Recognition and transport of ferric enterobactin in Escherichia coli

Abstract

The specificity of the outer membrane protein receptor for ferric enterobactin transport in Escherichia coli and the mechanism of enterobactin-mediated transport of ferric ions across the outer membrane have been studied. Transport kinetic and inhibition studies with ferric enterobactin and synthetic structural analogs have mapped the parts of the molecule important for receptor binding. The ferric complex of the synthetic structural analog of enterobactin, 1,3,5-N,N',N"-tris-(2,3-dihydroxybenzoyl)triaminomethylbenzene (MECAM), was transported with the same maximum velocity as was ferric enterobactin. A double-label transport assay with [59Fe, 3H]MECAM showed that the ligand and the metal are transported across the outer membrane at an identical rate. Under the growth conditions used, large fractions of the transported complexes were available for exchange across the outer membrane when a large excess of extracellular complex was added to the cell suspension; at least 60% of the internalized [59Felenterobactin exchanged with extracellular [55Fe]enterobactin.Internalized [59Fe, 3H1MECAM was released from the cell as the intact complex when either unlabeled Fe-MECAM or Fe-enterobactin was added extracellularly. The results suggest a mechanism of active transport of unmodified coordination complex across the outer membrane with possible accumulation in the periplasm.

Enterobactin Protonation and Iron Release: Structural Characterization of the Salicylate Coordination Shift in Ferric Enterobactin1

“…14,15 However, synthetic analogs of enterobactin, which are not susceptible to this specific hydrolysis, can be 5% as effective as enterobactin in delivering iron to the cell, and thus sufficient for growth promotion. 16,17 This result implies the existence of a secondary pathway for intracellular iron release. While ferric enterobactin at neutral pH cannot release iron by reduction, 18,19 protonation of ferric enterobactin makes reduction much easier.…”

Enterobactin: An archetype for microbial iron transport

“…1 Upper Left). The early studies of receptor binding and transport of FeEnt using a wide range of synthetic analogs showed that the coordinated catechol amide groups are essential and cannot be significantly changed without disrupting recognition, whereas the triserine macrocycle is not recognized and can be replaced with very different molecular scaffolds (34,61).…”

“…Iron release is through the action of the cytoplasmic esterase (32,33). However, synthetic analogs of enterobactin, which are not susceptible to hydrolysis, are Ϸ5% as effective in delivering iron (34), implying a secondary pathway of iron release. Unfortunately, perhaps because of previous errors, there continues to be confusion on this subject.…”

Recognition of Ferric Catecholates by FepA

“…Such concentrations, which may vary with diet, are insufficient for bacterial growth (34,36,47), but microorganisms capture iron from these environments (19) by the elaboration of high-affinity iron chelators (siderophores) (48), or by direct acquisition of the iron from eukaryotic proteins (19,37)). Certain gram-negative bacterial outer membrane (OM) proteins transport ferric siderophores (9,20,27,32,46,64), whereas others strip iron from transferrin and lactoferrin or use the heme within hemoglobin. These processes display specificity.…”