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{{BioPsy}}
 
{{BioPsy}}
[[Image:Potassium channels shut and open.png|thumb|400px|Bacterial potassium channels shut (left, PDB code=1k4c) and open (right, 1lnq). They can sense voltage differences across membrane, then change conformation
+
[[Image:Potassium channel1.png|thumb|300px|Top view of purple potassium ions moving through potassium channel (PDB code = 1BL8)]]
  +
  +
[[Image:Potassium channels shut and open.png|thumb|300px|Bacterial potassium channels shut (left, PDB code=1k4c) and open (right, 1lnq). They can sense voltage differences across membrane, then change conformation
 
[http://www.pdb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb38_1.html more details...]]]
 
[http://www.pdb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb38_1.html more details...]]]
In [[cell biology]], '''potassium channels''' are the most common type of [[ion channel]]. They form [[potassium]]-selective [[pore]]s that span [[cell membrane]]s. Potassium channels are found in most [[cell (biology)|cells]] and control cell function. In excitable cells such as [[neuron]]s, they shape [[action potential]]s and set the [[resting potential|resting membrane potential]]. By contributing to the regulation of the [[Cardiac action potential|action potential]] duration in [[cardiac muscle]], malfunction of potassium channels may cause life-theatening [[Cardiac arrhythmia|arrhythmias]]. They also regulate cellular processes such as the secretion of [[hormones]] (e.g. [[insulin]] release from the [[Beta cell|beta-cells]] in the [[pancreas]]) so their malfunction can lead to diseases (such as [[Diabetes mellitus type 2|diabetes]]).
+
In [[cell biology]], '''potassium channels''' are the most common type of [[ion channel]]. They form [[potassium]]-selective [[pore]]s that span [[cell membrane]]s. Potassium channels are found in most [[cell (biology)|cells]] and control cell function.
   
==Types of potassium channels==
+
==Functions==
  +
In excitable cells such as [[neuron]]s, they shape [[action potential]]s and set the [[resting potential|resting membrane potential]].
   
Some [[potassium in biology|potassium]] channels are [[voltage-gated ion channel]]s that open or close in response to changes in the [[membrane potential|transmembrane]] [[voltage]]. They can also open in response to the presence of [[calcium]] ions or other signalling molecules. Others are constitutively open or possess high basal activation, such as the [[resting ion channel|resting potassium 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]].
+
By contributing to the regulation of the [[Cardiac action potential|action potential]] duration in [[cardiac muscle]], malfunction of potassium channels may cause life-theatening [[Cardiac arrhythmia|arrhythmias]].
   
* [[Voltage-gated potassium channel]]
+
They also regulate cellular processes such as the secretion of [[hormones]] (e.g. [[insulin]] release from the [[Beta cell|beta-cells]] in the [[pancreas]]) so their malfunction can lead to diseases (such as [[Diabetes mellitus type 2|diabetes]]).
** [[HERG]]
+
** [[KvLQT1]]
+
==Types==
* [[Calcium-activated potassium channel]]
+
  +
* [[Voltage-gated potassium channel]] - are [[voltage-gated ion channel]]s that open or close in response to changes in the [[membrane potential|transmembrane]] [[voltage]]. Examples include:
  +
** [[hERG]] (K<sub>v</sub>11.1)
  +
** [[KvLQT1]] (K<sub>v</sub>7.1)
  +
* [[Calcium-activated potassium channel]] - open in response to the presence of [[calcium]] ions or other signalling molecules.
 
** [[BK channel]]
 
** [[BK channel]]
  +
** [[SK channel]]
 
* [[Inward-rectifier potassium ion channel|Inwardly rectifying potassium channel]]
 
* [[Inward-rectifier potassium ion channel|Inwardly rectifying potassium channel]]
* [[Tandem pore domain 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]].
   
==Potassium channel structure==
+
==Structure==
   
 
There are over 80 [[mammalian]] [[genes]] that encode potassium channel [[subunit]]s. The pore-forming subunits of potassium channels have a homo- or hetero[[tetramer]]ic 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.
 
