Negative Cooperativity in a Protein Ion Channel Revealed by Current Noise, Conductance and Selectivity Experiments

Abstract

Cooperativity is a phenomenon of universal importance in biophysics and has been extensively reported in many systems including enzymes, protein receptors and protein ion channels, among others. The concept of positive cooperativity appeared in the study of oxygen uptake by hemoglobin to explain that binding of a molecule of oxygen makes it easier the subsequent binding of other molecules. In contrast, negative cooperativity is found when the presence of the first molecule makes the binding of the second molecule more difficult. In particular, we study here the pH titration of the OmpF channel through measurement of current noise amplitude, conductance and ion selectivity. The steep pH dependence found both in channel conductance and Reversal Potential, together with the wide peak found in current noise amplitude are analyzed in terms of the Hill formalism. In all cases, Hill coefficients lower than unity are found, suggesting a negative cooperative behavior. Although OmpF porin is a trimer, previous studies demonstrate that each monomer is identical and both structural and functionally independent. The origin of cooperativity in each monomeric is subtle and does not necessarily demand the existence of different binding domains or subunits. Finally, experiments performed at different electrolyte concentrations evidence that salt cations play a major role in the observed channel features.