The Selectivity Filter - Structure and Function
For clarity, the following pictures only show half of the selectivity filter.
The selectivity filter of the potassium channel uses the oxygen atoms of the amino acid sequence Thr-Val-Gly-Tyr-Gly.
The potassium ions start out octahedrally coordinated by six water ligands.
In the selectivity filter, the four equatorial water ligands are replaced by the oxygen atoms of the protein's amino acid sequence.
The potassium ion is now relatively strongly bound to the protein; further ion flow is stopped.
There are, however, two potassium ions in the selectivity filter, and the repulsion between these two ions is stronger than the ion-protein bond.
The potassium ions move quickly through the filter and are again surrounded by a hydrate shell (aqua ligands).