Figure 3.

Results for the 1K1P loop. (A) The fraction of phosphorylated S₁ (z axis) as a function of r₁ and [S₂]₀. Note that for the purpose of display, we have set r₁ = r₂ in this case. The total concentration of [S₂] is normalized by its K_m (which is identical for both the kinase and phosphatase) and is plotted on a log scale. (B) The fraction of phosphorylated S₁ as a function of r₁ and the number of additional substrates in the loop (N, see Fig. 2A). All substrates are below saturating concentrations ([S_i]₀ = 0.1 × K_m). As in A, for the purpose of display, the r and K_m parameters have been set to be equal for all substrates. Note that in both panels A and B, the fraction S₁^∗ responds to r₁ with increasing ultrasensitivity as the total saturation of the enzymes (represented by [S₂]₀/K_m or N, respectively) increases.