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CHANNELS, RECEPTORS, AND ELECTRICAL SIGNALING |
1 Rush University Medical Center
2 Miller School of Medicine, University of Miami,
3 Univ of Pannonia, Veszprém, Hungary
4 Brigham Young University
* To whom correspondence should be addressed. E-mail: beisenbe{at}rush.edu.
Submitted on January 29, 2007
Revised on April 5, 2007
Accepted on 17 May 2007
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Abstract |
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p. Selectivity arises from the balance of electrostatic forces and steric repulsion by excluded volume of ions and side-chains of the channel protein in the highly concentrated and charged (~30 M) selectivity filter resembling an ionic liquid. Ions and structural side-chains are described as mobile charged hard spheres that assume positions of minimal free energy. Water is a dielectric continuum. Size selectivity (ratio of Na+ occupancy to K+ occupancy) and charge selectivity (Na+ to Ca2+ ) are computed in 10-5 M Ca2+. In general, small R reduces ion occupancy and favors Na+ over K+ because of steric repulsion. Small p increases occupancy and favors Na+ over Ca2+ because protein polarization 'amplifies' the pore's net charge. Size selectivity depends on R and is independent of p; charge selectivity depends on both R and p. Thus, small R and p make an efficient Na channel that excludes K+ and Ca2+ while maximizing Na+ occupancy. Selectivity properties depend on interactions that cannot be described by qualitative or verbal models or by quantitative models with a fixed free energy landscape.
Key Words: Ca Channel, DEKA, Electrostatics, Monte Carlo, Na channel, Simulations
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