Valinomycin is an antibiotic produced by the bacterium Streptomyces (Figure 10.23). Valinomycin is a cyclic polypeptide-like molecule, involving three repeats of the sequence (D-valine)–(L-lactate)–(L-valine)–(D-hydroxyisovalerate). Its folded conformation presents an outside surface rich in CH3 groups and an interior cluster of nitrogens and oxygens that is well-suited to chelating cations.

The dimensions of the interior cavity nicely accommodate a K⁺ ion but do not fit other cations as well. This structure is exactly what is needed for a cation carrier: The outer surface is hydrophobic, making the molecule soluble in the lipid bilayer, whereas the inside mimics in some ways the hydration shell that the cation would have in aqueous solution. A molecule like valinomycin can diffuse to one surface of a membrane, pick up an ion, and then diffuse to the other surface and release it. There is no directed flow, but the carrier, in effect, increases the solubility of the ion in the membrane.