FIGURE 3.

ATP and vanadate dependences of Na⁺,K⁺-ATPase activity of M8 aspartate mutants. ATPase activity was measured at 37 °C in 130 mm NaCl, 20 mm KCl, 3 mm MgCl₂, 30 mm histidine (pH 7.4), 1 mm EGTA, 10 μm ouabain, and the indicated ATP concentrations (upper panels) or 3 mm ATP and the indicated vanadate concentrations (lower panels). Each line represents the best fit of a Hill function (see Equations 1 and 2 of “Experimental Procedures”) with Hill coefficient ranging between 0.9 and 1.1. Extracted K_0.5 values are listed in Table 1. The dotted line in the left panel reproduces the data corresponding to wild type from the corresponding right panel for direct comparison in each panel. Error bars indicate mean ± S.E. (seen only when larger than the size of the symbols).