The oligomeric nature of Na/K-transport ATPase

Since the discovery of Na/K-ATPase, evidence has accumulated to suggest tha t 1 mol of ATP hydrolysis occurs via the Na+-occluded ADP-sensitive phospho enzyme, the K+-sensitive phosphoenzyme and the K+-occluded enzyme accompany ing active transport of 3Na(+) and 2K(+) according the Post-Albers scheme. However, some controversial issues have arisen concerning whether the funct ional unit of the enzyme is an alpha beta -protomer or a much higher oligom er, which would be related to the mechanism of transport, either sequential or simultaneous. Detailed studies of oligomer interaction and the reactivi ty of the enzyme and a comparison of the extent of phosphorylation with lig and-binding capacities in the presence or absence of ATP hydrolysis and oth ers strongly suggest that the functional unit of the enzyme in the membrane is a tetraprotomer, (alpha beta)(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 da ta may be useful not only to answer the long-standing question of whether t he mechanism functions in the presence of both Na+ and K+ but also contribu te to a better understanding of the mechanism of P-type pump ATPase in gene ral.