Can we use rate constants and state models to describe ion transport through gramicidin channels?
Can we use rate constants and state models to describe ion transport through gramicidin channels? Maybe, but only if rate constants are just proportionality constants between rates and probabilities of observing states of the channel. This approach is natural if the system of channel plus ions (plus water) is almost always in one or another of a small number of identifiable states. Many features of ion transport through gramicidin, including the conductance-concentration relationship, concentration-dependent permeability ratios, anomalous mole fraction effect and to some extent flux ratio exponents, are consistent with a description in which there are four occupation 'states' of the pore: only water; an ion at one end; an ion at the other; and ions at both ends. Current-voltage relationships can (and must) also be fitted, but until there is a theory to predict the potential dependence of the rate constants this success will remain hollow. Other features have resisted interpretation. These include the failures to determine 'binding constants' consistent with all the data; the variation of flux ratio exponents with ion type; and, probably, the variation of the currents with asymmetrical ion concentrations. Nevertheless, state models still have one attractive feature, they allow consideration of the effects that one ion within the pore has on the movements of another.
Hladky
S B
SB
Department of Pharmacology, University of Cambridge, UK.
D000900:Anti-Bacterial Agents / Q000737:chemistry*; D006096:Gramicidin / Q000737:chemistry*; D007473:Ion Channels; D017136:Ion Transport; D007700:Kinetics; D008055:Lipids / Q000737:chemistry; D008956:Models, Chemical; D015233:Models, Statistical
D000900:Anti-Bacterial Agents; D007473:Ion Channels; D008055:Lipids; D006096:Gramicidin