TETRAETHYLAMMONIUM BINDING TO THE OUTER MOUTH OF THE KcsA POTASSIUM CHANNEL: IMPLICATIONS FOR ION PERMEATION

ABSTRACT

Extracellular tetraethylammonium (TEA⁺) inhibits the current carried out by K⁺ ions in potassium channels. Structural models of wild-type (WT) and Y82C KcsA K⁺ channel/TEA⁺ complexes are here built using docking procedures, electrostatics calculations and molecular dynamics simulations. The calculations are based on the structure determined by Doyle et al.¹¹Our calculations suggest that in WT, the TEA⁺ cation turns binds at the outer mouth of the selectivity filter, stabilized by electrostatic and hydrophobic interactions with the four Tyr⁸² side chains. Replacement of Tyr⁸² with Cys causes a decrease of the affinity of the cation for the channel, consistently with the available site-directed mutagenesis data¹⁶. An MD simulation in which K⁺ replaces TEA⁺ provides evidence that TEA⁺ binding site can accommodate a potassium ion, in agreement with the high-resolution structure recently reported by Zhou et al.²⁰

Key Words: :

• Potassium channel
• Ion binding site
• Tetraethylammonium block
• KcsA
• Molecular modeling

Notes

*D_2d TEA⁺ in water. TEA⁺ was immersed in a water cubic box (∼40 Å edge) along with a Cl⁻ counterion. The system was equilibrated for 0.5 ns carrying out MD calculations at room conditions.

^†Standard structural parameters were imposed.

^‡During the dynamics, the two ions turned out to be separated by at least 15 Å; thus, the presence of Cl⁻ is expected not to affect the structural properties of TEA⁺ and its hydration sphere.

^§The four Cys side-chains, initially located into the bulk aqueous solvent, approached the hydrophobic Tyr-78 aromatic rings within 0.2 ns.

**Instead, TEA⁺ does not form cation-π interactions with the protein, as previously postulated.[4]