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Journal Article

Estimation of Na+ dwell time in the gramicidin A channel. Na+ ions as blockers of H+ currents.


Heinemann,  S.H.
Department of Membrane Biophysics, MPI for biophysical chemistry, Max Planck Society;

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Heinemann, S., & Sigworth, F. J. (1989). Estimation of Na+ dwell time in the gramicidin A channel. Na+ ions as blockers of H+ currents. Biochimica et Biophysica Acta (BBA) - Biomembranes, 987(1), 8-14. doi:10.1016/0005-2736(89)90448-3.

Cite as: https://hdl.handle.net/11858/00-001M-0000-002B-B7C4-C
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.