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Figure 1.
Figure 1. Mechanism of permeation in potassium channels. (a)
Two diagonal subunits of KcsA with the selectivity filter
highlighted in black (residues 74 -80). Potassium ions are green
spheres. (b) Model of permeation29. The selectivity filter is
shown schematically with ions numbered 1 -4. Oxygen ligands are
red crosses (water) or circles (carbonyl or hydroxyl). The two
states of the selectivity filter (1/3 and 2/4) are occupied
alternatively during permeation. A dehydration transition site
for potassium is observed experimentally on the outside of the
channel, but not on the inside (green arrow). (c) Structure of
the selectivity filter in low permeant ion concentration. Left,
low potassium (PDB entry 1K4D). Residues Thr74 -Asp80 are in
stick representation. Oxygen, nitrogen and carbon are red, blue
and gray, respectively. Middle, low thallium (PDB entry 1R3K).
Thallium ions are purple spheres. Right, schematic
representation of the structure observed at low permeant ion
concentration with one ion alternating between sites 1 and 4.
(d) Structure of the selectivity filter in high permeant ion
concentrations. Left, high potassium (PDB entry 1K4C). The
residues in the selectivity filter are labeled at the carbonyl
level. Middle, high thallium (PDB entry 1R3J). Right, schematic
representation with two ions alternating between the 1/3 and 2/4
states. All figures were generated with MolScript42 and
Raster3D^43.
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