Sandbox Reserved 1069: Difference between revisions

Binding site A is in the center of the transmembrane domain, attached and confined via residues from the TM2 and TM5 helices. The TM2 domain has <scene name='69/694236/Asp45/1'>Asp45</scene> and Asp49, and the TM5 has His153 and Asp157, which facilitate the binding and releasing of Zn²⁺ within the sites. The TM5 helix is significantly shorter than the other 5 helices around it, and this length forms a cavity in the membrane. In turn, this cavity is able to bind a Zn²⁺ ion. This site is the protein's active site, meaning that this is where the Zn²⁺ is able to attach and eventually exit the cell via proton transport. This particular site has an ideal tetrahedron among its residues which is preferred for Zn²⁺, thus making it the perfect active site for Zn²⁺ to bind.

It is important to note that the structure of this binding site is rigid because of the coordination of the Zn²⁺ between the four residues. This rigidity is indicative that any slight shift on either of the helices will cause a drastic readjustment of the coordination of Zn²⁺. In addition, there are no outer-shell constraints to hold the residues in place, which means that with a readjustment of the molecule, there is no energy being expended to bind or release another Zn²⁺ molecule. Therefore, the Zn²⁺ is able to rapidly release and a new Zn²⁺ can bind again with a simple reorientation or shift of the molecule. This rapid on off bind and release mechanism is the regulator of homeostatic levels of Zn²⁺ in the cell. This regulation is so rapid in fact, it is significantly faster than other Zn²⁺ exchange rate proteins- by several orders of magnitude.