Sodium Ion Channel

Sodium ion channels are large transmembrane structures whose function is to actively transfer sodium ions from one side of the membrane to the other, thus effectively changing the ion concentrations. Particularly present in the membrane of excitable cells (for example neurons and muscles cells), this transfer of cations enables a change in the membrane potential through the accumulation of positive charges on the inside of the cells.

This transfer of sodium ions across the membrane can be triggered through a change of membrane voltage (voltage-gated sodium channel) or the binding of a specific ligand (ligand-gated sodium channels).

Structurally, a sodium ion channel comprises a very large (260 kDa) main subunit, called subunit α alpha , responsible for the active transfer itself, and a varying amount of subunits beta, responsible for the voltage-based or ligand-based trigger (Figure 1).

Figure 1: The alpha subunit includes 4 transmembrane domains forming the pore, while additional beta domains provide additional functions.

Within the α alpha subunit, there are 24 transmembrane alpha; helices clustered in 4 domains of 6 helices each. Each helix and each domain are linked together through pore loops outside the membrane. The actual channel is located in the central space created by these 4 domains.

So far 9 different mammalian sodium ion channels have been identified, with low variation in the structure of the core α alpha subunit. Sodium ion channels are part of the superfamily of cation channels alongside calcium and potassium channels.