Item

Abstract

The permeation of Na+ through gramicidin A channels shows a simple saturation with increasing Na+ concentration that can be described by two different models. The first model assumes that one Na+ binds to the channel with high affinity (≈ 30 M−1) and that conduction occurs by a ‘knock-on’ mechanism requiring double occupancy of the channel; the other model assumes that Na+ binding is of low affinity (< 1 M−1), and that double occupancy of the channel is rare. NMR measurements have shown tight Na+ binding, favoring the first model, but measurements of flux ratios and water transport support the second model. We present here a relatively model-independent measurement of the dwell time of Na+ inside the channel, in which we characterize the fluctuations in H+ current through the channel induced by ‘block’ from the more slowly permeating Na+ ions. The mean Na+ dwell time inside the channel is estimated to be ≈ 10 ns at a membrane potential of 200 mV. This result is inconsistent with tight Na+ binding, thus favoring the second model.