| Research Abstract |
Since the discovery of Na/K-ATPase, evidence has accumulated to suggest that 1 mol of ATP hydrolysis occurs via the Na^+-occluded ADP-sensitive phosphoenzyme, the K^+- sensitive phoshphoenzyme and the K^+-occluded enzyme accompanying active transport of 3Na^+ and 2K^+ according the Post-Albers sheme. However, some controversial issues have arisen concerning whether the functional unit of the enzyme is an αβ-protomer or a much higher oligomer, which would be related to the mechanism of transport, either sequential or simultaneous. Detailed studies of oligomer interaction and the reactivity of the enzyme and a comparison of the extent of phosphorylation with ligand-binding capacities in the presence or absence of ATP hydrolysis and others strongly suggest that the functional unit of the enzyme in the membrane is a tetraprotomer, (αβ)_4. They also suggest that each reaction intermediate of the Post-Albers scheme, respectively, reflects half of the site property of the intermediate and that another half binds ATP.These data may be useful not only to answer the long-standing question of whether the mechanism functions in the presence of both Na^+ and K^+ but also contribute to a better understanding of the mechanism of P-type pump ATPase in general.
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