Roles of different amino-acid residues towards binding and selective transport of K+ through KcsA K+-ion channel

Abstract

The roles of different amino acid residues towards binding and selective transport of K⁺ ion in the selectivity filter of the bacterial K⁺ ion channel, KcsA, were explored. The selectivity filter of KcsA contains amino acids threonine (THR-75), valine (VAL-76), glycine (GLY-77) and tyrosine (TYR-78) from intracellular to extracellular regions. The hydroxyl-hydrogen and carbonyl-oxygen of THR-75 situated at the intracellular region are engaged in intra-residue hydrogen bonding. Thus, the hydroxyl-oxygens orient away from and the hydroxyl-hydrogens orient towards the center of the selectivity filter. Hence, this site attracts and acts as a ‘gate’ for the incoming K⁺ ion. The binding of K⁺ ion is first observed with eight carbonyl-oxygens of THR-75 and VAL-76 (S3 site). However, binding at this site breaks the intra-residue hydrogen bonding of THR-75, which in turn reorients the hydroxyl-hydrogens away from the selectivity filter. Accordingly, binding of K⁺ ion at this site prevents the instantaneous approach of another K⁺ ion. Thus, VAL-76 acts as a ‘hinge’ to open and close the ‘gate’. The next amino acid, GLY-77, shows low affinity towards K⁺ ion, thus acting as a ‘regulator’. TYR-78 situated at the extracellular region shows strong binding affinity towards K⁺ ion, and thus assists in the transport of K⁺ ion from the intracellular to the extracellular region. The release of K⁺ ion to form hydrated K(H₂O)₈⁺ in the extracellular region is endothermic (4.9 kcal mol⁻¹). Hence, the process of K⁺ ion transport is likely to be kinetically controlled. Moreover, the highest binding energy of Na⁺ is observed at the center of four carbonyl-oxygens of VAL-76 (−68.8 kcal mol⁻¹), which is even higher than the highest binding energy of K⁺ ion at S3 (−63.9 kcal mol⁻¹). Na⁺ binding energy decreases from intracellular to extracellular region. Hence, intracellular Na⁺ ion would block the passage of K⁺ ion. The K⁺ ion concentration is high in the intracellular region, and it moves from intracellular to extracellular region, while Na⁺ ion concentration is high in the extracellular region, and it moves from extracellular to intracellular region. Thus, the selective transport of K⁺/Na⁺ ion in KcsA can be attributed to the competition between K⁺ ion exit and Na⁺ ion entry at the center of four carbonyl-oxygens of TYR-78.