Molecular Dynamics Investigation of Anesthetic Halothane Interactions with the Proton-Gated Ion Channel GLIC

Abstract

Experimental studies found that general anesthetics inhibited the function of GLIC. To investigate the underlying inhibition mechanism, we studied the interaction of halothane with GLIC and determined the potential of mean force (PMF) for transporting a sodium ion across the GLIC channel in the presence and absence of halothane. Multiple halothane binding sites were identified through docking and multi-nanosecond molecular dynamics (MD) simulations; three important ones are: A) inter-subunit site near TM2-3 loop; B) inter-subunit site near D86; and C) intra-subunit site near W160. MD simulations demonstrated that halothane altered GLIC dynamics and electrostatic interactions between critical residues. Halothane near W160 reduced the stability of salt-bridges between D32 and R192, whose homologous electrostatic interaction was suggested to be important for channel gating in the ligand-gated ion channels. To assess potential impact of halothane binding to channel conductance, we calculated PMF using adaptive biased force method. Protonation state of E222 is a determinant for the PMF. PMF profile with five deprotonated E222 residues in the pentamer showed a significantly deeper energy well than that with three deprotonated E222, suggesting that five deprotonated E222 may trap an ion and that partial protonation of E222 is necessary for ion leaving the trap. While halothane near sites A and B introduced comparatively small effects on PMF, a profound PMF change near E222 was observed in the presence of halothane at W160. This change may likely affect the single channel conductance. Taken together, halothane may modulate GLIC function through altering salt bridges crucial for gating, coupled with a breaking of the fivefold symmetry of the pore-lining helices, leading to a change in the energy profile for ion passage through the channel. Supported by NIH (R01GM66358 and R01GM56257) and NCSA through PSC.