FIGURE 6.

ATP dependence of the rate of dephosphorylation of E₂P for mutant E439A. Dephosphorylation of the phosphoenzyme formed in the presence of ³²P_i was followed at various ATP concentrations at pH 7.0 in the absence of Mg²⁺ as described under “Experimental Procedures.” Examples of the decay curves are shown under supplemental Fig. S7. The dephosphorylation rate constants are shown here as a function of the ATP concentration. The parameters derived by fitting a hyperbolic function as described under “Experimental Procedures” are as follows (in each case, the total number of data points included in the fit is indicated in parentheses): wild type, K_0.5 = 34 ± 9 μm, k₀ = 2.0 min⁻¹, k_max = 4.7 min⁻¹ (n = 22); E439A, K_0.5 = 3115 ± 2171 μm, k₀ = 19.0 min⁻¹, k_max = 5.6 min⁻¹ (n = 19).