Gramicidin

The antibiotic gramicidin A is an open chain polypeptide and manages to interfere with the ionic permeability of cell membranes. In this case it forms a tube in the lipid bilayer (the other one, valinomycin, uses the carrier mechanism). Net effect is, that the mitochondria use the energy generated by electron transport to accumulate K⁺ instead of making energy in the form of ATP. So essentially it robs away the required energy for the cell to survive.
To give you an idea about the effectivity of a bit of gramicidin: with a concentration of 10^-9 M, the conductance of the membrane to K⁺ increases more than 10⁴-fold.

            O   H
            \\ /
             C							  O
             |							  ||
            HN-Val-Gly-Ala-Leu-Ala-Val-Val-Val-Trp-Leu-Trp-Leu-Trp-Leu-Trp-C
								  |
								  N-H
								  |
								  CH2
								  |
								  CH2
								  |
								  OH

Val = valine		Trp = tryptophan
Gly = glycine		Leu = leucine
Ala = alanine