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Berlin 2008 – scientific programme

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SYMP: Symposium Computational Soft matter physics

SYMP 1: Computational soft matter physics

SYMP 1.6: Invited Talk

Thursday, February 28, 2008, 11:45–12:15, H 0105

Role of fluctuations in the selectivity mechanism for the KcsA potassium channel — •Michael E. Paulaitis1, Dilip Asthagiri2, and Lawrence R. Pratt31Department of Chemical and Biomolecular Engineering, Ohio State University — 2Department of Chemical and Biomolecular Engineering, Johns Hopkins University — 3Department of Chemical and Biomolecular Engineering, Tulane University

The KcsA potassium channel belongs to a class of channels that is selective for K+ over Na+ at rates of K+ transport approaching the diffusion limit. Selectivity is explained thermodynamically in terms of the favorable partitioning of K+ relative to Na+ in a narrow selectivity filter in the channel. One mechanism for this selectivity invokes the size difference between K+ and Na+, and the molecular complementarity of the selectivity filter with the larger K+ ion. An alternative view holds that size-based selectivity is precluded because the conformational fluctuations of the selectivity filter are greater than the size difference between these two ions. A quasi-chemical theory implemented on the basis of molecular simulation is derived to examine these hypotheses by calculating the distribution of binding energies for Na+ and K+ in a model of the selectivity filter of the KcsA channel. We find that Na+ binds strongly to the selectivity filter with a mean binding energy substantially more favorable than that for K+. The difference is comparable to the difference in hydration free energies of Na+ and K+ in bulk aqueous solution. Thus, the average filter binding energies do not discriminate Na+ from K+ when measured from the baseline of the difference in bulk hydration free energies. Strong binding of the smaller Na+ also constricts the selectivity filter, consistent with a negative partial molar volume of Na+ in water in contrast to a positive partial molar volume of K+ in water. Discrimination in favor of K+ can be attributed to the scarcity of favorable binding configurations for Na+ relative to K+. That relative scarcity is quantified as enhanced binding energy fluctuations, which reflect both the energetically stronger binding of Na+ and the constriction of the filter induced by Na+ binding.

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