The Kdp-ATPase of Escherichia coli Mediates an ATP-Dependent, K+-Independent Electrogenic Partial Reaction

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Abstract
Charge transport by the K+ transporting Kdp-ATPase from Escherichia coli was investigated using planar lipid membranes to which liposomes reconstituted with the enzyme were adsorbed. To study reactions in the absence of K+, given some contamination of solutions with K+, we used a mutant of Kdp whose affinity for K+ was 6 mM instead of the wild-type whose affinity is 2 microM. Upon rapid release of ATP from caged ATP, a transient current occurred in the absence of K+. In the presence of K+, a stationary current was seen. On the basis of their structural similarity, we propose a kinetic model for the Kdp-ATPase analogous to that of the Na+K+-ATPase. In this model, the first, K+-independent step is electrogenic and corresponds to the outward transport of a negative charge. The second, K+-translocating step is probably also electrogenic and corresponds to transport of positive charge to the intracellular side of the protein.