|
FepA is an integral bacterial outer membrane porin protein, which is involved in the active transport of iron from the extracellular space, into the periplasm of Gram-negative bacteria. A Gram-negative bacterium secretes iron-binding proteins called siderophores, which bind strongly to ferric ions. The FepA receptor is present at the outer membrane and binds to ferric siderophores, and actively transport it into the cell. 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, FepA has been studied to determine the mechanism by which energy is brought to the outer membrane to drive the transport. It was found that the energy to drive the active transport originates from the proton motive force (electrochemical gradient) generated in the inner membrane complex TonB–ExbB–ExbD, and this force is relayed physically to FepA through the TonB subunit. == Structure == Using X-ray crystallography the structure of FepA was found to be a 724-residue 22-stranded β-barrel. The barrel contains loops that act as high-affinity and high-specificity ligand-binding sites outside the cell. The N-terminus forms a smaller barrel domain inside the hydrophilic barrel pore, effectively closing the pore; from studies of FhuA, a similar TonB-dependent outer membrane transporter, the interaction of the N-terminus domain to the inner walls of the pore is thought to be strengthened by nine salt-bridges and over 60 hydrogen bonds. The N-terminus also has a further 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 comprises a region called the TonB box, which includes at least a proline and glycine residue. The TonB box interacts with a proline-rich motif consisting of a conserved Gln160 residue and the C-terminus 48 residues. When they interact, the conformation of the N-terminal domain is changed so as to open the pore. In vivo crosslinking confirms that this interaction is physical. It is however energetically nonsensical to remove the whole of the N-terminal domain for translocation, because this requires the breakage of the salt bridges and numerous hydrogen bonds, and so it is assumed that the displace is only slight, just large enough for transport of FeEnt. The role of the N-terminus is revealed by using a deletion mutation of the N-terminal plug; the protein was still able to be inserted into the membrane, but acts as a non-selective pore for larger molecules, exhibited by increased permeability of the cell to maltotetraose, maltopentaose, ferrichrome, as well as several antibiotics including albomycin, vancomycin and bacitracin. However, this have to be treated with caution, as the conformation of the barrel may change in the absence of the N-terminal plug. Enterobactin is a cyclic tri-ester of 2,3-dihydroxybenzoylserine with a molecular mass of 719 Da. It binds ferric ions using six oxygens from three catechol groups, giving an overall charge of −3. Like the binding catechol, enterobactin is thought to also have a three-fold symmetry dissecting the metal centre. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「FepA」の詳細全文を読む スポンサード リンク
|