Enterobactin

Enterobactin (also known as Enterochelin) is a powerful siderophore used to acquire free iron for microbial systems. It is primarily found in gram-negative bacteria, such as Escherichia coli and Salmonella typhimurium.

Enterobactin is the strongest siderophore known, binding strongly to the ferric ion (Fe3+) with extremely high affinity (K = 1052 M-1). This value is substantially larger than even some synthetic metal chelators, such as EDTA (Kf,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] While a number of possible stereoisomers are possible due to the chirality of the serine residues, only the Δ-cis isomer is metabolically active.



Mechanism
Iron deficiency in a bacterial cell triggers secretion of enterobactin into the extracellular environment, causing formation of an adduct (FeEnt) as the ferric ion is chelated. 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.

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 which compose the molecule. It is known that reduction of the iron occurs (Fe3+ to Fe2+) in conjunction with this cleavage, but no FeEnt bacterial reductase enzyme has been identified, and the mechanism for this process is still unclear.