(Na/ + K+)ATPase has one functioning phosphorylation site per alpha subunit.

(Na+ + K+)ATPase contains two different subunits, a catalytic subunit (alpha) and a subunit with uncertain function (beta). The enzyme binds ATP, ouabain and vanadate, and can be phosphorylated by ATP as well as by inorganic phosphate. From the previously reported maximal binding and phosphorylation capacities of 3.5--4.3 nmol P per mg protein (based on Lowry protein determination) and the earlier molecular weight value of approximately 250,000, a molar binding and phosphorylation capacity of 0.87--1.07 mol per mol enzyme was derived. As it is generally agreed that the enzyme molecule contains two alpha subunits or even a multiple of two, it has been suggested that the enzyme operates by means of a so-called "half-of-the-sites mechanism" whereby only of the two alpha subunits can be phosphorylated at any one time. We now present evidence that every alpha subunit can be phosphorylated simultaneously, which rules out the operation of such a mechanism.