The Kdp-ATPase of Escherichia coli mediates an ATP-Dependelb, K+-independent electrogenic partial reaction

Charge transport by the K+ transporting Kdp-ATPase from Escherichia coli wa s investigated using planar lipid membranes to which liposomes reconstitute d with the enzyme were adsorbed. To study reactions in the absence of K+, g iven 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 mu M . 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. O n 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 fi rst, K+-independent step is electrogenic and corresponds to the outward tra nsport of a negative charge. The second, K+-translocating step is probably also electrogenic and corresponds to transport of positive charge to the in tracellular side of the protein.