Thermodynamic Analysis of Voltage-Sensing Mechanisms

Abstract

Voltage-gated ion channels (VGIC) form a large superfamily of ion channels and the activation of these channels underlie electrical and chemical signaling in a variety of cell types. Structure-function studies are widely used to deduce the energetic effects of a mutation by measuring macroscopic currents and fitting their voltage-dependence to a Boltzmann function. However, in absence of detailed kinetic models, this approach can introduce serious errors in free-energy estimates because of the inherent assumption that the channel activation is a two-state process. We recently developed analytical tools that allows us to calculate the free energies required for activation of voltage-dependent processes without any prior knowledge of the underlying gating scheme. Our method involves measurement of conjugate displacement associated with the force that drives the activation of these channels. In the case of voltage-gated ion channels, gating charge movement is the conjugate displacement and the force is voltage across the membrane. We show that by measuring the median voltage of charge transfer, VM, and the total gating charge per channel, we can calculate the chemical free energy difference between the resting and activated state of the channels. These free-energy measurements can be extended to other members of the VGIC superfamily to obtain a measure of interaction energies between voltage- and ligand-dependent pathways. Development of well-defined model-independent metrics of interaction energies will be crucial for delineating molecular interaction pathways and understand the mechanisms of voltage-transduction.