 |
Inhaltsübersicht | Nanomaschinen | Moleküle | Programme | Kurse | Fun | Links |
 |
| > |
Potassium Channels
Exploring the Structure
The remarkable ability of the potassium channel to pass only potassium ions is accomplished by a selectivity filter at one end of the pore, as shown here from PDB entry 1k4c. For clarity, only two of the four protein molecules, one either side of the channel, are shown in a stick representation. The little green spheres are potassium ions passing through the selectivity filter. Normally potassium ions float around encased in a cushion of water, like the one at the bottom of the stack of ions. Notice that it is surrounded by eight water molecules, shown as red spheres. In order to pass through the selectivity filter, each potassium ion has to shed these water molecules. This is how the selectivity filter works: the dimensions of the channel are designed to mimic this shell of water. Protein oxygen atoms that line the pore (colored in red) are oriented toward the center of the channel. Eight of these oxygen atoms surround each potassium ion, and act as a perfect replacement for the normal layer of water molecules. During transport, the ions march from one site to the next along the pore. Once the potassium ions cross this filter, they are again enclosed by water molecules. Sodium ions, on the other hand, are slightly smaller in size, so they fail to interact with the oxygen atoms lining the pore wall. They are far more comfortable with their normal shell of water than they are inside the pore, so they are not efficiently ferried across the membrane.This illustration was created with RasMol. You can create similar pictures by clicking on the accession code above and choosing one of the options under View Structure. You will find that this PDB file is a little tricky: it includes only one of the four chains in the pore and also includes a large antibody fragment that is bound to the channel. To get coordinates for the whole pore, look under Other Sources when you are in the Structure Explorer, and go to the EBI MSD Macromolecule File Server. There, you can obtain a "mmol" file that contains all four chains of the channel. You might also look at PDB entry 1bl8, which includes all four chains and several potassium ions, but not the water molecules.
A list of all potassium channel structures in the PDB as of February, 2003, is available here. For additional information on potassium channels and how they work, click here.
Next: Interaktive 3D-Animation
Previous: A Poisonous Aside
Last changed by: A.Honegger,