Biology:Potassium channel

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Short description: Ion channel that selectively passes K+
Potassium channel Kv1.2, structure in a membrane-like environment. Calculated hydrocarbon boundaries of the lipid bilayer are indicated by red and blue lines.

Potassium channels are the most widely distributed type of ion channel found in virtually all organisms.[1] They form potassium-selective pores that span cell membranes. Potassium channels are found in most cell types and control a wide variety of cell functions.[2][3]

Function

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).[4] 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 cardiac 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.

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).

Some toxins, such as dendrotoxin, are potent because they block potassium channels.[5]

Types

There are four major classes of potassium channels:

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).

For more examples of pharmacological modulators of potassium channels, see potassium channel blocker and potassium channel opener.

Potassium channel classes, function, and pharmacology.[6]
Class Subclasses Function Blockers Activators
Calcium-activated
6T & 1P
  • inhibition in response to rising intracellular calcium
Inwardly rectifying
2T & 1P
  • recycling and secretion of potassium in nephrons
  • final repolarization phase and stabilising the resting potential of the action potential in cardiac myocytes[16]
  • mediate the inhibitory effect of many GPCRs
  • ML-297 (VU0456810)[27]
  • ATP-sensitive (Kir6.x)
  • close when ATP is high to promote insulin secretion
Tandem pore domain
4T & 2P
Voltage-gated
6T & 1P

Structure

Top view of a potassium channel with potassium ions (purple) moving through the pore (in the center). (PDB: 1BL8​)

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,[55][56] profound insights have been gained into how potassium ions pass through these channels and why (smaller) sodium ions do not.[57] The 2003 Nobel Prize for Chemistry was awarded to Rod MacKinnon for his pioneering work in this area.[58]

Selectivity filter

Crystallographic structure of the bacterial KcsA potassium channel (PDB: 1K4C​).[59] In this figure, only two of the four subunits of the tetramer are displayed for the sake of clarity. The protein is displayed as a green cartoon diagram. In addition backbone carbonyl groups and threonine sidechain protein atoms (oxygen = red, carbon = green) are displayed. Finally potassium ions (occupying the S2 and S4 sites) and the oxygen atoms of water molecules (S1 and S3) are depicted as purple and red spheres respectively.

Potassium ion channels remove the hydration shell from the ion when it enters the selectivity filter. The selectivity filter is formed by a five residue sequence, TVGYG, termed the signature sequence, within each of the four subunits. This signature sequence is within a loop between the pore helix and TM2/6, historically termed the P-loop. This signature sequence is highly conserved, with the exception that a valine residue in prokaryotic potassium channels is often substituted with an isoleucine residue in eukaryotic channels. This sequence adopts a unique main chain structure, structurally analogous to a nest protein structural motif. The four sets of electronegative carbonyl oxygen atoms are aligned toward the center of the filter pore and form a square antiprism 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. Sodium ions, however, are too small to fill the space between the carbonyl oxygen atoms. Thus, it is energetically favorable for sodium ions to remain bound with water molecules in the extracellular space, rather than to pass through the potassium-selective ion pore.[60] This width appears to be maintained by hydrogen bonding and van der Waals forces within a sheet of aromatic amino acid residues surrounding the selectivity filter.[55][61] 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 center of the protein with the extracellular solution.[62]

Selectivity mechanism

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,[63] toy models of ion binding,[64] thermodynamic calculations,[65] topological considerations,[66][67] and structural differences[68] between selective and non-selective channels.

The mechanism for ion translocation in KcsA has been studied extensively by theoretical calculations and simulation.[62][69] The prediction of an ion conduction mechanism in which the two doubly occupied states (S1, S3) and (S2, S4) play an essential role has been affirmed by both techniques. Molecular dynamics (MD) simulations suggest the two extracellular states, Sext and S0, reflecting ions entering and leaving the filter, also are important actors in ion conduction.

Hydrophobic region

This region neutralizes 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.

Regulation

File:038-PotassiumChannels.tiff The flux of ions through the potassium channel pore is regulated by two related processes, termed gating and inactivation. Gating is the opening or closing of the channel in response to stimuli, while inactivation is the rapid cessation of current from an open potassium channel and the suppression of the channel's ability to resume conducting. While both processes serve to regulate channel conductance, each process may be mediated by a number of mechanisms.

