
''Another, unrelated ion channeling process is part of ion implantation.'' Ion channels are present in the cell membranes that surround all cell (biology)s. By conducting and controlling the flow of ions, these pore-forming proteins help establish the small negative voltage that all cells possess at rest (see cell potential). == Basic features == An ion channel is an integral membrane protein or more typically an assembly of several proteins. Such "multi-protein subunit" assemblies usually involve a circular arrangement of identical or homologous proteins closely packed around a water-filled pore through the plane of the membrane or lipid bilayer. While large-pore channels permit the passage of ions more or less indiscriminately, the archetypal channel pore is just one or two atoms wide at its narrowest point, it conducts a specific species of ion, such as sodium or potassium, and conveys them through the membrane single file--nearly as fast as the ions move through free fluid. Access to the pore is governed by "gates," which may be opened or closed by chemical or electrical signals, or mechanical force, depending on the variety of channel. == Biological role == Because "voltage-gated" channels underlie the nerve impulse and because "transmitter-gated" channels mediate conduction across the synapses, channels are especially prominent components of the nervous system. Indeed, most of the offensive and defensive toxins that organisms have evolved for shutting down the nervous systems of predators and prey (e.g. the venoms produced by spiders, scorpions, snakes, fish, bees, sea snails and others) work by plugging ion channel pores. But ion channels figure in a wide variety of biological processes that involve rapid changes in cells. In the search for any drug, ion channels are a favorite target. === Diversity and activation === *Voltage-gated ion channel sense the transmembrane potential and open or close in response to action potential or action potential, respectively. Examples include the sodium and potassium channel voltage-gated channels of nerve and muscle, and the voltage-gated calcium channels that control neurotransmitter release in synapse. *Ligand-gated ion channel open in response to a specific ligand molecule on the external face of the membrane in which the channel resides. Examples include the Acetylcholine receptor, AMPA receptor and other neurotransmitter-gated channels. *Cyclic nucleotide-gated ion channel, ''Calcium-activated'' channels and others open in response to internal solutes and mediate cellular responses to second messengers. *Stretch-activated ion channel open or close in response to mechanical forces that arise from local stretching or compression of the membrane around them; for example when their cells swell or shrink. Such channels are believed to underlie touch sensation and the transduction of acoustic vibrations into the sensation of sound. *G-protein-gated ion channel open in response to G protein-activation via its receptor. *Inward-rectifier potassium ion channel allow potassium to flow into the cell in an inwardly rectifying manner. They are involved in important physiological processes such as the pacemaker activity in the heart, insulin release, and potassium uptake in glial cells. *Light-gated channels like channelrhodopsin are directly opened by the action of light Certain channels respond to multiple influences. For instance, the NMDA receptor is partially activated by interaction with its ligand, glutamate, but is also voltage-sensitive and only conducts when the membrane is depolarized. Some calcium-sensitive potassium channels respond to both calcium and depolarization, with an excess of one apparently being sufficient to overcome an absence of the other. == Detailed structure == Channels differ with respect to the ion they let pass (for example, Na+, K+, Cl−), the ways in which they may be regulated, the number of subunits of which they are composed and other aspects of structure. Channels belonging to the largest class, which includes the voltage-gated channels that underlie the nerve impulse, consists of four subunits with six transmembrane helix each. On activation, these helices move about and open the pore. Two of these six helices are separated by a loop that lines the pore and is the primary determinant of ion selectivity and conductance in this channel class and some others. The channel subunits of one such other class, for example, consist of just this "P" loop and two transmembrane helices. The determination of their molecular structure by Roderick MacKinnon using crystallography won a share of the 2003 Nobel Prize in Chemistry. Because of their small size and the difficulty of crystallizing integral membrane proteins for X-ray analysis, it is only very recently that scientists have been able to directly examine what channels "look like." Particularly in cases where the crystallography required removing channels from their membranes with detergent, many researchers regard images that have been obtained as tentative. An example is the long-awaited crystal structure of a