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.
Potassium channels function to conduct potassium ions down their electrochemical gradient, doing so both rapidly (up to the diffusion rate of K+ ions in bulk water) and selectively (excluding, most notably, sodium despite the sub-angstrom difference in ionic radius). Biologically, these channels act to set or reset the resting potential in many cells. In excitable cells, such as neurons, the delayed counterflow of potassium ions shapes the action potential.
By contributing to the regulation of the action potential duration in cardiac muscle, malfunction of potassium channels may cause life-threatening arrhythmias. Potassium channels may also be involved in maintaining vascular tone.
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.
- 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).
6T & 1P
2T & 1P
|Tandem pore domain
4T & 2P
6T & 1P
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.
Potassium ion channels remove the hydration shell from the ion when it enters the selectivity filter. The selectivity filter is formed by a highly conserved five residue sequence (TVGYG, termed the signature sequence) in the P loop of each subunit, which have their electro-negative carbonyl oxygen atoms aligned toward the centre of the filter pore and form a square 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, providing an energetically favorable route for de-solvation of the ions. 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 mechanism of potassium channel selectivity remains under continued debate. 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 neighboring sites occupied by ions. Proposals for the mechanism of selectivity have been made based on molecular dynamics simulations, toy models of ion binding, thermodynamic calculations, and structural differences between selective and non-selective channels.
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 (characterized 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.
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.
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.
These blockers work in a way to inhibit the flow of potassium ions through the KcsA channel. They either compete with potassium when binding to KcsA channel or they bind to the outside of the channel on an external binding site causing a conformational change, which closes the channel. An example of one of these competitors is quaternary ammonium ions, thought to be toxins that inhibit the binding of potassium. In a study it was found that Tyr82 residues provides a good external binding site for blockers, especially the quaternary ammonium ions.
Muscarinic 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, cause an outward current of potassium, which slows down the heart rate.
Potassium channels in fine art
Roderick MacKinnon commissioned Birth of an Idea, a 5-foot (1.5 m) tall sculpture based on the KcsA potassium channel. The artwork contains a wire object representing the channel's interior with a blown glass object representing the main cavity of the channel structure.
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