Generally, gating is thought to be mediated by additional structural domains which sense stimuli and in turn open the channel pore. These domains include the RCK domains of BK channels,[70][71][72] and voltage sensor domains of voltage gated K+ channels. These domains are thought to respond to the stimuli by physically opening the intracellular gate of the pore domain, thereby allowing potassium ions to traverse the membrane. Some channels have multiple regulatory domains or accessory proteins, which can act to modulate the response to stimulus. While the mechanisms continue to be debated, there are known structures of a number of these regulatory domains, including RCK domains of prokaryotic[73][74][75] and eukaryotic[70][71][72] channels, pH gating domain of KcsA,[76] cyclic nucleotide gating domains,[77] and voltage gated potassium channels.[78][79]

N-type inactivation is typically the faster inactivation mechanism, and is termed the "ball and chain" model.[80] N-type inactivation involves interaction of the N-terminus of the channel, or an associated protein, which interacts with the pore domain and occludes the ion conduction pathway like a "ball". Alternatively, C-type inactivation is thought to occur within the selectivity filter itself, where structural changes within the filter render it non-conductive. There are a number of structural models of C-type inactivated K+ channel filters,[81][82][83] although the precise mechanism remains unclear.

Pharmacology

Blockers

Main page: Chemistry:Potassium channel blocker

Potassium channel blockers inhibit the flow of potassium ions through the channel. They either compete with potassium binding within the selectivity filter or bind outside the filter to occlude ion conduction. An example of one of these competitors is quaternary ammonium ions, which bind at the extracellular face[84][85] or central cavity of the channel.[86] For blocking from the central cavity quaternary ammonium ions are also known as open channel blockers, as binding classically requires the prior opening of the cytoplasmic gate.[87]

Barium ions can also block potassium channel currents,[88][89] by binding with high affinity within the selectivity filter.[90][91][92][93] This tight binding is thought to underlie barium toxicity by inhibiting potassium channel activity in excitable cells.

Medically potassium channel blockers, such as 4-aminopyridine and 3,4-diaminopyridine, have been investigated for the treatment of conditions such as multiple sclerosis.[49] Off target drug effects can lead to drug induced Long QT syndrome, a potentially life-threatening condition. This is most frequently due to action on the hERG potassium channel in the heart. Accordingly, all new drugs are preclinically tested for cardiac safety.

Activators

Main page: Chemistry:Potassium channel opener

Muscarinic potassium channel

Birth of an Idea (2007) by Julian Voss-Andreae. The sculpture was commissioned by Roderick MacKinnon based on the molecule's atomic coordinates that were determined by MacKinnon's group in 2001.

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.[94][95] 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.[96][97]

In fine art

Roderick MacKinnon commissioned Birth of an Idea, a 5-foot (1.5 m) tall sculpture based on the KcsA potassium channel.[98] The artwork contains a wire object representing the channel's interior with a blown glass object representing the main cavity of the channel structure.

