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Enterobactin
IUPAC name	N,N',N''-((3S,7S,11S)-2,6,10- trioxo-1,5,9-trioxacyclododecane- 3,7,11-triyl)tris(2,3-dihydroxybenzamide)
Identifiers
CAS number
PubChem	34231
SMILES	C1C(C(=O)OCC(C(=O)OCC(C(=O)O1)NC(=O)C2=C(C(=CC=C2)O)O)NC(=O)C3=C(C(=CC=C3)O)O)NC(=O)C4=C(C(=CC=C4)O)O
ChemSpider ID	31543
Properties
Molecular formula	C30H27N3O15
Molar mass	669.54648 g/mol
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Enterobactin (also known as Enterochelin) is a high affinity siderophore that acquires iron for microbial systems. It is primarily found in gram-negative bacteria, such as Escherichia coli and Salmonella typhimurium. ¹

Enterobactin is the strongest siderophore known, binding to the ferric ion (Fe³⁺) with the affinity (K = 10⁵² M^-1).² This value is substantially larger than even some synthetic metal chelators, such as EDTA (K_f,Fe3+ ~ 10²⁵ M^-1). ^[3] Due to its high affinity, enterobactin is capable of chelating even in environments where the concentration of ferric ion is held very low, such as within living organisms. Pathogenic bacteria can steal iron from other living organisms using this mechanism, even though the concentration of iron is kept extremely low due to the toxicity of free iron.

Structure and Biosynthesis

Chorismic acid, an aromatic amino acid precursor, is converted to 2,3-dihydroxybenzoic acid (DHB) by a series of enzymes, EntA, EntB and EntC. An amide linkage of DHB to L-serine is then catalyzed by EntD, EntE, EntF and EntB. Three molecules of the DHB-Ser formed undergo intermolecular cyclization, yielding enterobactin. ^[5] Whereas a number of possible stereoisomers are possible due to the chirality of the serine residues, only the Δ-cis isomer is metabolically active. ³

Mechanism

Iron deficiency in bacterial cells triggers secretion of enterobactin into the extracellular environment, causing formation of an coordination complex "FeEnt" wherein ferric ion is chelated to the conjugate base of enterobactin. In Escherichia coli, FepA in the bacterial outer membrane then allows entrance of FeEnt to the bacterial periplasm. FepB,C,D and G all participate in transport of the FeEnt through the inner membrane by means of an ATP-binding cassette transporter.⁴

Due to the extreme iron binding affinity of enterobactin, it is necessary to cleave FeEnt with ferrienterobactin esterase to remove the iron. This degradation yields three 2,3-dihyroxybenzoyl-L-serine units. Reduction of the iron (Fe³⁺ to Fe²⁺) occurs in conjunction with this cleavage, but no FeEnt bacterial reductase enzyme has been identified, and the mechanism for this process is still unclear. ⁵

History

Enterochelin was discovered by the Gibson group, who named the siderophore "enterochelin." These initial studies established the structure and its relationship to 2,3-dihydroxybenzoic acid.⁶

References

1. ^ Dertz, Emily A., Jide Xu, Alain Stintzi, and Kenneth N. Raymond (2006). "Bacillibactin-Mediated Iron Transport in Bacillus Subtilis". J. Am. Chem. Soc. 128: 22–23. doi:10.1021/ja055898c. 
2. ^ Carrano, Carl J., and Kenneth N. Raymond (1979). "Ferric Ion Sequestering Agents. 2. Kinetics and Mechanism of Iron Removal From Transferrin by Enterobactin and Synthetic Tricatechols". J. Am. Chem. Soc. 101: 5401–5404. doi:10.1021/ja00512a047. 
3. ^ Walsh, Christopher T., Jun Liu, Frank Rusnak, and Masahiro Sakaitani (1990). "Molecular Studies on Enzymes in Chorismate Metabolism and the Enterobactin Biosynthetic Pathway". Chemical Reviews 90: 1105–1129. doi:10.1021/cr00105a003. 
4. ^ Raymond, Kenneth N., Emily A. Dertz, and Sanggoo S. Kim (2003). "Bioinorganic Chemistry Special Feature: Enterobactin: an Archetype for Microbial Iron Transport". Proc. Nat. Acad. Sci. 100: 3584–3588. doi:10.1073/pnas.0630018100. PMID 12655062. 
5. ^ Ward, Thomas R., Andreas Lutz, Serge P. Parel, Jurgen Eusling, Philipp Gutlich, Peter Buglyo, and Chris Orvig (1999). "An Iron-Based Molecular Redox Switch as a Model for Iron Release From Enterobactin Via the Salicylate Binding Mode". Inorg. Chem. 38: 5007–5017. doi:10.1021/ic990225e. 
6. ^ I. G. O'Brien, G. B. Cox, F. Gibson (1970). "Biologically active compounds containing 2,3-dihydroxybenzoic acid and serine formed by Escherichia coli". Biochim Biophys Acta 201: 453–60. 

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