Selectivity sequences in a model calcium channel: role of electrostatic field strength |
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Authors: | Daniel Krauss Bob Eisenberg Dirk Gillespie |
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Institution: | (1) Department of Molecular Biophysics and Physiology, Rush University Medical Center, Chicago, IL, USA;(2) Grinnell College, Grinnell, IA, USA; |
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Abstract: | The energetics that give rise to selectivity sequences of ionic binding selectivity of Li+, Na+, K+, Rb+, and Cs+ in a model of a calcium channel are considered. This work generalizes Eisenman’s classic treatment (Biophys J 2(Suppl. 2):259,
1962) by including multiple, mobile binding site oxygens that coordinate many permeating ions (all modeled as charged, hard spheres).
The selectivity filter of the model calcium channel allows the carboxyl terminal groups of glutamate and aspartate side chains
to directly interact with and coordinate the permeating ions. Ion dehydration effects are represented with a Born energy between
the dielectric coefficients of the selectivity filter and the bath. High oxygen concentration creates a high field strength
site that prefers small ions, as in Eisenman’s model. On the other hand, a low filter dielectric constant also creates a high
field strength site, but this site prefers large ions, contrary to Eisenman’s model. These results indicate that field strength
does not have a unique effect on ionic binding selectivity sequences once entropic, electrostatic, and dehydration forces
are included in the model. Thus, Eisenman’s classic relationship between field strength and selectivity sequences must be
supplemented with additional information about selectivity filters such as the calcium channel that has amino acid side chains
mixing with ions to make a crowded permeation pathway. |
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