25. Self-Sustaining Chemical        Systems: Living Cells   Previous PageNext Page
       Eucaryotic Cell Membrane

Most of these antibiotics are closed-ring compounds with many -O- or C=O groups, as in nonactin (right).

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, Rb, or Cs. 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.

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