Fast charge translocations associated with partial reactions of the Na,K-pump: II. Microscopic analysis of transient currents.

Nonstationary pump currents which have been observed in K+-free Na+ media after activation of the Na,K-ATPase by an ATP-concentration jump (see the preceding paper) are analyzed on the basis of microscopic reaction models. It is shown that the behavior of the current signal at short times is governed by electrically silent reactions preceding phosphorylation of the protein; accordingly, the main information on charge-translocating processes is contained in the declining phase of the pump current. The experimental results support the Albers-Post reaction scheme of the Na,K-pump, in which the translocation of Na+ precedes translocation of K+. The transient pump current is represented as the sum of contributions of the individual transitions in the reaction cycle. Each term in the sum is the product of a net transition rate times a "dielectric coefficient" describing the amount of charge translocated in a given reaction step. Charge translocation may result from the motion of ion-binding sites in the course of conformational changes, as well as from movement of ions in access channels connecting the binding sites to the aqueous media. A likely interpretation of the observed nonstationary currents consists in the assumption that the principal electrogenic step is the E1-P/P-E2 conformational transition of the protein, followed by a release of Na+ to the extracellular side. This conclusion is supported by kinetic data from the literature, as well as on the finding that chymotrypsin treatment which is known to block the E1-P/P-E2 transition abolishes the current transient. By numerical simulation of the Albers-Post reaction cycle, the proposed mechanism of charge translocation has been shown to reproduce the experimentally observed time behavior of pump currents.