FepA

Many bacteria secrete small iron-binding molecules called siderophores, which bind strongly to ferric ions. FepA is an integral bacterial outer membrane porin protein, which is involved in the active transport of iron bound by the siderophore enterobactin from the extracellular space, into the periplasm of Gram-negative bacteria. FepA has also been shown to transport vitamin B12, and colicins B and D as well. This protein belongs to family of ligand-gated protein channels.

Because no energy is directly available to the outer membrane, the energy to drive the transport of ferric-enterobactin by FepA originates from the proton motive force (electrochemical gradient) generated by the inner membrane complex TonB–ExbB–ExbD. This force is relayed physically to FepA through direct interaction between FepA and TonB.

Using X-ray crystallography the structure of FepA was found to be a 724-residue 22-stranded β-barrel. The extracellular side of the barrel contains loops that act as high-affinity and high-specificity ligand-binding sites for ferric-enterobactin. The N-terminus forms a smaller plug domain inside the hydrophilic barrel, effectively closing the pore. Studies of FhuA, a similar TonB-dependent outer membrane transporter, show that the interaction of the N-terminus domain to the inner walls of the pore is strengthened by nine salt-bridges and over 60 hydrogen bonds. The N-terminus also has two extracellular loops in the pore, which are thought to aid in the signal transduction between ligand-binding and TonB-mediated transport, though the precise mechanism is not clear. Residues 12 to 18 of the N-terminus domain of FepA comprise a region called the TonB box, which includes at least a proline and glycine residue.

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