Ion movement through gramicidin A channels. Studies on the diffusion-controlled association step

Area of Science:

• Biophysics
• Membrane Biophysics
• Ion Channel Function

Background:

• Gramicidin A channels are widely studied models for ion transport.
• Previous models assumed negligible resistance from aqueous regions at channel entrances.
• This assumption is critical for interpreting ion channel permeability data.

Purpose of the Study:

• To re-evaluate the assumption of negligible aqueous convergence region barriers in gramicidin A channels.
• To investigate the influence of aqueous diffusion on ion movement at high potentials.
• To assess the implications for kinetic modeling of ion channel function.

Main Methods:

• Analysis of gramicidin A single-channel current-voltage characteristics up to 500 mV.
• Measurement of ion currents at varying permeant ion concentrations.
• Experimental manipulation of aqueous diffusion coefficients.

Main Results:

• Ion currents reached a voltage-independent limiting value at low concentrations, proportional to ion concentration.
• This limiting current magnitude decreased with reduced aqueous diffusion.
• Both high-potential and ohmic permeability data indicated aqueous diffusion limitations.

Conclusions:

• Aqueous diffusion in convergence regions significantly limits ion movement into gramicidin A channels.
• The commonly used two-site-three-barrier model may yield erroneous results.
• Accurate kinetic models require explicit consideration of aqueous diffusion limitations for gramicidin A and biological ion channels.