Three paradoxes of ferric enterobactin uptake

Bacteria elaborate iron chelators that scavenge iron from the environment, including their human and animal hosts, and iron acquisition is a determinant of pathogenicity. One such iron chelate, the siderophore ferric enterobactin, enters Gram-negative bacteria through the FepA protein of the outer membrane. The ferric enterobactin transport process is a high-affinity, multi-specific, multi-component, energy dependent reaction, that is a paradigm of ligand-gated transport: FeEnt binding activates FepA to transport competency. On the basis of the FepA, FhuA, FecA and BtuB crystal structures, and in light of recent molecular biological, biochemical, and biophysical findings, this review considers the mechanism of ferric enterobactin uptake. The discussion focuses on three preeminent questions about the transport reaction: the function of the N-terminal globular domain that resides within the FepA channel, the mechanistic contributions of TonB to the activities of ligand-gated porins, and the energy dependence of metal transport reactions through the OM bilayer. Available data points to the idea that the N-terminal globular domains of these receptor proteins dynamically exit their pores during transport, creating a suction-force that pulls ligands through the surface loops into the periplasm. The functions of TonB and energy in these processes remain unknown.