Side-chain ionization states in a potassium channel

Theoretical and computational models of ion channels

Computational studies can make meaningful contributions to our understanding of biological ion channels. A wide variety of methods, at different levels of approximation, can be used. Over the past few years, progress in the experimental determination of three-dimensional structures has given a fresh impetus to the theorists. Noteworthy progress has been made in carefully constructing realistic models of a number of complex biological channels to address important questions about their function.

Intrinsic flexibility and gating mechanism of the potassium channel KcsA

The gating mechanism of the potassium channel KcsA was studied by normal mode analysis. The results provided an atomic description of the locations of the pivot points and the motional features of key structural elements in the gating process. Two pivot points were found in the motions of the inner TM2 helical bundle that directly modulate the size of the central channel pore. One point is an intrasubunit hinge point that sharply divides the structural flexibility between the more rigid selectivity filter and the more mobile peripheral transmembrane helices. Such a division is vital for KcsA because it permits the large-scale motions of transmembrane helices required for the gating and, in the meantime, maintains the rigidity of the filter region essential for the selectivity. The other pivot point is an intersubunit one at which all four TM2 helices are bundled together. During the gating process, each TM2 helix undergoes a lever-like swinging motion pivoting on the intrasubunit hinge, and the entire TM2 bundle undergoes a concerted rotational motion around the central channel axis constrained around the intersubunit bundle point. This series of motions leads to a dramatic enlargement of the intracellular gate without loosening up the structural integrity.

Tetraethylammonium binding to the outer mouth of the KcsA potassium channel: implications for ion permeation

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. (11) Our 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 data (16). 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)

The ionization state and the conformation of Glu-71 in the KcsA K(+) channel

The side chain of Glu-71 of the KcsA K(+) channel, an important residue in the vicinity of the selectivity filter, was not resolved in the crystallographic structure of Doyle et al. (Doyle, D. A., J. M. Cabral, R. A. Pfuetzner, A. Kuo, J. M. Gulbis, S. L. Cohen, B. T. Chait, and R. MacKinnon. 1998. Science. 280:69-77). Its atomic coordinates are undetermined and its ionization state is unknown. For meaningful theoretical and computational studies of the KcsA K(+) channel, it is essential to address questions about the conformation and the ionization state of this residue in detail. In previous MD simulations in which the side chain of Glu-71 is protonated and forming a strong hydrogen bond with Asp-80 it was observed that the channel did not deviate significantly from the crystallographic structure (Bernèche, S., and B. Roux. 2000. Biophys. J. 78:2900-2917). In contrast, we show here that the structure of the selectivity filter of the KcsA channel is significantly disrupted when these side chains are fully ionized on each of the four monomers. To further resolve questions about the ionization state of Glu-71 we calculated the pK(a) value of this residue using molecular dynamics free energy simulations (MD/FES) with a fully flexible system including explicit solvent and membrane and finite-difference Poisson-Boltzmann (PB) continuum electrostatics. It is found that the pK(a) of Glu-71 is shifted by approximately +10 pK(a) units. These results strongly suggest that Glu-71 is protonated under normal conditions.

Conducting-state properties of the KcsA potassium channel from molecular and Brownian dynamics simulations

The mechanisms underlying transport of ions across the potassium channel are examined using electrostatic calculations and three-dimensional Brownian dynamics simulations. We first build open-state configurations of the channel with molecular dynamics simulations, by pulling the transmembrane helices outward until the channel attains the desired interior radius. To gain insights into ion permeation, we construct potential energy profiles experienced by an ion traversing the channel in the presence of other resident ions. These profiles reveal that in the absence of an applied field the channel accommodates three potassium ions in a stable equilibrium, two in the selectivity filter and one in the central cavity. In the presence of a driving potential, this three-ion state becomes unstable, and ion permeation across the channel is observed. These qualitative explanations are confirmed by the results of three-dimensional Brownian dynamics simulations. We find that the channel conducts when the ionizable residues near the extracellular entrance are fully charged and those near the intracellular side are partially charged. The conductance increases steeply as the radius of the intracellular mouth of the channel is increased from 2 A to 5 A. Our simulation results reproduce several experimental observations, including the current-voltage curves, conductance-concentration relationships, and outward rectification of currents.

Brownian dynamics study of an open-state KcsA potassium channel

Three-dimensional Brownian dynamics simulations are used to study conductance of the KcsA potassium channel using the known crystallographic structure. Employing an open-state channel created by molecular dynamics simulations, current-voltage and current-concentration curves broadly consistent with experimental measurements are obtained. In the absence of an applied potential, the channel houses three potassium ions at positions that are in close agreement with X-ray diffraction maps.