Gramicidin (1GMA) - This small dimeric protein is a channel-forming ionophore that kills bacteria. Each subunit consists of a 15 amino acid helix. Two subunits together span the bacterial membrane as seen in the side view of the dimer (left). Notice that most of the atoms on the outside of the helix are nonpolar carbon atoms (gray) that interact with the nonpolar interior of the membrane. Viewed end-on, the hole in the center of the molecule through which ions pass is clearly visible (right view). The polar oxygen (red) and nitrogen (blue) atoms exposed in the central hole facilitate the movement of polar ions across the membrane. The ability to generate differences in ion concentrations across membranes (ion gradients) is essential to the life of a cell. For example, bacterial cells need ion gradients to make ATP. Gramicidin destroys this ability by allowing free movement of ions.
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