PfeA is an outer membrane TonB-dependant-transporter involved in Iron acquisition via the uptake of enterobactin (AKA enterochelin) secreted by bacteria in the Enterobacteriaceae family such as *Escherichia coli* and *Salmonella enterica*. ![[5MZS.png|250]] PDB 5MZS PfeA *pseudomonas aeruginosa* Extracellular loops: bottom ### Regulation The *pfeA* promotor contains a Fur box meaning that in high iron conditions *pfeA* is down regulated to avoid over-saturation of iron in the cell. [[Regulation by Fur]] Further to this ferri-enterobactin is also involved in gene regulation via a [[Two component signal transduction system]]. PfeS forms a homodimer and is the histidine kinase/ sensing component of the system, each monomer likely contains two membrane spanning domains. PfeR is the response regulator, when it is phosphorylated it is released and can go on to regulate and promote target genes (such as *pfeA* and *pfeE*). When Ferri-enterobactin is taken up via PfeA it binds to PfeS in the periplasm resulting in autophosphorylation of PfeS, which results in phosphorylation and release of PfeR on the cytoplasmic side of the membrane. ### Uptake of the ferri-siderophore complex PfeA has 4 large extracellular loops- L3, L4, L7 and L10 however it is seen that when the complex of Ferri-enterobactin interacts with PfeA it binds predominantly with loops L2, L3, L4 and L11 and each catechol ring forms distinct H-bonds. L4 undergoes a large conformational change mainly facilitated by the highly flexible nature of this loop. The internal polar void of this protein enlarges upon ligand binding and the TonB box becomes disordered, likely allowing for its interaction with TonB. the enlargement of the internal cavity allows for the internalisation of the ferri-siderophore complex resulting in the complex interacting with the plug domain which likely induces further conformational changes resulting in the passage of Ferri-enterobactin into the periplasm where it can then interact with PfeS. (Moynié et al., 2019) Further to ferri-enterobactin are azotochelin (di-catechol containing siderophore) and protochelin (tri-catechol containing siderophore) which interact with the extracellular loops differently but still result in the same conformational changes and disorderment of the TonB box allowing for their uptake. Although it has not yet been determined whether these siderophores interact with PfeS, since their uptake does result the discrete upregulation of PfeA specifically it is mildly indicative that they may :p **Release of iron form enterobactin** because of the high-affinity of enterobactin for ferric iron it is not released from enterobactin via the usual mechanism (reductive release) instead it is released via hydrolysis of enterobactin into 3 identical molecules; 2,3-dihydroxybenzoylserine. which is also the exact building block of enterobactin itself ( [[Siderophore#Enterobactin]]). This hydrolysis does not complete the release of iron from enterobactin and there is likely a protein involved in the reduction of Fe3+ to completely liberate iron that has yet to be identified. **References** - [Dean, C.R., Neshat, S. and Poole, K. (1996). PfeR, an enterobactin-responsive activator of ferric enterobactin receptor gene expression in Pseudomonas aeruginosa. _Journal of bacteriology_, 178(18), pp.5361–5369. doi:https://doi.org/10.1128/jb.178.18.5361-5369.1996.](https://journals.asm.org/doi/epdf/10.1128/jb.178.18.5361-5369.1996?src=getftr&utm_source=sciencedirect_contenthosting&getft_integrator=sciencedirect_contenthosting) - [Dean, C.R. and Poole, K. (1993). Expression of the ferric enterobactin receptor (PfeA) of Pseudomonas aeruginosa: involvement of a two-component regulatory system. _Molecular Microbiology_, 8(6), pp.1095–1103. doi:https://doi.org/10.1111/j.1365-2958.1993.tb01654.x.](https://doi.org/10.1111/j.1365-2958.1993.tb01654.x) - [Gasser, V., Baco, E., Olivier Cunrath, Pamela Saint August, Perraud, Q., Zill, N., Schleberger, C., Schmidt, A., Paulen, A., Bumann, D., Gaëtan L. A. Mislin and Schalk, I.J. (2016). Catechol siderophores repress the pyochelin pathway and activate the enterobactin pathway in _P__seudomonas aeruginosa_: an opportunity for siderophore–antibiotic conjugates development. _Environmental Microbiology_, 18(3), pp.819–832. doi:https://doi.org/10.1111/1462-2920.13199.](https://enviromicro-journals.onlinelibrary.wiley.com/doi/full/10.1111/1462-2920.13199?saml_referrer) - [Moynié, L., Milenkovic, S., Mislin, G.L.A., Gasser, V., Malloci, G., Baco, E., McCaughan, R.P., Page, M.G.P., Schalk, I.J., Ceccarelli, M. and Naismith, J.H. (2019). The complex of ferric-enterobactin with its transporter from Pseudomonas aeruginosa suggests a two-site model. _Nature Communications_, 10(1). doi:https://doi.org/10.1038/s41467-019-11508-y.](https://www.nature.com/articles/s41467-019-11508-y) - [Perraud, Q., Moynié, L., Gasser, V., Munier, M., Julien Godet, Françoise Hoegy, Yves Mély, Gaëtan. L. A. Mislin, Naismith, J.H. and Schalk, I.J. (2018). A Key Role for the Periplasmic PfeE Esterase in Iron Acquisition _via_ the Siderophore Enterobactin in _Pseudomonas aeruginosa_. _ACS Chemical Biology_, 13(9), pp.2603–2614. doi:https://doi.org/10.1021/acschembio.8b00543.](https://pubs.acs.org/doi/10.1021/acschembio.8b00543)