voltage-gated potassium channel, which was reported in May 2003. One inevitable ambiguity about these structures relates to the strong evidence that channels change conformation as they operate (they open and close, for example), such that the structure in the crystal could represent any one of these operational states. Most of what researchers have deduced about channel operation so far they have established through electrophysiology, biochemistry, gene sequence comparison and mutagenesis. == History == The existence of ion channels was hypothesized by the British biophysicss Alan Hodgkin and Andrew Huxley as part of their Nobel Prize in Physiology or Medicine-winning theory of the nerve impulse, published in 1952. Channel's existence was confirmed in the 1970s with an electrophysiology known as the "patch clamp," which led to a Nobel Prize to Erwin Neher and Bert Sakmann, the technique's inventors. Hundreds if not thousands of researchers continue to pursue a more detailed understanding of how these enzymes work. ==See also== * transmembrane receptor * passive transport * active transport * Action potential ==External links== *[http://physrev.physiology.org/cgi/content/full/80/2/555 The Voltage Sensor in Voltage-Dependent Ion Channels] *[http://www.cellbio.wustl.edu/faculty/huettner/69.pdf X-ray crystal structure of a potassium channel] *[http://www.ionchannels.org Ion Channel, Biophysics and Electrophysiology Resources] membrane biologyBiochemistry
"common currency of biological energy, ATP" -- someone please expand (and link) this acronym! - What about non-gated ion channels? (Leak-channels), appreciate some expansion on that subject / User:Afx ==Ion channels as enzymes== The Classification of Membrane Transport Proteins is described here: http://www.chem.qmul.ac.uk/iubmb/mtp/intro.html Note that this classification work is performed by the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology. Here is the definition of “channel” from their glossary: “membrane protein (or an oligomeric cluster) involved in specific transport of ions or uncharged molecules down their chemical or electrochemical potential gradient. Most channels exist in two conformational states, open and closed. The opening can be accomplished (a) by a spreading electric field (potential-gated channels), (b) by binding a specific ligand (chemically gated channels), (c) by mechanical stress or strain (mechanically gated channels). When open, the specific site in the channel can transiently bind solutes from both sides of the membrane.” Note that being an enzyme is not part of the definition. You can search the membrane transport protein data base at http://tcdb.ucsd.edu/tcdb/ Put 1.A.1.10 into the TC# lookup box and you will be shown some major types of ion channels such as the Voltage-sensitive Na+ channel. 3.A.3 will direct you to the P-type ATPase (P-ATPase) Superfamily which includes the “sodium pump”(discussed further below). This class of membrane transport proteins (3.A.3) provide examples of enzymes because of their associated ATP-splitting (or forming) activities. In contrast with membrane transport proteins discussed above, enzymes have been classified in accordance with the recommendations of the Enzyme Commission of the International Union of Biochemistry (see http://www.chem.qmul.ac.uk/iupac/jcbn/index.html#6 for details) You can search the IUBMB Enzyme listings at http://www.chem.qmul.ac.uk/iubmb/enzyme/search.html First, note that some membrane transport proteins do have an associated enzyme activity. Try putting “ion” into the enzyme search engine (URL given above). Go down the resulting list to EC 3.6.3.7, one of the most famous ion transport proteins, the “sodium pump”. This ion transport protein is an enzyme because it splits ATP molecules and uses the chemical energy of the terminal phosphate bond of ATP to change the conformation of the pump protein so that the pumped ions can be moved against a concentration gradient. Try putting “channel” into the enzyme search engine. The first choice should be EC 3.6.3.49, another famous ion transport protein, the one that is defective in the disease Cystic Fibrosis. This ion transport protein is an enzyme because it also catalyses the typical ATPase reaction: ATP + H2O = ADP + phosphate. Note that you will not find proteins such as the Voltage-sensitive Na+ channel in the enzyme data base. Summary. Enzymologists like to limit the term “enzyme” to proteins and other macromolecules that catalyze chemical reactions that involve changes in specific chemical bonds. Some ion transport proteins are enzymes, but not all. In particular, most proteins that form transmembrane ion channels are not enzymes. There is nothing wrong with an analogy between ion channels and enzymes that points out how ion channels can provide a low energy path across a cell membrane for the transport of an ion, but there is no need to insist that ion channels are enzymes.User:68.109.166.14 03:56, 4 Dec 2004 (UTC) Why do these count as enzymes? They typically don't catalyze a chemical reaction, do they? User:AxelBoldt 01:24 Mar 11, 2003 (UTC) ;Ions diffuse across a pure lipid bilayer at a very low rate. Ion channels lower the activation energy barrier for their diffusion through the bilayer core and so accelerate their rate of crossing. Since diffusion falls under the heading of chemistry and since the acceleration is brought about in the same way (stearics, stabilization of the substrate, lowering of the activation energy etc) as in more stereotypical catalytic scenarios, many enzymologists and biophysicists count channels as enzymes, and many other scientists who haven't reflected on it would concede the point when pressed, even if they wouldn't want to start calling them enzymes themselves. User:168... 