Despite extensive study of individual enzymes and their organization into pathways, the means by which enzyme networks control metabolite concentrations and fluxes in cells remains incompletely understood. Here, we examine the integrated regulation of central nitrogen metabolism in Escherichia coli through metabolomics and ordinary-differential-equation-based modeling. Metabolome changes triggered by modulating extracellular ammonium centered around two key intermediates in nitrogen assimilation, a-ketoglutarate and glutamine. Many other compounds retained concentration homeostasis, indicating isolation of concentration changes within a subset of the metabolome closely linked to the nutrient perturbation. In contrast to the view that saturated enzymes are insensitive to substrate concentration, competition for the active sites of saturated enzymes was found to be a key determinant of enzyme fluxes. Combined with covalent modification reactions controlling glutamine synthetase activity, such active-site competition was sufficient to explain and predict the complex dynamic response patterns of central nitrogen metabolites.

We present a new method for probing cellular metabolic fluxes that is based on the kinetics of assimilation of isotope-labeled nutrient into a diversity of downstream metabolites. In the case of nitrogen assimilation, half-maximal labeling of most metabolites occurs in 10-300 s. Fluxes measured on the basis of the kinetics of nitrogen assimilation in exponentially growing E. coli agree well with those fluxes predicted to allow optimal biomass production.