Efficacy of external tetraethylammonium block of the KcsA potassium channel: Molecular and Brownian dynamics studies

Blockade of the KcsA potassium channel by externally applied tetraethylammonium is investigated using molecular dynamics calculations and Brownian dynamics simulations. In KcsA, the aromatic rings of four tyrosine residues located just external to the selectivity filter create an attractive energy well or a binding cage for a tetraethylammonium molecule. We first investigate the effects of re-orienting the four tyrosine residues such that the centers of the aromatic rings face the tetraethylammonium molecule directly. Then, we systematically move the residues inward in both orientations so that the radius of the binding cage formed by them becomes smaller. For each configuration, we construct a one-dimensional free energy profile by bringing in a tetraethylammonium molecule from the external reservoir toward the selectivity filter. The free energy profile is then converted to a one-dimensional potential energy profile, taking the available space between the tyrosine residues and the tetraethylammonium molecule into account. Incorporating this potential energy profile into the Brownian dynamics algorithm, we determine the conductance properties of the channel under various conditions, construct the current-tetraethylammonium-concentration curve and compare it with the experimentally determined inhibitory constant k_i for externally applied tetraethylammonium. We show that the experimentally determined binding affinity for externally applied tetraethylammonium can be replicated when each of the four tyrosine residues is moved inward by about 0.7 Å, irrespective of orientation of their aromatic rings.