See also

References

  1. ↑ "Ion channels and synaptic organization: analysis of the Drosophila genome". Neuron 26 (1): 35–43. April 2000. doi:10.1016/S0896-6273(00)81135-6. PMID 10798390. 
  2. ↑ Hille, Bertil (2001). "Chapter 5: Potassium Channels and Chloride Channels". Ion channels of excitable membranes. Sunderland, Mass: Sinauer. pp. 131–168. ISBN 978-0-87893-321-1. 
  3. ↑ "Chapter 6: Ion Channels". Principles of Neural Science (4th ed.). New York: McGraw-Hill. 2000. pp. 105–124. ISBN 978-0-8385-7701-1. 
  4. ↑ "Chapter 10. Potassium Versus Sodium Selectivity in Monovalent Ion Channel Selectivity Filters". The Alkali Metal Ions: Their Role in Life. Metal Ions in Life Sciences (Springer) 16: 325–347. 2016. doi:10.1007/978-3-319-21756-7_9. PMID 26860305. 
  5. ↑ indirectly cited from reference number 3,4,5,6 in "Purification and subunit structure of a putative K+-channel protein identified by its binding properties for dendrotoxin I". Proceedings of the National Academy of Sciences of the United States of America 85 (13): 4919–4923. July 1988. doi:10.1073/pnas.85.13.4919. PMID 2455300. Bibcode: 1988PNAS...85.4919R. 
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 6.13 Rang, HP (2015). Pharmacology (8 ed.). Edinburgh: Churchill Livingstone. p. 59. ISBN 978-0-443-07145-4. 
  7. ↑ "Electrostatic interaction between charybdotoxin and a tetrameric mutant of Shaker K(+) channels". Biophysical Journal 78 (5): 2382–2391. May 2000. doi:10.1016/S0006-3495(00)76782-8. PMID 10777734. Bibcode: 2000BpJ....78.2382T. 
  8. ↑ "A strongly interacting pair of residues on the contact surface of charybdotoxin and a Shaker K+ channel". Neuron 16 (1): 123–130. January 1996. doi:10.1016/S0896-6273(00)80029-X. PMID 8562075. 
  9. ↑ "Peptide toxins and small-molecule blockers of BK channels". Acta Pharmacologica Sinica 37 (1): 56–66. January 2016. doi:10.1038/aps.2015.139. PMID 26725735. 
  10. ↑ "Mode of action of iberiotoxin, a potent blocker of the large conductance Ca(2+)-activated K+ channel". Biophysical Journal 63 (2): 583–590. August 1992. doi:10.1016/S0006-3495(92)81630-2. PMID 1384740. Bibcode: 1992BpJ....63..583C. 
  11. ↑ "An apamin-sensitive Ca2+-activated K+ current in hippocampal pyramidal neurons". Proceedings of the National Academy of Sciences of the United States of America 96 (8): 4662–4667. April 1999. doi:10.1073/pnas.96.8.4662. PMID 10200319. Bibcode: 1999PNAS...96.4662S. 
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  14. ↑ 14.0 14.1 Patnaik, Pradyot (2003). Handbook of inorganic chemicals. McGraw-Hill. pp. 77–78. ISBN 978-0-07-049439-8. https://archive.org/details/Handbook_of_Inorganic_Chemistry_Patnaik. 
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  18. ↑ "Class III antiarrhythmic drug dronedarone inhibits cardiac inwardly rectifying Kir2.1 channels through binding at residue E224". Naunyn-Schmiedeberg's Archives of Pharmacology 387 (12): 1153–1161. December 2014. doi:10.1007/s00210-014-1045-6. PMID 25182566. 
  19. ↑ "Dual Mechanism for Inhibition of Inwardly Rectifying Kir2.x Channels by Quinidine Involving Direct Pore Block and PIP2-interference". The Journal of Pharmacology and Experimental Therapeutics 361 (2): 209–218. May 2017. doi:10.1124/jpet.116.238287. PMID 28188270. 
  20. ↑ "Flecainide increases Kir2.1 currents by interacting with cysteine 311, decreasing the polyamine-induced rectification". Proceedings of the National Academy of Sciences of the United States of America 107 (35): 15631–15636. August 2010. doi:10.1073/pnas.1004021107. PMID 20713726. 
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  28. ↑ "Glibenclamide, a blocker of K+(ATP) channels, shows antileishmanial activity in experimental murine cutaneous leishmaniasis". Antimicrobial Agents and Chemotherapy 50 (12): 4214–4216. December 2006. doi:10.1128/AAC.00617-06. PMID 17015627. 
  29. ↑ "Gliclazide produces high-affinity block of KATP channels in mouse isolated pancreatic beta cells but not rat heart or arterial smooth muscle cells". Diabetologia 44 (8): 1019–1025. August 2001. doi:10.1007/s001250100595. PMID 11484080. 
  30. ↑ 30.0 30.1 30.2 30.3 30.4 30.5 "Molecular background of leak K+ currents: two-pore domain potassium channels". Physiological Reviews 90 (2): 559–605. April 2010. doi:10.1152/physrev.00029.2009. PMID 20393194. http://repo.lib.semmelweis.hu//handle/123456789/8205. 
  31. ↑ 31.0 31.1 31.2 31.3 31.4 31.5 "Biophysical, pharmacological, and functional characteristics of cloned and native mammalian two-pore domain K+ channels". Cell Biochemistry and Biophysics 47 (2): 209–256. 2007. doi:10.1007/s12013-007-0007-8. PMID 17652773. 
  32. ↑ "A neuronal two P domain K+ channel stimulated by arachidonic acid and polyunsaturated fatty acids". The EMBO Journal 17 (12): 3297–3308. June 1998. doi:10.1093/emboj/17.12.3297. PMID 9628867. 
