Cells continually pump sodium ions out and potassium ions in, powered by ATP
Medicine for the Heart
Exploring the Structure
Sodium-Potassium Pump (PDB entry 2zxe)
The sodium-potassium pump (PDB entry 2zxe ) is a protein machine with many moving parts. The helices that run through the membrane contain the binding sites for the sodium ions and potassium ions, and the large lobes that stick into the cytoplasm contain the machinery for linking the cleavage of ATP to the pumping cycle. The typical cycle occurs in several steps. First, the pump binds ATP and three sodium ions from the cytoplasm. The ATP then phosphorylates the pump and it shifts in shape, creating an opening towards the outside of the cell. The sodium is released and two potassium ions are picked up. Finally, the phosphate is cleaved off and the pump shifts back, releasing the potassium inside the cell. The structure shown here has captured the pump in the middle of the cycle, when the pump has just picked up its payload of potassium ions. The two potassium ions (shown here in green) are surrounded on all sides by oxygen atoms from the protein. Click on the image above for an interactive JSmol view of this interaction.
Topics for Further Discussion
- The sodium-potassium pump is able to distinguish sodium ions from potassium ions. How might a protein distinguish between these two ions, or between other types of ions?
- The portion of the sodium-potassium pump that crosses the membrane is composed of a bundle of alpha helices. Many other membrane-bound proteins have similar bundles of alpha helices. Can you find other examples in the PDB, and why is this a particularly effective approach for building membrane-bound proteins?
October 2009, David Goodselldoi:10.2210/rcsb_pdb/mom_2009_10