There are over 80 [[mammalian]] [[genes]] that encode potassium channel [[subunit]]s. The pore-forming subunits of potassium channels have a homo- or hetero[[tetramer]]ic 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 [[bacterium|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 (since sodium ions have greater [[charge density]], they have a larger shell of [[water]] [[molecule]]s surrounding them and thus are more bulky). The [[2003]] [[Nobel Prize for Chemistry]] was awarded to [[Roderick MacKinnon|Rod MacKinnon]] for his pioneering work on this subject.
+
Potassium channels found in [[bacterium|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 (since sodium ions have greater [[charge density]], they have a larger shell of [[water]] [[molecule]]s surrounding them and thus are more bulky). The [[2003]] [[Nobel Prize for Chemistry]] was awarded to [[Roderick MacKinnon|Rod MacKinnon]] for his pioneering work on this subject.
  +
  +
==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 procaryotic 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 form together with the five
  +
filter residues 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.
  +
==Blockers==
  +
Potassium channel blockers, such as [[4-Aminopyridine]] and [[3,4-Diaminopyridine]], have been investigated for the treatment of conditions such as [[multiple sclerosis]].
   
 
==See also==
 
==See also==
Line 26: Line 27:
   
 
==External links==
 
==External links==
* [http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v423/n6935/full/423021a_fs.html Potassium channels - Life's Transistors]
+
* [http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v423/n6935/full/423021a_fs.html Potassium channels - Life's Transistors] at [[Nature (journal)|Nature]]
 
* {{MeshName|Potassium+Channels}}
 
* {{MeshName|Potassium+Channels}}
* [http://www.neuro.wustl.edu/neuromuscular/mother/chan.html#k Overview at wustl.edu]
+
* [http://www.neuro.wustl.edu/neuromuscular/mother/chan.html#k Overview] at [[Washington University in St. Louis]]
*[http://opm.phar.umich.edu/families.php?superfamily=8 Spatial positions of potassium channels in membranes]
+
* {{UMichOPM|families|superfamily|8}}
   
 
== References ==
 
== References ==
  +
* Hellgren M, Sandberg L, Edholm O. A comparison between two prokaryotic potassium channels (KirBac1.1 and KcsA) in
  +
a molecular dynamics (MD) simulation study. Biophys Chem. 2006 Mar 1;120(1):1-9. Epub 2005 Oct 25.
 
* [[Eric R. Kandel|Kandel ER]], Schwartz JH, Jessell TM. ''[[Principles of Neural Science]]'', 4th ed. McGraw-Hill, New York (2000). ISBN 0-8385-7701-6
 
* [[Eric R. Kandel|Kandel ER]], Schwartz JH, Jessell TM. ''[[Principles of Neural Science]]'', 4th ed. McGraw-Hill, New York (2000). ISBN 0-8385-7701-6
 
* Bertil Hille. Ion Channels of Excitable Membranes, 3rd Edition. Sinauer Associates, Sunderland, MA (2001). ISBN 0-87893-321-2.
 
* Bertil Hille. Ion Channels of Excitable Membranes, 3rd Edition. Sinauer Associates, Sunderland, MA (2001). ISBN 0-87893-321-2.
+
* Nobel Prize in Chemistry 2003 [[http://nobelprize.org/nobel_prizes/chemistry/laureates/2003/]]
 
{{Ion channels}}
 
{{Ion channels}}
  +
 
[[Category:Ion channels]]
 
[[Category:Ion channels]]
 
[[Category:Electrophysiology]]
 
[[Category:Electrophysiology]]
 
[[Category:Integral membrane proteins]]
 
[[Category:Integral membrane proteins]]
+
[[Category:Potassium]]
[[de:Kaliumkanal]]
+
:de:Kaliumkanal
 
 
{{enWP|Potassium channel}}
 
{{enWP|Potassium channel}}

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Potassium channel1
Top view of purple potassium ions moving through potassium channel (PDB code = 1BL8)
Dr Joe KiffAdded by Dr Joe Kiff
Potassium channels shut and open
Bacterial potassium channels shut (left, PDB code=1k4c) and open (right, 1lnq). They can sense voltage differences across membrane, then change conformation more details...
Dr Joe KiffAdded by Dr Joe Kiff

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.

FunctionsEdit

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-theatening arrhythmias.

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

TypesEdit

StructureEdit

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, 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). The 2003 Nobel Prize for Chemistry was awarded to Rod MacKinnon for his pioneering work on this subject.

Selectivity filterEdit

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 procaryotic 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 form together with the five filter residues 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.

BlockersEdit

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

See alsoEdit

External linksEdit


References Edit

  • Hellgren M, Sandberg L, Edholm O. A comparison between two prokaryotic potassium channels (KirBac1.1 and KcsA) in

a molecular dynamics (MD) simulation study. Biophys Chem. 2006 Mar 1;120(1):1-9. Epub 2005 Oct 25.

de:Kaliumkanal
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