Passage (Potassium Channel Selectivity Filter)
33" h x 28" w
”For a cell to exist, it must have an inside and an outside. The cell membrane is the boundary that separates these two regions, and without which, there is no cell. But the cell membrane must allow passage of substances such as nutrients and minerals. The potassium channel is a cell membrane protein whose selectively filter allows the entry of potassium ions and only potassium. Even sodium, with the same +1 charge and an even smaller diameter, cannot pass through this filter. The basis for this selectivity was a mystery, solved by MacKinnon whose work won the 2003 Nobel Prize for Chemistry (along with Agre - for work on the water channel.) This stained glass piece is a view of the potassium channel selectivity filter. (Oddly, a parallel for the solution is seen in the “spider climb,” an obstacle from the American Ninja Reality TV show - - see below.)
Potassium channel selectivity filter: 1J95.PDB
How can the potassium channel allow passage of positively charged potassium atoms, but exclude positively charged sodium ions which are smaller?
Both ions, in solution, are bound to several water molecules which are polar, and carry slight negative charges near the oxygens. In this “hydrated” state, both potassium and sodium are too large to pass through the potassium channel. However, the center of the channel is lined with hydrophilic amino acids. Potassium ions are able to separate from their polar water molecules by forming electrostatic bonds with polar amino acids on both sides of the channel, then “walk” down the channel passage by moving from one polar amino acid to another. Because of the smaller size of sodium ions, a sodium ion would only be able to form bonds with polar amino acids on one side of the channel. This would not allow the sodium to release all its water of hydration, and in this form it is too large to fit through the channel. (In the “spider climb” obstacle, an athlete must ascend a channel between two parallel walls, pressing their hands and feet against the walls and moving up with small jumps. If the walls were too far apart, or the athlete 's arms and legs too short, they would not be able to interact with both sides of the channel and would fall - - analogous to the smaller sodium ion.)