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  35. ↑ "The two-pore domain K+ channel, TRESK, is activated by the cytoplasmic calcium signal through calcineurin". The Journal of Biological Chemistry 279 (18): 18550–18558. April 2004. doi:10.1074/jbc.M312229200. PMID 14981085. 
  36. ↑ "Local anesthetic inhibition of baseline potassium channels with two pore domains in tandem". Anesthesiology 90 (4): 1092–1102. April 1999. doi:10.1097/00000542-199904000-00024. PMID 10201682. 
  37. ↑ 37.0 37.1 37.2 "Functional characterisation of human TASK-3, an acid-sensitive two-pore domain potassium channel". Neuropharmacology 40 (4): 551–559. March 2001. doi:10.1016/S0028-3908(00)00189-1. PMID 11249964. 
  38. ↑ "Amide local anesthetics potently inhibit the human tandem pore domain background K+ channel TASK-2 (KCNK5)". The Journal of Pharmacology and Experimental Therapeutics 306 (1): 84–92. July 2003. doi:10.1124/jpet.103.049809. PMID 12660311. 
  39. ↑ "Inhibition of human TREK-1 channels by bupivacaine". Anesthesia and Analgesia 96 (6): 1665–1673. June 2003. doi:10.1213/01.ANE.0000062524.90936.1F. PMID 12760993. 
  40. ↑ "TWIK-1, a ubiquitous human weakly inward rectifying K+ channel with a novel structure". The EMBO Journal 15 (5): 1004–1011. March 1996. doi:10.1002/j.1460-2075.1996.tb00437.x. PMID 8605869. 
  41. ↑ "TASK, a human background K+ channel to sense external pH variations near physiological pH". The EMBO Journal 16 (17): 5464–5471. September 1997. doi:10.1093/emboj/16.17.5464. PMID 9312005. 
  42. ↑ "Cloning and expression of a novel pH-sensitive two pore domain K+ channel from human kidney". The Journal of Biological Chemistry 273 (47): 30863–30869. November 1998. doi:10.1074/jbc.273.47.30863. PMID 9812978. 
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  45. ↑ 45.0 45.1 "Inhibition of the human two-pore domain potassium channel, TREK-1, by fluoxetine and its metabolite norfluoxetine". British Journal of Pharmacology 144 (6): 821–829. March 2005. doi:10.1038/sj.bjp.0706068. PMID 15685212. 
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  59. ↑ "Chemistry of ion coordination and hydration revealed by a K+ channel-Fab complex at 2.0 A resolution". Nature 414 (6859): 43–48. November 2001. doi:10.1038/35102009. PMID 11689936. Bibcode: 2001Natur.414...43Z. 
  60. ↑ Molecular Cell Biology (8th ed.). New York, NY: W. H. Freeman and Company. 2016. p. 499. ISBN 978-1-4641-8339-3. 
  61. ↑ "Protein interactions central to stabilizing the K+ channel selectivity filter in a four-sited configuration for selective K+ permeation". Proceedings of the National Academy of Sciences of the United States of America 108 (40): 16634–16639. October 2011. doi:10.1073/pnas.1111688108. PMID 21933962. Bibcode: 2011PNAS..10816634S. 
  62. ↑ 62.0 62.1 "A comparison between two prokaryotic potassium channels (KirBac1.1 and KcsA) in a molecular dynamics (MD) simulation study". Biophysical Chemistry 120 (1): 1–9. March 2006. doi:10.1016/j.bpc.2005.10.002. PMID 16253415. 
  63. ↑ "Importance of hydration and dynamics on the selectivity of the KcsA and NaK channels". The Journal of General Physiology 129 (2): 135–143. February 2007. doi:10.1085/jgp.200609633. PMID 17227917. 
  64. ↑ "Control of ion selectivity in potassium channels by electrostatic and dynamic properties of carbonyl ligands". Nature 431 (7010): 830–834. October 2004. doi:10.1038/nature02943. PMID 15483608. Bibcode: 2004Natur.431..830N. 
  65. ↑ "Tuning ion coordination architectures to enable selective partitioning". Biophysical Journal 93 (4): 1093–1099. August 2007. doi:10.1529/biophysj.107.107482. PMID 17513348. Bibcode: 2007BpJ....93.1093V. 
  66. ↑ "The predominant role of coordination number in potassium channel selectivity". Biophysical Journal 93 (8): 2635–2643. October 2007. doi:10.1529/biophysj.107.108167. PMID 17573427. Bibcode: 2007BpJ....93.2635T. 
  67. ↑ "Selectivity in K+ channels is due to topological control of the permeant ion's coordinated state". Proceedings of the National Academy of Sciences of the United States of America 104 (22): 9260–9265. May 2007. doi:10.1073/pnas.0700554104. PMID 17519335. Bibcode: 2007PNAS..104.9260B. 
  68. ↑ "Tuning the ion selectivity of tetrameric cation channels by changing the number of ion binding sites". Proceedings of the National Academy of Sciences of the United States of America 108 (2): 598–602. January 2011. doi:10.1073/pnas.1013636108. PMID 21187421. Bibcode: 2011PNAS..108..598D. 
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  70. ↑ 70.0 70.1 "Structure of the human BK channel Ca2+-activation apparatus at 3.0 A resolution". Science 329 (5988): 182–186. July 2010. doi:10.1126/science.1190414. PMID 20508092. Bibcode: 2010Sci...329..182Y. 
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  72. ↑ 72.0 72.1 "Structure of the RCK domain from the E. coli K+ channel and demonstration of its presence in the human BK channel". Neuron 29 (3): 593–601. March 2001. doi:10.1016/S0896-6273(01)00236-7. PMID 11301020. 
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