Estimation of Na+ dwell time in the gramicidin A channel. Na+ ions as blockers of H+ currents.
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 (approximately 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 (less than 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 approximately 10 ns at a membrane potential of 200 mV. This result is inconsistent with tight Na+ binding, thus favoring the second model.
Heinemann
S H
SH
Max-Planck-Institut für biophysikalische Chemie, Abteilung Membranbiophysik, Göttingen, F.R.G.
Sigworth
F J
FJ
D001692:Biological Transport; D004553:Electric Conductivity; D005989:Glycerides; D006096:Gramicidin / Q000378:metabolism*; D007473:Ion Channels / Q000378:metabolism*; D008051:Lipid Bilayers / Q000378:metabolism; D008954:Models, Biological; D011522:Protons; D012964:Sodium / Q000378:metabolism*; D013185:Squalene
D005989:Glycerides; D007473:Ion Channels; D008051:Lipid Bilayers; D011522:Protons; D006096:Gramicidin; D013185:Squalene; D012964:Sodium; C005953:monoolein