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= Voltage-gated ion channel = From Wikipedia, the free encyclopedia

Voltage-gated ion channels are a class of transmembrane protein ion channels that are activated by changes in electrical membrane potential near the channel. The electrical membrane potential, or the difference in electrical charge on either side of a cell membrane, causes conformational changes in the proteins that allow ions to move between either side of the membrane. Cell membranes are generally impermeable to ions, thus they must diffuse through the membrane through transmembrane protein channels. These types of ion channels play a key role in excitable cells such as neurons by contributing to the generation and propagation of nerve impulses. Found along the axon and at the synapse, voltage-gated ion channels directionally propagate electrical signals. Voltage-gated ion-channels are ion-specific, and channels specific to sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-) ions have been identified. The opening and closing of the channels are triggered by the relative changes of concentration between two sides of the membrane.

Contents
[hide]
 * 1Structure
 * 2Mechanism
 * 3References
 * 4See also
 * 5External links

Structure[edit source | edit]
Voltage-gated ion channels are generally composed of several subunits arranged in such a way that there is a central pore through which ions can travel down their electrochemical gradients. The channels tend to be ion-specific, although similarly sized and charged ions may sometimes travel through them. The functionality of voltage-gated ion channels is attributed to its three main discrete units: the voltage sensor, the pore or conducting pathway, and the gate.[3] Na+, K+, and Ca2+ channels are composed of four identical transmembrane subunits arranged around a central pore.[4] The membrane-spanning segments, designated S1-S6, all take the form of alpha helices with specialized functions. The fifth and sixth transmembrane segments (S5 and S6) and pore loop serve the principle role of ion conduction, comprising the gate and pore of the channel, while S1-S4 serve as the voltage-sensing region.[3]

Mechanism[edit source | edit]
Crystallographic structural studies of a potassium channels have shown that, when a potential difference is introduced over the membrane, the associated electric field induces a conformational change in the potassium channel. The conformational change distorts the shape of the channel proteins sufficiently such that the cavity, or channel, opens to allow influx or efflux to occur across the membrane. This movement of ions down their concentration gradients subsequently generates an electric currentsufficient to depolarize the cell membrane.

Voltage-gated sodium channels and calcium channels are made up of a single polypeptide with four homologous domains. Each domain contains 6 membrane spanning alpha helices. One of these helices, S4, is the voltage sensing helix.[5] The S4 segment contains many positive charges such that a high positive charge outside the cell repels the helix, keeping the channel in its closed state.

In general, the voltage sensing portion of the ion channel is responsible for the detection of changes in transmembrane potential that triggers the opening or closing of the channel. The S1-4 alpha helices are thought to serve this role. In potassium and sodium channels, voltage-sensing S4 helices contain positively-charged lysine or arginine residues in repeated motifs. In its resting state, half of each S4 helix is in contact with the cell cytosol. Upon depolarization, the positively-charged residues on the S4 domains are oriented toward the exoplasmic surface of the membrane. It is thought that the first 4 arginines account for the gating current, moving toward the exracellular environment upon channel activation in response to membrane depolarization. The movement of 10-12 of these protein-bound positive charges triggers a conformational change in the protein that opens the channel. The exact mechanism by which this movement occurs is not currently agreed upon, however the canonical, transporter, paddle, and twisted models are examples of current theories.

Movement of the voltage-sensor triggers a conformational change of the gate of the conducing pathway, controlling the flow of ions through the channel.

The main functional part of the voltage-sensitive protein domain of these channels contains a region composed of S3b and S4 helices, known as the "paddle" due to its shape, which appears to be a conserved sequence, interchangeable across a wide variety of cells and species. A similar voltage sensor paddle has also been found in a family of voltage sensitive phosphatases in various species.[5] Genetic engineering of the paddle region from a species of volcano-dwelling archaebacteria into rat brain potassium channels results in a fully functional ion channel, as long as the whole intact paddle is replaced.[6] This "modularity" allows use of simple and inexpensive model systems to study the function of this region, its role in disease, and pharmaceutical control of its behavior rather than being limited to poorly characterized, expensive, and/or difficult to study preparations.[7]

Although voltage-gated ion channels are typically activated by membrane depolarization, some channels, such as inward-rectifier potassium ion channels, are activated instead by hyperpolarization.

The gate is thought to be coupled to the voltage sensing regions of the channels and appears to contain a mechanical obstruction to ion flow. While the S6 segment has been agreed upon as the segment acting as this obstruction, its exact mechanism is unknown. Possible explanations include: the S6 segment makes a scissor-like movement allowing ions to flow through, the S6 segment breaks into two segments allowing of passing of ions through the channel , or the S6 channel serving as the gate itself. The mechanism by which the movement of the S4 segment affects that of S6 is still unknown, however it is theorized that there is a S4-S5 linker whose movement allows the opening of S6.

Inactivation of ion channels occurs within milliseconds after opening. Inactivation is thought to be mediated by an intracellular gate that controls the opening of the pore on the inside of the cell. This gate is modeled as a ball tethered to a flexible chain (see Figure 2). During inactivation, the chain folds in on itself and the ball blocks the flow of ions through the channel. Inactivation is directly linked to the activation caused by intramembrane movements of the S4 segments, though the mechanism linking movement of S4 and the engagement of the inactivation gate is unknown.

Sodium (Na+) Channels
Sodium channels have similar functional properties across many different cell types. While ten human genes encoding for sodium channels have been identified, their function is typically conserved between species and different cell types.[16]

Calcium (Ca2+) Channels
With sixteen different identified genes for human calcium channels, this type of channel differs in function between cell types. Ca2+ channels give rise to action potentials similarly to Na+ channels in some neurons. In most cells, Ca2+ channels regulate a wide variety of biochemical processes due to their role in controlling intracellular Ca2+ concentrations.[12]

Potassium (K+) Channels
Potassium channels are the largest and most diverse class of voltage-gated channels, with over 100 encoding human genes. These types of channels differ significantly in their gating properties; some inactivating extremely slowly and others inactivating extremely quickly. This difference in activation time influences the duration and rate of action potential firing, which has a significant effect on electrical conduction along an axon as well as synaptic transmission. Potassium channels differ in structure from the other channels in that they contain four separate polypeptide subunits, while the other channels contain four homologous domain but on a single polypeptide unit.[6]

Chloride (Cl-) Channels
Chloride channels are present in every type of neuron. With the chief responsibility of controlling excitability, chloride channels contribute to the maintenance of cell resting membrane potential and help to regulate cell volume.[1]

Examples include:
 * the sodium and potassium voltage-gated channels of nerve and muscle.
 * the voltage-gated calcium channels that play a role in neurotransmitter release in pre-synaptic nerve endings.

See also[edit source | edit]

 * Ion channel
 * Voltage-dependent calcium channel
 * Voltage-gated proton channel
 * Voltage-gated potassium channel
 * Sodium ion channel
 * Potassium channel
 * Catecholaminergic polymorphic ventricular tachycardia

External links[edit source | edit]

 * IUPHAR-DB Voltage-gated ion channel subunits
 * The IUPHAR Compendium of Voltage-gated Ion Channels 2005
 * Voltage-Dependent Anion Channels at the US National Library of Medicine Medical Subject Headings