D. Research Applications:

2. Regioselectivity in Iron Uptake by E. Coli
Research by K. Raymond (University of California Berkeley)
b. Contrasting the ability of E. coli to recognize several synthetic analogs of [Δ-Fe(ent)]3-


The Raymond group synthesized and tested the ability of a number of different molecules to either inhibit the iron uptake by E. coli or be taken up by E. coli. Here we present a subset of the molecules tested that collectively illustrate that recognition is regioselective to the metal tris chelate face of the molecule.

These molecules are NOT recognized by the fepA receptor

These molecules are recognized by the fep A receptor


Proposed Structure for Δ-[Rh(III)Cat3]3-
Viewing Tools

A racemic mixture of the Δ and Λ forms DOES NOT inhibit uptake of [Δ-Fe(ent)]3- by the fep receptor
Reference: Ecker, D.J., Loomis, L.D., Cass, M.E.,
Raymond, K.N.; J. Am. Chem. Soc. Vol 110, 1988, 2457-2464. DOI
Conclusion: The metal tris chelate region alone is insufficient for recognition
This is the Proposed Structure for [Δ-Rh(III)DMB3]3-
Viewing Tools:

A racemic mixture of the Δ and Λ forms DOES inhibit uptake of [Δ-Fe(ent)]3- by the fep receptor
Reference: Ecker et al.
Conclusion: At least some portion of the amide group attached to the catechols is necessary for recognition

Racemic mixtures of the Δ and Λ forms of different Fe(III) complexes of substituted MECAM ligands (where R = neutral CH3 or charged SO3-) DO NOT inhibit uptake of [Δ-Fe(ent)]3- by the fep receptor and ARE NOT transported into the cytoplasm. Substitution at different positions on the catechol ring (o, m and p) consistently shut down inhibition or transport.
Conclusion: substituents on the metal-ligand tris chelate face block recognition by FepA
This is the Known Structure for Δ-[Fe(III)TRENCAM]3-
Reference/More information
Viewing Tools:

A racemic mixture of Λ and Δ forms DOES inhibit uptake of [Δ-Fe(ent)]3- by the fep receptor
and is transported into the cytoplasm. However does not promote growth in E. coli since it lacks an ester backbone that can be hydrolyzed by the esterase (Fes).
Reference: Thulasiraman et al.
Conclusion: Confirms that recognition by FepA occurs at the metal chelate region, however release of iron via a pathway that requires hydrolysis of the ligand backbone
The collected observations established that
  • Recognition by the fepA protein receptor was regioselective to the metal tris chelate region of the the molecules (which includes the three catechol binding subunits and some if not all of each amide linkage) and is highlighted for the naturally occurring Fe(ent)3-
  • Recognition by FepA is not selective for the chirality at the metal center
  • Hydrolysis by the Fes esterase is stereoselective for the L-Serine backbone
  • Further experiments using a similar siderophore SGC that has a longer linker arm to the catechols and a naturally occurring tri-L-serine backbone, produces only the Λ isomer when complexed with Fe(III). [Λ-Fe(SGC)]3- is hydrolyzed by the Fes esterase of Shigella flezneri. The synthetic mirror image [Δ-Fe(D-SGC)]3-] is not hydrolyzed by Fes
[Λ-Iron(III)enantio-enterobactin]3- is transported into the cytoplasm! (Reference: Thulasiraman et al)
but DOES NOT promote bacterial growth (References: Rastetter et al. and Thulasiraman et al.
Conclusion: The recognition by FepA is NOT Stereoselective, but hydrolysis by the Fes esterase is selective for the L-serine backbone
Return to Index

Resources developed by Marion E. Cass, Carleton College and updated in 2014. Computations and content done in consultation with Henry S. Rzepa, Imperial College, London