enterobactin 10, using optically active natural L-seiine.

There are two catecholate siderophores which may be chosen as model compounds for synthesis the cyclic enterobactin and the linear parabactin precursor N. N8-bis(2,3-dihydroxybenzoyl)spermidine. Both of these natural products are capable of the rapid removal of iron from transferrin, the human iron transport protein 89-90). The synthesis of these and other catecholate ligands routinely requires protection of the phenolic oxygens (for example, by methyl, benzyl or acetyl groups). Very few preparations of catechol-containing siderophores have appeared in which the unprotected 2,3-dihydroxybenzoyl group is used in the synthesis 91,92). 

---