Metabolite concentrations, fluxes and free energies imply efficient enzyme usage. Author Junyoung Park, Sara Rubin, Yi-Fan Xu, Daniel Amador-Noguez, Jing Fan, Tomer Shlomi, Joshua Rabinowitz Publication Year 2016 Type Journal Article Abstract In metabolism, available free energy is limited and must be divided across pathway steps to maintain a negative ΔG throughout. For each reaction, ΔG is log proportional both to a concentration ratio (reaction quotient to equilibrium constant) and to a flux ratio (backward to forward flux). Here we use isotope labeling to measure absolute metabolite concentrations and fluxes in Escherichia coli, yeast and a mammalian cell line. We then integrate this information to obtain a unified set of concentrations and ΔG for each organism. In glycolysis, we find that free energy is partitioned so as to mitigate unproductive backward fluxes associated with ΔG near zero. Across metabolism, we observe that absolute metabolite concentrations and ΔG are substantially conserved and that most substrate (but not inhibitor) concentrations exceed the associated enzyme binding site dissociation constant (Km or Ki). The observed conservation of metabolite concentrations is consistent with an evolutionary drive to utilize enzymes efficiently given thermodynamic and osmotic constraints. Keywords Animals, Mice, Escherichia coli, Cell Line, Thermodynamics, Saccharomyces cerevisiae, Enzymes Journal Nat Chem Biol Volume 12 Issue 7 Pages 482-9 Date Published 2016 Jul ISSN Number 1552-4469 DOI 10.1038/nchembio.2077 Alternate Journal Nat Chem Biol PMCID PMC4912430 PMID 27159581 PubMedPubMed CentralGoogle ScholarBibTeXEndNote X3 XML