<Tr> <Th> OPM superfamily </Th> <Td> 8 </Td> </Tr> <Tr> <Th> OPM protein </Th> <Td> 2a79 </Td> </Tr> <P> Voltage - gated ion channels are a class of transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential near the channel . The membrane potential alters the conformation of the channel proteins, regulating their opening and closing . Cell membranes are generally impermeable to ions, thus they must diffuse through the membrane through transmembrane protein channels . They have a crucial role in excitable cells such as neuronal and muscle tissues, allowing a rapid and co-ordinated depolarization in response to triggering voltage change . Found along the axon and at the synapse, voltage - gated ion channels directionally propagate electrical signals . Voltage - gated ion - channels are usually ion - specific, and channels specific to sodium (Na), potassium (K), calcium (Ca), and chloride (Cl) ions have been identified . The opening and closing of the channels are triggered by changing ion concentration, and hence charge gradient, between the sides of the cell membrane . </P> <P> 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 . Na, K, and Ca channels are composed of four transmembrane domains arranged around a central pore; these four domains are part of a single α - subunit in the case of most Na and Ca channels, whereas there are four α - subunits, each contributing one transmembrane domain, in most K channels . 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 principal role of ion conduction, comprising the gate and pore of the channel, while S1 - S4 serve as the voltage - sensing region . The four subunits may be identical, or different from one another . In addition to the four central α - subunits, there are also regulatory β - subunits, with oxidoreductase activity, which are located on the inner surface of the cell membrane and do not cross the membrane, and which are coassembled with the α - subunits in the endoplasmic reticulum . </P>

Hannels within the structure of the cell membrane are composed of