Voltage gated potassium channels

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Voltage-gated Potassium Channels are channels in a cell membrane that allow potassium ions to cross the plasma membrane. The channels are activated when the membrane is depolarised, but they activate at a slower rate than voltage gated sodium channels. Similarly, they have a closed state, an open state and an inactivated state. The channel inactivates automatically after a random period of being in the open state, and closes when the membrane is repolarised.

Structure of the channel

The channel protein is a tetrameric structure comprising 4 subunits, clustered around a central pore. They have negatively charged regions which exclude negatively charged ions by repulsion, and a selectivity filter of carbonyl oxygen molecules which are lined areound the pore at the exact distance required to select for the small potassium ion. Dense concentrations of negatively charged amino acids are located near the entrance and exit of the pore, and a pore helix carrying a negative charge is located on each constituent subunit. At the N-terminal of the subunit, a ball of about 20 amino acids is attached by a short length of unfolded polypeptide, and is free to move around.

Mechanism of action

The filter works via the principle of forcing any potassium ion to lose all of its bound water molecules in its hydration shell, thus forcing it to interact with the polar carbonyl oxygen molecules lining the pore. The spacing of the carbonyl oxygen is so precise that the potassium ion is literally pulled through, leaving the water molecules behind, but a smaller sodium ion would not have its associated water molecules removed and thus cannot benefit from the interaction of the carbonyl oxygen and is therefore excluded from passing through the pore. The removal of water molecules requires the expenditure of energy, and the interaction with oxygen molecules provides that energy by serving as substitute water molecules.

The channel has a suggested mechanism of operation where a 'ball' of 20 amino acids in the terminal N region of the protein move into and thus block the pore after a period of time. Each channel has 4 such balls, and is attached to the pore itself by a short chain of unfolded polypeptide[1].

References

  1. Alberts B et al, 2008, Molecular Biology of the Cell, 5th edition pages 671-678

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