User:Pballieu/sandbox

The X-ray structure of the KcsA Potassium channel revealed that selectivity filter is lined by backbone carbonyl groups groups from the residues of signature amino acid sequence TVGYG, highly conserved all known potassium channels. The selectivity filter is a about 3 Å in diameter

The most important local interactions are the very strong electrostatic attraction between the cation and the carbonyl groups. Thus the selectivity is best explained by the field strength concept developed by George Eisenman based on Coulomb's Law. At physiological pH and in water, the low field carbonyl oxygens sites are better at coordinating with low field cations with larger ionic radii such as K+ than with high field cations with smaller ionic radii such as Na+. This selectivity is based inversely on the sum of the radii of the anionic site and the radii of the cation, thus being independent of their relative position and of structural changes.

An alternative explanation for the selectivity of KcsA is based on the close-fit model also known as the snug-fit model developed by Benzanilla and Armstrong. This model describes the importance for passage of the dehydration of the potassium ion. The main chain carbonyl oxygen atoms that make up the selectivity filter are held at a precise position that allows them to substitute for water molecules in the hydrated shell of the potassium ion, but they are too far from a sodium ion.

Molecular dynamics simulations on KcsA suggest the subunits are not rigidly bound to one another, and the result is a fairly flexible (liquid-like) pore structure. The precise mechanism of potassium channel selectivity continues to be studied and debated and multiple models are used to described different aspects of the selectivity.