Binding of Potassium Ions in Access Channel at Cytoplasmic Side of Na+,K+ ATPase

Binding of potassium ions at the cytoplasmic side of the Na+,K+-ATPase controlled by pH and magnesium ions has been studied. The investigations were performed by the measurement of small increments of the admittance (capacitance and conductance) of bilayer lipid membranes with adsorbed membrane fragments containing Na+,K+-ATPase, using the method developed earlier. The change of the membrane capacitance triggered by release of protons from a bound form (caged H+) by a UV flash was stepwise in the absence of Mg2+ ions. In the presence of Mg2+ the kinetics was more complex and consisted of a fast step followed by a slow relaxation into a new steady-state value within 1-2 s. This kinetics depended on pH and the concentration of K+ ions. The dependence of the fast stepwise change of the capacitance on the K+ concentration can be explained by a model of the competitive binding of protons and Na+ or K+ as developed by us earlier. The effect of Mg2+ can be explained by an influence of these ions bound to the Na+,K+-ATPase on the binding of K+ due to modifying either the protein conformation or electrostatic fields in the ion access channel at the cytoplasmic side.