Potassium channel

In cell biology, potassium channels are the most common type of ion channel. They form potassium-selective pores that span cell membranes. Potassium channels are found in most cells and control cell function.^[1][2]

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Functions

In excitable cells such as neurons, they shape action potentials and set the resting membrane potential.

By contributing to the regulation of the action potential duration in cardiac muscle, malfunction of potassium channels may cause life-threatening arrhythmias.

They also regulate cellular processes such as the secretion of hormones (e.g., insulin release from beta-cells in the pancreas) so their malfunction can lead to diseases (such as diabetes).

Types

• Voltage-gated potassium channel - are voltage-gated ion channels that open or close in response to changes in the transmembrane voltage.
• Calcium-activated potassium channel - open in response to the presence of calcium ions or other signalling molecules.
• Inwardly rectifying potassium channel
• Tandem pore domain potassium channel - are constitutively open or possess high basal activation, such as the "resting potassium channels" or "leak channels" that set the negative membrane potential of neurons. When open, they allow potassium ions to cross the membrane at a rate which is nearly as fast as their diffusion through bulk water.

Structure

There are over 80 mammalian genes that encode potassium channel subunits. The pore-forming subunits of potassium channels have a homo- or heterotetrameric arrangement. Four subunits are arranged around a central pore. All potassium channel subunits have a distinctive pore-loop structure that lines the top of the pore and is responsible for potassium selectivity.

Potassium channels found in bacteria are amongst the most studied of ion channels, in terms of their molecular structure. Using X-ray crystallography,^[4][5] profound insights have been gained into how potassium ions pass through these channels and why (smaller) sodium ions do not (since sodium ions have greater charge density, they have a larger shell of water molecules surrounding them and thus are more bulky).^[6] The 2003 Nobel Prize for Chemistry was awarded to Rod MacKinnon for his pioneering work in this area.^[7]

Selectivity filter

Potassium ion channels remove the hydration shell from the ion when it enters the selectivity filter. The selectivity filter is formed by five residues (TVGYG-in prokaryotic species) from each subunit which have their electro-negative carbonyl oxygen atoms aligned towards the centre of the filter pore and form an anti-prism similar to a water solvating shell around each potassium binding site. The distance between the carbonyl oxygens and potassium ions in the binding sites of the selectivity filter is the same as between water oxygens in the first hydration shell and a potassium ion in water solution. The selectivity filter opens towards the extracellular solution, exposing four carbonyl oxygens in a glycine residue (Gly79 in KcsA). The next residue towards the extracellular side of the protein is the negatively charged Asp80 (KcsA). This residue together with the five filter residues form the pore that connects the water filled cavity in the centre of the protein with the extracellular solution.^[9]

The carbonyl oxygens are strongly electro-negative and cation attractive. The filter can accommodate potassium ions at 4 sites usually labelled S1 to S4 starting at the extracellular side. In addition one ion can bind in the cavity at a site called SC or one or more ions at the extracellular side at more or less well defined sites called S0 or Sext. Several different occupancies of these sites are possible. Since the X-ray structures are averages over many molecules, it is, however, not possible to deduce the actual occupancies directly from such a structure. In general, there is some disadvantage due to electrostatic repulsion to have two neighbouring sites occupied by ions. The mechanism for ion translocation in KcsA has been studied extensively by simulation techniques. A complete map of the free energies of the 2⁴=16 states (characterised by the occupancy of the S1, S2, S3 and S4 sites) has been calculated with molecular dynamics simulations resulting in the prediction of an ion conduction mechanism in which the two doubly occupied states (S1, S3) and (S2, S4) play an essential role. The two extracellular states, S_ext and S₀, were found in a better resolved structure of KcsA at high potassium concentration. In free energy calculations the entire ionic pathway from the cavity, through the four filter sites out to S₀ and S_ext was covered in MD simulations. The amino acids sequence of the selectivity filter of potassium ion channels is conserved with the exception that an isoleucine residue in eukaryotic potassium ion channels often is substituted with a valine residue in prokaryotic channels.^[9]

Blockers

Potassium channel blockers, such as 4-Aminopyridine and 3,4-Diaminopyridine, have been investigated for the treatment of conditions such as multiple sclerosis.

Muscarinic potassium channel

Some of the types of potassium channels are activated by muscarinic receptors, and these are called muscarinic potassium channels (K_ACh).

Examples are potassium channels in the heart, which, when activated by parasympathetic signals through M2 muscarinic receptors, causes an inward current of potassium which slows down the heart rate.

See also

• Sodium ion channel

References