Only 2 of the 4 subunits are shown. The filter contains 2 K ions (purple) and a water molecule (oxygen molecule in red). Each fully dehydrated K ion is coordinated by 8 carbonyl oxygens derived from the peptide backbone, creating two K binding sites. Electrostatic repulsion between the 2 bound K ions is reduced significantly due to the partial negative charges of the carbonyl oxygens and presence of the intervening water molecule. As a result, the two binding sites may have a relatively high affinity for K ions. At physiological temperature, the optimal coordination chemistry of the oxygen cage may degrade due to thermal movement of the backbone. This can significantly decrease affinity and allow the bound K ions to dissociate.

The idea that affinity for K ions can vary dramatically due to small alterations of the filter conformation leads us to propose a specific mechanism by which the filter can function as a gate: