13. Sodium Potassium Pump

It is a vital transmembrane ATPase found in animal cells. It moves sodium ions out of cells & potassium ions into cells against steep conc. gradients.

Active transport

Active transport protein

1. Pump generates conc. gradients between cells & their environment. This is important in the gut, where Na gradient aids glucose absorption. 2. Na-K pump moves 3+ ions out of cell for every 2 that are pumped in. A potential difference is created & is essential for the generation of the resting potential in nerve cells.

Digoxin from the foxglove - acts as potent toxins but also have medicinal uses at very low concentrations

1. High affinity for intracellular Na+ 2. High affinity for extracellular K+

Stage 1 - Na-K pump has high affinity for Na+ in one conformational state. - 3 Na+ binding sites exposed to cytosol by pump - 3 Na+ move in & bind to these sites Stage 2 - When 3 Na+ are attached, protein able to hydrolyse ATP molecule to ADP & phosphate. - Phosphate is not liberated into cytosol at this stage, but is bonded to part of protein. - ATP therefore powers the pump by transferring a phosphate group to the protein - This phosphorylation causes a conformational change (shape) to protein. Stage 3 - 2nd conformation has lower affinity for Na+ ions but it can only release Na+ ions to extracellular fluid, thus pumping 3 Na+ out of cell. Stage 4 - 2nd conformation has higher affinity for K+ ions so K+ from extracellular fluid attach to 2 K+ binding sites. - This triggers release of phosphate group from protein. Stage 5 - Dephosphorylation restores protein to its original conformation. Stage 6 - Original conformation has low affinity for K+ so it releases 2 K+ into cell. - Protein's affinity for Na+ is high again & cycle repeats.

They differ in affinity for sodium and potassium.