Increased Salt Concentration Promotes Negative Cooperativity in OmpF Channel

Abstract

The concept of positive cooperativity appeared in the study of oxygen uptake by hemoglobin to explain that when a molecule of oxygen binds makes it easier for a second molecule to bind. Quite the reverse, negative cooperativity refers to the situation where the presence of the first molecule makes it more difficult for the second molecule to bind. We study here the effect of salt on the pH titration of the OmpF channel, paying attention to the current noise, conductance and ion selectivity that are analyzed in terms of the Hill formalism. In all cases, values lower than 1 are found, suggesting a negative cooperativity. Although OmpF porin is a trimer, it was shown by a number of different methods that each monomer is identical and functionally independent. Thus, the slowed-down channel titration is a property of each monomer. Surprisingly, we find that increasing salt concentration promotes negative cooperativity, which is seen as a salt-induced decrease of the Hill coefficient. This observation seems to exclude direct electrostatic interactions between protonation sites as the source of the phenomenon, suggesting another, more subtle mechanism(s). The binding of cations to certain acidic residues has a crucial effect at low pH because results in an inhibition of channel conductance that additionally provides an anionic selectivity to the channel. This suggests that the binding site could play a certain role in the protection of the bacteria against acidic media