Metabolic imbalance limits fermentation in microbes engineered for high-titer ethanol production

Microbial strains engineered for high-titer ethanol production often stop fermenting while substantial substrate remains, limiting industrial performance. We investigated this limitation in engineered strains of Escherichia coli and Thermoanaerobacterium saccharolyticum and the native ethanologen Zymomonas mobilis. By combining high-titer fermentations with intracellular metabolomics, we are able to see how intracellular metabolite concentrations change as product formation stops. We then used max-min driving force (MDF) thermodynamic analysis to understand how these changes in intracellular metabolite levels can limit flux and to identify key enzymes that might be responsible for these limitations. In engineered strains, cessation of ethanol production coincided with strong pyruvate accumulation and MDF values near or below zero at the pyruvate kinase step, implying that the pyruvate consuming enzyme(s) (pyruvate decarboxylase for E. coli and pyruvate ferredoxin oxidoreductase, or associated electron transfer enzymes for T. saccharolyticum) might limit flux. By contrast, Z. mobilis maintained positive driving forces without pyruvate buildup, suggesting that its titer is limited by processes outside central carbon metabolism, such as substrate uptake. These results establish a generalizable framework linking metabolite concentrations to pathway thermodynamics and demonstrate how thermodynamic analysis can diagnose where metabolic constraints emerge during high-titer fermentation.IMPORTANCEHigh-titer fermentation is essential for economically viable biofuel production, yet even extensively-engineered microbes frequently stop producing ethanol before the substrate is exhausted. Furthermore, the causes of titer limitations are often poorly understood. A particular challenge is identifying the location of titer limitations in multi-enzyme pathways. Here, we show that MDF analysis can assist in the interpretation of metabolomic data. These findings provide a systems-level explanation for "stuck" fermentations in bacteria and identify thermodynamic driving force as a quantitative diagnostic metric that reveals where biological design targets emerge for metabolic engineering of ethanol and other bioproducts.