Potassium Channel

Overall Structure

The potassium channel looks like an upside down tepee. Click on the button below to see the structure of the potassium channel. In the final orientation, the channel lies in a horizontal membrane, and potassium ions will travel from the exterior of the cell (oriented at the top) to the interior of the cell (bottom).

In the following view, the channel is rotated forward 90° so you are looking down the center of the channel from the exterior of the cell.

Subunit Structure

The potassium channel consists of four identical subunits. Each subunit contains two long helices (an inner helix and an outer helix) and a half helix (called the pore helix) that span the membrane (Doyle et al, 1998). In the image below, helices are colored in salmon, and the displayed sidechains are important for guiding the K⁺ ions to the channel opening.

How it Works

The exterior surface of the potassium channel has a wide, funnel-shaped opening that is lined with polar residues that help to draw the K⁺ ions into the channel.

Pore helices are aligned so that the carboxyl oxygen atoms point downward toward the center of the channel. This dipole further assists in drawing the K⁺ ions through the channel. K⁺ ions are usually hydrated with eight water molecules; carbonyl oxygen atoms in the selectivity filter strip the water molecules off the ions so they don't pass through the pore. These carbonyl oxygen atoms (colored pink in image) replace the water molecules and hydrate the ions as they pass through the pore.

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In the next view, one of the subunits is removed to better view the inside of the channel. This structure, from 1bl8, contains purple K⁺ ions, in the channel.

Selectivity Filter

Tryptophan and tyrosine residues form a hydrophobic sheet at the top of the selectivity filter. MacKinnon's lab (Doyle, 1998) proposed that this sheet acts 'like a layer of springs' to maintain the pore at a precise diameter. The carbonyl oxygens in the selectivity filter strip the water hydrating the K⁺ ions. Na⁺ ions are too small to interact with all the carbonyl oxygens at a specific level in the filter and are therefore excluded. The hydrophobic cuff surrounding the selectivity filter prevents it from flexing inward to accommodate Na ions. The potassium channel allows only 1 Na⁺ ion to cross the membrane for every 10,000 K⁺ ions transported.

The potassium channel is embedded in the membrane, and as such, contains hydrophobic residues on its exterior surface. (Backbone of hydrophobic residues is colored yellow.) Tryptophan and tyrosine residues (displayed in ball and stick and colored cpk) form a ring at the upper and lower boundaries of the phospholipid tails, effectively anchoring the channel in the membrane.

Stabilization in the Membrane

Scorpion Toxin

Scorpion toxin inhibits the potassium channel by binding to the external surface. A lysine residue blocks the channel and flashes in purple in this image.

References

Doyle, Declan A., Cabral, João Morais, Pfuetzner, Richard A., Kuo, Anling, Gulbis, Jacqueline M., Cohen, Steven L., Chait, Brian T. and MacKinnon Roderick. 1998. The Structure of the Potassium Channel: Molecular Basis of K⁺ Conduction and Selectivity. Science 280:69-77.'