The
sodium-potassium pump transports sodium out of and potassium into the
cell in a repeating cycle of conformational (shape) changes. In each
cycle, three sodium ions exit the cell, while two potassium ions enter.
This process takes place in the following steps:
- To begin, the pump is open to the inside of the cell. In this form, the pump really likes to bind (has a high affinity for) sodium ions, and will take up three of them.
- When the sodium ions bind, they trigger the pump to hydrolyze (break down) ATP. One phosphate group from ATP is attached to the pump, which is then said to be phosphorylated. ADP is released as a by-product.
- Phosphorylation makes the pump change shape, re-orienting itself so it opens towards the extracellular space. In this conformation, the pump no longer likes to bind to sodium ions (has a low affinity for them), so the three sodium ions are released outside the cell.
- In its outward-facing form, the pump switches allegiances and now really likes to bind to (has a high affinity for) potassium ions. It will bind two of them, and this triggers removal of the phosphate group attached to the pump in step 2.
- With the phosphate group gone, the pump will change back to its original form, opening towards the interior of the cell.
- In its inward-facing shape, the pump loses its interest in (has a low affinity for) potassium ions, so the two potassium ions will be released into the cytoplasm. The pump is now back to where it was in step 1, and the cycle can begin again.
This
may seem like a complicated cycle, but it just involves the protein
going back and forth between two forms: an inward-facing form with high
affinity for sodium (and low affinity for potassium) and an
outward-facing form with high affinity for potassium (and low affinity
for sodium). The protein can be toggled back and forth between these
forms by the addition or removal of a phosphate group, which is in turn
controlled by the binding of the ions to be transported.
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