There are several antibiotics called ionophores, most notably nonactin and
valinomycin, that coordinate with metal cations in a manner similar to that of
crown ether.
Normally, cells must maintain a gradient between the concentrations of sodium
and potassium ions inside and outside the cell wall.
Potassium ions are “pumped” in; sodium ions are pumped out.
The cell membrane, in its interior, is like a hydrocarbon, because it consists in
this region primarily of the hydrocarbon portions of lipids.
The transport of hydrated sodium and potassium ions through the cell membrane
is slow, and this transport requires an expenditure of energy by the cell.
Nonactin upsets the concentration gradient of these ions by coordinating more
strongly with potassium ions than with sodium ions.
Because the potassium ions are bound in the interior of the nonactin, this
host-guest complex becomes hydrocarbonlike on its surface and passes readily
through the interior of the membrane.
The cell membrane thereby becomes permeable to potassium ions, and the
essential concentration gradient is destroyed.
