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Potassium Ion Channel

Potassium ion channels are complex transmembrane proteins whose purpose is to enable the transfer of potassium ions across the lipid bilayer of the membrane.

The structure of the potassium ion channel family is usually highly conserved, and not unlike sodium ion channels a channel is usually comprised of 4 subunits, or domains, themselves built from 6 transmembrane helices, with the space between the 4 domains forming the actual pore.

Forty different versions of such domains - called Kv - have been identified so far in humans, usually categorized into 12 families. However, a potassium ion channel can be built out of a combination of different domains, not necessarily identical tetramers of the same subunit, which will impact the properties of that channel. For example, some specific combinations have been identified to trigger a fast deactivation of the ion channel or on the opposite a delayed deactivation, which will directly impact the repeated firing of action potentials across the axon membrane (Figure 1).

Voltage dependent activation kinetics for the potassium ion channel can be split into fast kinetics and slow kinetics, and into low voltage-activated and high voltage-activated. Fast kinetics are shown by potassium ion channels K v 1, K v 3, and K v 4. Slow kinetics are shown by potassium ion channels K v 7 and K v 2. In a low voltage-activated channel, step 1, the voltage increases from minus 90 to minus 30 millivolts, step 2, the voltage remains constant at minus 30 millivolts, step 3, the voltage decreases to minus 70 millivolts. K v 1, K v 4, and K v 7 are low voltage-activated channels. K v 1 has a rapid increase in current at step 1, remains constant, then at step 3 there is a rapid decrease. K v 4 has a rapid increase in current at step 1, then there is an exponential decrease. K v 7 has a gradual increase in current from step 1, then a steep decrease at step 3. In a high voltage-activated channel, step 4, the voltage increases from minus 70 to 0 millivolts, step 5, the voltage remains constant at 0 millivolts, step 6, the voltage decreases to minus 70 millivolts. K v 3 and K v 2 are high voltage-activated channels. K v 3 has a rapid increase in current at step 4, remains constant, then decreases rapidly at step 6. K v 2 has a gradual increase in current at step 4, remains constant, then has a rapid decrease at step 6.

Figure 1. Different combinations of domains around the pore will trigger different activation conditions and kinetics

Referred from:

Article
Ion channels