Movement of Molecules Across Cell Membranes
101
123
4
4
1
3
2
(a)
(b)
Subunit
Figure 4–5
Model of an ion channel composed of fi ve polypeptide subunits. (a) A channel subunit consisting of an integral membrane protein containing
four transmembrane segments (1, 2, 3, and 4), each of which has an alpha helical confi guration within the membrane. Although this model
has only four transmembrane segments, some channel proteins have as many as 12. (b) The same subunit as in (a) shown in three dimensions
within the membrane, with the four transmembrane helices aggregated together. (c) The ion channel consists of fi ve of the subunits illustrated
in (b), which form the sides of the channel. As shown in cross section, the helical transmembrane segment 2 (light purple) of each subunit
forms each side of the channel opening. The presence of ionized amino acid side chains along this region determines the selectivity of the
channel to ions. Although this model shows the fi ve subunits as identical, many ion channels are formed from the aggregation of several
different types of subunit polypeptides.
(c)
Ion channel
Subunit
Cross section
Aqueous pore
of ion channel
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