Sandbox Reserved 1069: Difference between revisions

<scene name='69/694236/Bsa/2'>Binding site A</scene> is in the center of the transmembrane domain, attached and confined via residues from the TM2 and TM5 helices. The TM2 helix has <scene name='69/694236/Bsa/4'>Asp45 and Asp49</scene>, and the TM5 has <scene name='69/694236/Bsa/5'>His153 and Asp157</scene>, which facilitate the binding and releasing of Zn²⁺ within the sites. The <scene name='69/694236/Shorttm5/1'>TM5 helix</scene> is significantly shorter than the other 5 helices around it, and this length forms a cavity in the membrane. Site A is one of Yiip's active sites, where 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⁺ binding.

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 bind a new Zn²⁺ 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, which is significantly faster than other Zn²⁺ exchange rate proteins by several orders of magnitude.