Theoretical models of conformational transitions and ion conduction in voltage-dependent ion channels: Bioferroelectricity and superionic conduction

Theoretical studies of the structure and function of ion channels are reviewed, with emphasis on the underlying physical phenomena. Nerve and muscle membranes exhibit behavior interpretable as ferroelectric: Studies show that conformational transitions in voltage-dependent ion channels can be understood in terms of transitions from a ferroelectric state to a superionically conducting state. The ferroelectric-superionic transition hypothesis is supported by observations of voltage-gated ion conduction, surface charge, hysteresis, pyroelectricity, piezoelectricity, transition temperatures and Curie-Weiss behavior in these channels. Macroscopically, the opening of the sodium (Na+) channel appears to involve a moving phase boundary traveling from the outer to the inner surface of the axonal membrane. Molecular biology and electrophysiology have provided a partial picture of the microscopic structure of the Na+ channel, demonstrating a pattern of charged residues that suggests a “voltage sensor” role for the four membrane-spanning S4 segments. The Na+ channel molecule is proposed to be a ferroelectric liquid crystal component of the membrane, which undergoes a SmC*-SmA transition under control of the voltage across the membrane. The open Na+ channel is proposed to be metalloprotein, in which the permeant ions form an integral part of the channel structure, with ion hopping from occupied to vacant sites as the mechanism of selective ion conduction. Ion-exchange reactions involving hydrogen bonds appear to play a fundamental role in the cooperative transitions that alter the conformation of the molecule.

Keywords:

• Ion channel
• conformational transition
• ferroelectric liquid crystal
• superionic conduction
• metaHoprotein