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channels shut and open.png|thumb|Bacterial potassium channels shut (left, PDB code=1k4c) and open (right, 1lnq). They can sense voltage differences across membrane, then change conformationhttp://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 pores that span cell membranes. Potassium channels are found in most cells and control cell 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 duraction 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).
Types of potassium channels Some potassium channels are voltage-gated ion channels that open or close in response to changes in the 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 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. Potassium channel 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, 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. See also | ||||||||
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