Na/K pump: Single-channel vs. Two-access-channel

Abstract

We re-studied the dialyzed Na/K pumps, the experimental foundation for the two-access-channel model. We found that the charge-movement pump currents, the major evidence for the two-access-channel, are only observed in certain situations. Once the stimulation pulse is high enough at low ionic concentration gradient, the backward pump currents disappeared. The two-directional charge movement pump currents become uni-directional pump currents showing that ions have passed through the channel across the cell membrane.\n\nA modified single-channel configuration is introduced to explain the pump currents as two-way charge movement current and the one-way current. The negatively charged amino acids deeply inside the pump channel constitute a spatial array that function as a collimator to align the moving ions in the channel lumen and an energy-well for the moving cations. If the stimulation pulse cannot drive ions to overcome the ionic concentration gradient, the ions will be entrapped into the energy-well as if the pump channel is obstructed showing two separated access-channels. Once the stimulation pulse is high enough to drive ions overcoming the ionic concentration gradient, ions will pass through the channel across the cell membrane so that Na/K pumps exhibit a single channel configuration.