Ionic interactions in multiply occupied channels

Area of Science:

• Biophysics
• Computational Biology
• Structural Biology

Background:

• Many ion channels utilize multi-ion mechanisms for physiological function.
• Ion-ion repulsion is crucial for ion permeation through these channels.
• Understanding these interactions is key to explaining channel selectivity and transport.

Purpose of the Study:

• To investigate the energetic consequences of multiple ion occupancy in ion channels.
• To analyze the role of ion-water and ion-peptide correlations in ion permeation.
• To compare these effects in idealized models of gramicidin A and KcsA K+ channels.

Main Methods:

• Application of the semi-microscopic Monte Carlo approach.
• Analysis of excess repulsion energy, accounting for ion-water and ion-peptide correlations.
• Modeling of double occupancy in gramicidin A and KcsA K+ channel systems.

Main Results:

• In gramicidin, pair occupancy is influenced by bulk solvent, channel water, and peptide backbone interactions.
• In KcsA, uneven spacing and large ion-water distances in the selectivity filter promote double occupancy.
• Channel binding pocket design in KcsA reduces net ionic repulsion compared to gramicidin-like structures.

Conclusions:

• Ion-water-peptide correlations significantly impact ion permeation energetics in multi-ion channels.
• The structural features of the KcsA channel's selectivity filter and binding sites are optimized to modulate ion repulsion.
• These findings provide insights into the molecular mechanisms governing ion transport through biological channels.