Sodium/Potassium Pump

Sections

NOTES
Full-Ltngth Text

NOTES

SODIUM/POTASSIUM PUMP

• Found in the membrane of all animal cells
• Active transport - uses ATP (around 30% of a cell's total ATP usage) breaking it down to ADP and phosphate
• Helps maintain membrane voltage (thought to contribute about 10% of total voltage)
• Maintains sodium and potassium concentration gradients
• Helps maintain cellular volute by regulating a cell's osmolarity
• Transports 3 sodium ions out of the cell and 2 potassium ions into the cell

The sodium and potassium ion gradients set up by the pump are required for numerous functions such as:

• Nerve cell action potentials
• Muscle contractions
• Glucose absorption by intestinal cells

SODIUM/POTASSIUM PUMP CYCLE

1. Intracellular sodium ions bind the protein

2. Protein becomes phosphorylated (phosphate added)

3. Conformational change in the protein due to the phosphorylation ejects the sodium ions to the now accessible extracellular space

4. Extracellular potassium binds to the protein

5. Protein is dephosphorylated (phosphate is removed)

6. Due to dephosphorylation, protein returns to original conformation and ejects the potassium ions intracellularly. The pump is now ready to start the cycle again at step 1.

Full-Ltngth Text

• Now we will look at a very specific example of active transport, the sodium/potassium pump.

• First, start a table to list some key features of the sodium/potassium pump.

• Denote that it is found in the membrane of all animal cells.

• Denote that its functions include:
  • Helps maintain membrane voltage. It is thought to contribute about 10 percent of the total voltage.
  • Maintain certain ion concentration gradients. Other transporters use this gradient as the driving force of their functions
  • Helps maintain cellular volume by regulating the osmolarity of cells.

Now let us look at the overall function of the sodium/potassium pump.

• Draw the phospholipid bilayer.

• Label the extracellular and cytosolic spaces.

• Indicate that the extracellular side is positively charged and the cytosolic side is negatively charged because there is a voltage across the cell membrane.

• Draw the sodium electrochemical gradient as positively charged molecules with a concentration gradient towards the cytoplasm.

• Draw the potassium electrochemical gradient as positively charged molecules with a concentration gradient towards the extracellular side.

• Now draw the sodium/potassium pump.
  • Because it is a transport protein, it spans both layers of the lipid bilayer.

• On the extracellular side, draw three sodium ions and indicate with an arrow that they are moved from the cytoplasmic side.

• On the cytoplasmic side, draw two potassium ions and indicate with an arrow that they are moved from the extracellular side.
  • This means one more positive charge leaves the cell than is gained, helping to create the slight negative charge on the cytoplasmic side of the membrane.

• Indicate that this movement against the electrochemical gradients requires the sodium/potassium pump to use ATP as an energy source, breaking it down to ADP and phosphate.
  • The use of ATP for active transport is known as primary active transport and it is thought that the sodium/potassium pump is responsible for about thirty percent of a cell's total ATP usage.

Now let us look more in depth into the working cycle of the sodium/potassium pump. For simplicity's sake we will use six discreet steps connected by arrows to illustrate the cycle, and we will only illustrate one ion moving in each direction. But remember, however, that three sodium ions are pumped out of the cell and two potassium ions are pumped into the cell.

• Draw the sodium/potassium pump in a cellular membrane with a channel to indicate that the binding site is accessible from the cytosolic side.

• Label the extracellular side and the cytosolic side.

• Label the binding site.
  • This is where the ions bind the protein.

• Draw a single sodium ion on the cytoplasmic side and indicate with an arrow that it moves to the binding site.

• Underneath the picture, indicate that this first step is where intracellular sodium binds to the protein.

• Now draw the sodium/potassium pump, this time with the sodium bound.

• Indicate that a molecule of ATP comes in and a phosphate is added to the sodium/potassium pump (aka phosphorylation).
  • This is the result of enzyme activity by the pump which produces a molecule of ADP (aka adenosine diphosphate).

• Indicate that this second step is where the protein becomes phosphorylated.

• Now draw the sodium/potassium pump where the binding site has changed shape and is accessible to the extracellular side of the membrane.

• Draw the sodium ion on the extracellular side and indicate with an arrow that it left the binding site.

• Indicate that during this third step the conformational change in the protein due to the phosphorylation during the previous step ejects the sodium ion to the now accessible extracellular space.

• Now draw the sodium/potassium pump.

• Draw an extracellular potassium ion and indicate with an arrow that it moves to the binding site.

• Indicate that during this fourth step, extracellular potassium binds to the protein.

• Now draw the sodium/potassium pump with the potassium bound.

• Draw the phosphate free of the protein and indicate with an arrow that the phosphate was removed from the protein (aka dephosphorylation).

• Indicate that the phosphate is removed from the protein during this fifth step.

• Finally, draw the sodium/potassium pump in the same conformation as the first step, with the binding site accessible to the cytoplasmic side of the membrane.

• Draw the potassium ion on the cytoplasmic side and indicate with an arrow that it left the binding site.

• Indicate that during this sixth step and because of the removal of the phosphate group in the previous step, the protein returns to its original conformation which causes the potassium to be ejected and allows for the cycle to start over.

• Finally, as a clinical correlation, denote that without the sodium/potassium pump, nothing that depends on the sodium or potassium gradients would function.
  • Denote that these include: nerve cell action potentials, muscle contractions, or glucose absorption by intestinal cells.