Potassium channel

In the field of cell biology, potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes. Furthermore potassium channels are found in most cell types and control a wide variety of cell functions.

Function
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. Potassium channnels may also be involved in maintaining vascular tone.

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
There are four major classes of potassium channels:
 * Calcium-activated potassium channel - open in response to the presence of calcium ions or other signalling molecules.
 * Inwardly rectifying potassium channel - passes current (positive charge) more easily in the inward direction (into the cell).
 * 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 that is nearly as fast as their diffusion through bulk water.
 * Voltage-gated potassium channel - are voltage-gated ion channels that open or close in response to changes in the transmembrane voltage.

The following table contains a comparison of the major classes of potassium channels with representative examples (for a complete list of channels within each class, see the respective class pages).

Structure


Potassium channels have a tetrameric structure in which four identical protein subunits associate to form a fourfold symmetric (C4) complex arranged around a central ion conducting pore (i.e., a homotetramer). Alternatively four related but not identical protein subunits may associate to form heterotetrameric complexes with pseudo C4 symmetry. All potassium channel subunits have a distinctive pore-loop structure that lines the top of the pore and is responsible for potassium selective permeability.

There are over 80 mammalian genes that encode potassium channel subunits. However potassium channels found in bacteria are amongst the most studied of ion channels, in terms of their molecular structure. Using X-ray crystallography, profound insights have been gained into how potassium ions pass through these channels and why (smaller) sodium ions do not. The 2003 Nobel Prize for Chemistry was awarded to Rod MacKinnon for his pioneering work in this area.

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) in the P loop from each subunit, which have their electro-negative carbonyl oxygen atoms aligned toward 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. Passage of sodium ions would be energetically unfavorable, since the strong interactions between the filter and pore helix would prevent the channel from collapsing to the smaller sodium ion size. The selectivity filter opens towards the extracellular solution, exposing four carbonyl oxygens in a glycine residue (Gly79 in KcsA). The next residue toward 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.

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, Sext 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 Sext 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.

Hydrophobic region
This region is used to neutralize the environment around the potassium ion so that it is not attracted to any charges. In turn, it speeds up the reaction.

Central cavity
A central pore, 10 Å wide, is located near the center of the transmembrane channel, where the energy barrier is highest for the transversing ion due to the hydrophobity of the channel wall. The water-filled cavity and the polar C-terminus of the pore helices ease the energetic barrier for the ion. Repulsion by preceding multiple potassium ions is thought to aid the throughput of the ions. The presence of the cavity can be understood intuitively as one of the channel's mechanisms for overcoming the dielectric barrier, or repulsion by the low-dielectric membrane, by keeping the K+ ion in a watery, high-dielectric environment.

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
See also G protein-coupled inwardly-rectifying potassium channel

Some types of potassium channels are activated by muscarinic receptors and these are called muscarinic potassium channels (IKACh). These channels are a heterotetramer composed of two GIRK1 and two GIRK4 subunits. Examples are potassium channels in the heart, which, when activated by parasympathetic signals through M2 muscarinic receptors, causes an outward current of potassium, which slows down the heart rate.