06:25 Mar 11, 2003 (UTC) ;;It took some searching on Google to find someone calling channels "enzymes," but here's a person: [http://depts.washington.edu/pbiopage/faculty/sgordon.html]User:168... 06:50 Mar 11, 2003 (UTC) Well done Axel, ion channels are not enzymes. This is really old hat, a hangover from the days when it was thought by some that electrical signalling was due to carriers or enzymes. Although similar in ways (as the tortuous argument above outlines) it is not useful to state identity between the two where there is only analogy -see below. I don't agree that most biophysicists would describe channels as enzymes. Enzymologists might, but anyway Google seems to disagree! This should be changed. One simple example of difference: Generally, each enzyme catalyses at most a few substrates at a time in it's catalytic cycle. Once an individual channel has opened, any number up to trillions of ions , which I think are identified above as the 'substrate' of the channel, can cross the membrane more easily than otherwise, before the channel closes again. Dependent, of course, on any chemical or electrical potential that may act on the ions. As Bertil Hille in *the* textbook on this subject* says: "Ion channels bear the same relation to electrical signalling in nerve, muscle and synapse as enzymes do to metabolism". ;;*'Ion Channels of Excitable Membranes' Bertil Hille (3rd ed., 2001, Sinauer Associates) User:Aplested 13:02, 22 Sep 2004 (UTC) == Question about external link == Is the newly added link to http://www.ionchannels.org useful? It seems to be a company website with ads and dead links to academic research labs that might provide information about ion channels...if they worked. User:Memenen 02:54, 14 Dec 2004 (UTC)
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Words begining with Ion_channel:
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Ion_channel
Ion_channels
Ion channel
''Another, unrelated ion channeling process is part of ion implantation.'' Ion channels are present in the cell membranes that surround all cell (biology)s. By conducting and controlling the flow of ions, these pore-forming proteins help establish the small negative voltage that all cells possess at rest (see cell potential). == Basic features == An ion channel is an integral membrane protein or more typically an assembly of several proteins. Such "multi-protein subunit" assemblies usually involve a circular arrangement of identical or homologous proteins closely packed around a water-filled pore through the plane of the membrane or lipid bilayer. While large-pore channels permit the passage of ions more or less indiscriminately, the archetypal channel pore is just one or two atoms wide at its narrowest point, it conducts a specific species of ion, such as sodium or potassium, and conveys them through the membrane single file--nearly as fast as the ions move through free fluid. Access to the pore is governed by "gates," which may be opened or closed by chemical or electrical signals, or mechanical force, depending on the variety of channel. == Biological role == Because "voltage-gated" channels underlie the nerve impulse and because "transmitter-gated" channels mediate conduction across the synapses, channels are especially prominent components of the nervous system. Indeed, most of the offensive and defensive toxins that organisms have evolved for shutting down the nervous systems of predators and prey (e.g. the venoms produced by spiders, scorpions, snakes, fish, bees, sea snails and others) work by plugging ion channel pores. But ion channels figure in a wide variety of biological processes that involve rapid changes in cells. In the search for any drug, ion channels are a favorite target. === Diversity and activation === *Voltage-gated ion channel sense the transmembrane potential and open or close in response to action potential or action potential, respectively. Examples include the sodium and potassium channel voltage-gated channels of nerve and muscle, and the voltage-gated calcium channels that control neurotransmitter release in synapse. *Ligand-gated ion channel open in response to a specific ligand molecule on the external face of the membrane in which the channel resides. Examples include the Acetylcholine receptor, AMPA receptor and other neurotransmitter-gated channels. *Cyclic nucleotide-gated ion channel, ''Calcium-activated'' channels and others