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Abstract

Charge transport by the K⁺ transporting Kdp-ATPase from Escherichiacoli 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 μM. 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.