Analysis of flux control through triose-phosphate isomerase and PRPP amidotransferase. (A) Substrate, product, and enzyme concentrations for the triose-phosphate isomerase reaction in glycolysis. The 25 experimental conditions are laid out across the X-axis as per Fig. 1. (B) Michaelis-Menten equation relating concentrations to flux and extent of agreement between measured fluxes (red) and the best Michaelis-Menten fit (blue). (C) Substrate, product, and enzyme concentrations for the PRPP aminotransferase reaction, the committed step of purine biosynthesis. (D) Michaelis-Menten equation relating concentrations to flux and extent of agreement between measured fluxes and the best Michaelis-Menten fit. (E) Concentrations of the reaction inhibitor AMP and extent of agreement between measured fluxes and the best Michaelis-Menten fit after including AMP as a regulator. DHAP, dihydroxyacetone phosphate; GAP, glyceraldehyde 3-phosphate; PRPP, phosphoribosyl pyrophosphate.