ATP and Acetyl Phosphate Induces Molecular Events near the ATP Binding Site and the Membrane Domain of Na+,K+-ATPase: THE TETRAMERIC NATURE OF THE ENZYME

Abstract

The addition of ATP to Mg2+-Na+-bound-probe labeled Na+,K+-ATPase preparations containing approximately 0.5 mol of pyridoxal 5'-diphospho-5'-adenosine (AP2PL) probe at Lys-480 and approximately 0.9 mol of fluorescein 5'-isothiocyanate (FITC) probe at Lys-501 showed a decrease and an increase in the AP2PL fluorescence intensity with neither significant ATP-dependent phosphorylation nor FITC fluorescence change. The rate constants for the fluorescence change increased nearly linearly with increasing ATP concentrations. The substitution of AcP for ATP decreased the FITC fluorescence rather monophasically, 8.5/s, which was followed by the half-site phosphorylation with same amount of components with different rate constant, 7.2 and 4.6/s, followed by a much slower increase in the two components of AP2PL fluorescence, 1.4 and 0.2/s. The addition of Na+ with increasing concentrations of ATP to the K+-bound AP2PL-FITC enzymes induced accelerations in the decrease and an increase in the AP2PL fluorescence intensity with two different increases in the FITC fluorescence intensity, showing that the same concentration of ATP is capable of inducing four different fluorescence changes. The addition of ATP to the Mg2+-Na+-bound enzymes modified with N-[p-(2-benzimidazolyl)phenyl]-maleimide (BIPM) at Cys-964 and retaining full Na+,K+-ATPase activity induced two different increases in BIPM fluorescence intensity. Each rate constant for the BIPM fluorescence change versus concentrations of ATP gave two intersecting straight lines. These data and the stoichiometries of fluorescence probe bindings and ATP- and AcP-dependent phosphorylation provide strong support for the conclusion that the functional membrane-bound Na+,K+-ATPase is a tetramer.