Sandbox Reserved 1066
Zn Transporter YiiP
This is a default text for your page Kyle Colston/Sandbox 1. Click above on edit this page to modify. Be careful with the < and > signs. You may include any references to papers as in: the use of JSmol in Proteopedia [1] or to the article describing Jmol [2] to the rescue. OrganismThis protein is found in E. coli StructureYiiP is a homodimer with transmembrane (TMD) and C-terminal (CTD) domains that are connected via a charge interlocking mechanism located on a flexible loop. There are 3 Zn2+ binding sites per unit of homodimer. Site A is located in the TMD, site C is located in the CTD, and site B is located at the junction of the domains join. Both TMD are composed of 6 helices, 4 of which (TM1,TM2,TM4,TM5) form a pore in which Zn2+ and H+ can reach binding Site A. Zn2+ binding at site C helps hold the CTD together and is thought to stabilize conformational changes in YiiP. Mechanism of TransportYiiP's ability to export Zn2+ from the cytoplasm is best described as an alternating access mechanism with Zn2+/H+ antiport. YiiP has 2 major structural conformations as shown by the crystallized structures 3H90 and 3J1Z (a YiiP homolog derived from Shewanella oneidensis). 3H90 shows YiiP in its outward-facing conformation and 3J1Z shows the YiiP homolog in an inward-facing conformation. When YiiP is saturated with Zn2+ it seems to favor the perplasmic/outward-facing conformation whereas when active sites are either empty or bound to H+ the inward facing conformation is favored. This drives the export of Zn2+ from the cytoplasm and enhances the coupling of the proton-motive force. Although YiiP exists as a homodimer both monomers can undergo conformation change independent of one other to produce the alternating access mechanism. Zn Induced Conformation ChangeConformation changes occur in the TMD and CTD, both of which are heavily influenced by the presence of Zn2+. Both of these conformation changes are focused around the charge interlocking mechanism that holds the dimer together. This is because a flexible loop that likes the CTD and the TMD which acts as a hinge for The conformation change directly involved with Zn2+/H+ antiport occurs in the TMD as helix pivoting controls what environment site A is available to. Conformation change occurs when the transmembrane helix pairs TM3-TM6 pivot around cation binding site. It is believed that the energy for TMD conformation change comes from energy of binding each substrate. Changing to the outward from the inward-facing conformation causes a shift in TM5 which disrupts the tetrahedral geometry of active site A. This in turn decreases binding affinity site A has for Zn2+ and causes Zn2+ to leave site A which then favors change back to inward-facing conformation. In contrast the main purpose of conformation change in the CTD is to stabilize the YiiP dimer and acts as a Zn2+ sensor. This is possible because of the flexible loop that links the TMD and the CTD. This loop harbors the charge interlock which serves as a hinge that allows movement of the CTD. Using FRET to measure the distance between the CTD of each monomer fluorescence quenching was observed as the concentration Zn2+ increased, which supports that idea that Zn2+ induces a stabilizing conformation change in the CTD.
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