open in response to internal solutes and mediate cellular responses to second messengers. *Stretch-activated ion channel open or close in response to mechanical forces that arise from local stretching or compression of the membrane around them; for example when their cells swell or shrink. Such channels are believed to underlie touch sensation and the transduction of acoustic vibrations into the sensation of sound. *G-protein-gated ion channel open in response to G protein-activation via its receptor. *Inward-rectifier potassium ion channel allow potassium to flow into the cell in an inwardly rectifying manner. They are involved in important physiological processes such as the pacemaker activity in the heart, insulin release, and potassium uptake in glial cells. *Light-gated channels like channelrhodopsin are directly opened by the action of light Certain channels respond to multiple influences. For instance, the NMDA receptor is partially activated by interaction with its ligand, glutamate, but is also voltage-sensitive and only conducts when the membrane is depolarized. Some calcium-sensitive potassium channels respond to both calcium and depolarization, with an excess of one apparently being sufficient to overcome an absence of the other. == Detailed structure == Channels differ with respect to the ion they let pass (for example, Na+, K+, Cl−), the ways in which they may be regulated, the number of subunits of which they are composed and other aspects of structure. Channels belonging to the largest class, which includes the voltage-gated channels that underlie the nerve impulse, consists of four subunits with six transmembrane helix each. On activation, these helices move about and open the pore. Two of these six helices are separated by a loop that lines the pore and is the primary determinant of ion selectivity and conductance in this channel class and some others. The channel subunits of one such other class, for example, consist of just this "P" loop and two transmembrane helices. The determination of their molecular structure by Roderick MacKinnon using crystallography won a share of the 2003 Nobel Prize in Chemistry. Because of their small size and the difficulty of crystallizing integral membrane proteins for X-ray analysis, it is only very recently that scientists have been able to directly examine what channels "look like." Particularly in cases where the crystallography required removing channels from their membranes with detergent, many researchers regard images that have been obtained as tentative. An example is the long-awaited crystal structure of a voltage-gated potassium channel, which was reported in May 2003. One inevitable ambiguity about these structures relates to the strong evidence that channels change conformation as they operate (they open and close, for example), such that the structure in the crystal could represent any one of these operational states. Most of what researchers have deduced about channel operation so far they have established through electrophysiology, biochemistry, gene sequence comparison and mutagenesis. == History == The existence of ion channels was hypothesized by the British biophysicss Alan Hodgkin and Andrew Huxley as part of their Nobel Prize in Physiology or Medicine-winning theory of the nerve impulse, published in 1952. Channel's existence was confirmed in the 1970s with an electrophysiology known as the "patch clamp," which led to a Nobel Prize to Erwin Neher and Bert Sakmann, the technique's inventors. Hundreds if not thousands of researchers continue to pursue a more detailed understanding of how these enzymes work. ==See also== * transmembrane receptor * passive transport * active transport * Action potential ==External links== *[http://physrev.physiology.org/cgi/content/full/80/2/555 The Voltage Sensor in Voltage-Dependent Ion Channels] *[http://www.cellbio.wustl.edu/faculty/huettner/69.pdf X-ray crystal structure of a potassium channel] *[http://www.ionchannels.org Ion Channel, Biophysics and Electrophysiology Resources] membrane biologyBiochemistry
Ion channel
"common currency of biological energy, ATP" -- someone please expand (and link) this acronym! - What about non-gated ion channels? (Leak-channels), appreciate some expansion on that subject / User:Afx ==Ion channels as enzymes== The Classification of Membrane Transport Proteins is described here: http://www.chem.qmul.ac.uk/iubmb/mtp/intro.html Note that this classification work is performed by the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology. Here is the definition of “channel” from their glossary: “membrane protein (or an oligomeric cluster) involved in specific transport of ions or uncharged molecules down their chemical or electrochemical potential gradient. Most channels exist in two conformational states, open and closed. The opening can be accomplished (a) by a spreading electric field (potential-gated channels), (b) by binding a specific ligand (chemically gated channels), (c) by mechanical stress or strain (mechanically gated channels). When open, the specific site in the channel can transiently bind solutes from both sides of the membrane.” Note that being an enzyme is not part of the definition. You can search the membrane transport protein data base at http://tcdb.ucsd.edu/tcdb/ Put 1.A.1.10 into the TC# lookup box and you will be shown some major types of ion channels such as the Voltage-sensitive Na+ channel. 