Molecule of the Month: Potassium Channels
Potassium channels allow potassium ions to pass, but block smaller sodium ions
Ion Channels in Nerve Signals
Open and Shut
A Poisonous Aside
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 blue 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. To explore this structure in more detail, click on the image for an interactive JSmol.
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- 1lnq: JIANG, Y., LEE, A., CHEN, J., CADENE, M., CHAIT, B.T., MACKINNON, R. (2002) Crystal structure and mechanism of a calcium-gated potassium channel. Nature 417: 515-522
- Yellen, G. (2002): The voltage-gated potassium channels and their relatives. Nature 419, pp. 35-42.
- 1f6g: Cortes, D.M., Cuello, L.G., Perozo, E. (2001) Molecular architecture of full-length KcsA: role of cytoplasmic domains in ion permeation and activation gating. J.Gen.Physiol. 117: 165-180
- Minor Jr.,D.L. (2001): Potassium channels: life in the post-structural world. Current Opinion in Structural Biology 11, pp. 408-414.
- 1k4c: Zhou, Y., Morais-Cabral, J.H., Kaufman, A., MacKinnon, R. (2001) Chemistry of ion coordination and hydration revealed by a K+ channel-Fab complex at 2.0 A resolution. Nature 414: 43-48
- 1bl8: Doyle, D.A., Morais Cabral, J., Pfuetzner, R.A., Kuo, A., Gulbis, J.M., Cohen, S.L., Chait, B.T., MacKinnon, R. (1998) The structure of the potassium channel: molecular basis of K+ conduction and selectivity. Science 280: 69-77
- 2crd: Bontems, F., Gilquin, B., Roumestand, C., Menez, A., Toma, F. (1992) Analysis of side-chain organization on a refined model of charybdotoxin: structural and functional implications. Biochemistry 31: 7756-7764
February 2003, Shuchismita Dutta, David Goodselldoi:10.2210/rcsb_pdb/mom_2003_2