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.