Page "Ion channel" Paragraph 48

Channels differ with respect to the ion they let pass ( for example, Na < sup >+</ sup >, K < sup >+</ sup >, Cl < sup >−</ sup >), 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 helices 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 existence and mechanism for ion selectivity was first postulated in the 1960s by Clay Armstrong.

He suggested that the pore lining could efficiently replace the water molecules that normally shield potassium ions, but that sodium ions were too small to allow such shielding, and therefore could not pass through.

This mechanism was finally confirmed when the structure of the channel was elucidated.

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 X-ray crystallography won a share of the 2003 Nobel Prize in Chemistry.