3.A.3 will direct you to the P-type ATPase (P-ATPase) Superfamily which includes the “sodium pump”(discussed further below). This class of membrane transport proteins (3.A.3) provide examples of enzymes because of their associated ATP-splitting (or forming) activities. In contrast with membrane transport proteins discussed above, enzymes have been classified in accordance with the recommendations of the Enzyme Commission of the International Union of Biochemistry (see http://www.chem.qmul.ac.uk/iupac/jcbn/index.html#6 for details) You can search the IUBMB Enzyme listings at http://www.chem.qmul.ac.uk/iubmb/enzyme/search.html First, note that some membrane transport proteins do have an associated enzyme activity. Try putting “ion” into the enzyme search engine (URL given above). Go down the resulting list to EC 3.6.3.7, one of the most famous ion transport proteins, the “sodium pump”. This ion transport protein is an enzyme because it splits ATP molecules and uses the chemical energy of the terminal phosphate bond of ATP to change the conformation of the pump protein so that the pumped ions can be moved against a concentration gradient. Try putting “channel” into the enzyme search engine. The first choice should be EC 3.6.3.49, another famous ion transport protein, the one that is defective in the disease Cystic Fibrosis. This ion transport protein is an enzyme because it also catalyses the typical ATPase reaction: ATP + H2O = ADP + phosphate. Note that you will not find proteins such as the Voltage-sensitive Na+ channel in the enzyme data base. Summary. Enzymologists like to limit the term “enzyme” to proteins and other macromolecules that catalyze chemical reactions that involve changes in specific chemical bonds. Some ion transport proteins are enzymes, but not all. In particular, most proteins that form transmembrane ion channels are not enzymes. There is nothing wrong with an analogy between ion channels and enzymes that points out how ion channels can provide a low energy path across a cell membrane for the transport of an ion, but there is no need to insist that ion channels are enzymes.User:68.109.166.14 03:56, 4 Dec 2004 (UTC) Why do these count as enzymes? They typically don't catalyze a chemical reaction, do they? User:AxelBoldt 01:24 Mar 11, 2003 (UTC) ;Ions diffuse across a pure lipid bilayer at a very low rate. Ion channels lower the activation energy barrier for their diffusion through the bilayer core and so accelerate their rate of crossing. Since diffusion falls under the heading of chemistry and since the acceleration is brought about in the same way (stearics, stabilization of the substrate, lowering of the activation energy etc) as in more stereotypical catalytic scenarios, many enzymologists and biophysicists count channels as enzymes, and many other scientists who haven't reflected on it would concede the point when pressed, even if they wouldn't want to start calling them enzymes themselves. User:168... 06:25 Mar 11, 2003 (UTC) ;;It took some searching on Google to find someone calling channels "enzymes," but here's a person: [http://depts.washington.edu/pbiopage/faculty/sgordon.html]User:168... 06:50 Mar 11, 2003 (UTC) Well done Axel, ion channels are not enzymes. This is really old hat, a hangover from the days when it was thought by some that electrical signalling was due to carriers or enzymes. Although similar in ways (as the tortuous argument above outlines) it is not useful to state identity between the two where there is only analogy -see below. I don't agree that most biophysicists would describe channels as enzymes. Enzymologists might, but anyway Google seems to disagree! This should be changed. One simple example of difference: Generally, each enzyme catalyses at most a few substrates at a time in it's catalytic cycle. Once an individual channel has opened, any number up to trillions of ions , which I think are identified above as the 'substrate' of the channel, can cross the membrane more easily than otherwise, before the channel closes again. Dependent, of course, on any chemical or electrical potential that may act on the ions. As Bertil Hille in *the* textbook on this subject* says: "Ion channels bear the same relation to electrical signalling in nerve, muscle and synapse as enzymes do to metabolism". ;;*'Ion Channels of Excitable Membranes' Bertil Hille (3rd ed., 2001, Sinauer Associates) User:Aplested 13:02, 22 Sep 2004 (UTC) == Question about external link == Is the newly added link to http://www.ionchannels.org useful? It seems to be a company website with ads and dead links to academic research labs that might provide information about ion channels...if they worked. User:Memenen 02:54, 14 Dec 2004 (UTC)
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