Most of these antibiotics
are closed-ring compounds with many -O- or C=O groups, as in nonactin
Nonactin wraps around a potassium ion, with its oxygen atoms coordinated
to the ion, and with hydrophobic groups exposed to the exterior.
It is the exact opposite of the oildrop model of a protein, in which
the protein has a hydrophobic interior and a polar exterior. The
inverted structure of the antibiotic presumably makes it possible
for nonactin and a K
ion to diffuse through the lipid bilayer of the membrane.
Nonactin, in effect, gives the ion a hydrophobic overcoat. Gramicidin
can carry all of the alkali metal ions through a membrane, but valinomycin
carries only K,
Such antibiotics are toxic because they make membranes susceptible
to alkali metal ions when they should not be. Cells waste their
ATP by pumping K
in and Na
out, only to find them leaking the wrong way again, with the aid
of these carrier molecules.