Figure 2.

Illustration of the difference between measuring flux in systems without and with regulatory constraints. The flux in a pathway with two irreversible Michaelis-Menten enzymes was calculated. The amount of the first enzyme e₁ was varied either independently of e₂, as it should be to measure its control coefficient, or in a system in which the total amount of enzyme e₁ + e₂ is constrained. In the latter system an optimal distribution of the enzymes is observed at e₁ = 0.4. In the neighborhood of that optimum e₁ has no apparent control on the flux (nor does e₂). Rate equations and parameter values used: v₁ = k₁e₁(S / K_S)/(1 + (S / K_S) + (X / K_SK_IX))v₂ = k₂e₂X / (K_X + X) k₁ = 2, k₂ = 1, K_S = 1, K_IX = 5, K_X = 2, and S = 5.