Diffusion of Potassium Ions through the KcsA K+ Channel
Ion channels assist and control the diffusion of ions through biological membranes. Potassium channels permit millions of potassium ions -- but not sodium ions -- to cross the membrane per second. How is such remarkable selectivity achieved given that the ionic radius of Na+ is 0.4 Å smaller than that of K+?
The protein stabilizes dehydrated potassium ions in the selectivity filter. The structure of the KcsA K+ channel reveals eight oxygens surrounding each ion binding site, with four oxygens above and four below each site. These eight oxygen ligands are organized as twisted cube and thus are a beautiful mimic of the inner hydration shell of the potassium ion.