STOICHIOMETRIC FLUX BALANCE MODELS QUANTITATIVELY PREDICT GROWTH AND METABOLIC BY-PRODUCT SECRETION IN WILD-TYPE ESCHERICHIA-COLI W3110

Flux balance models of metabolism use stoichiometry of metabolic pathw ays, metabolic demands of growth, and optimality principles to predict metabolic flux distribution and cellular growth under specified envir onmental conditions. These models have provided a mechanistic interpre tation of systemic metabolic physiology, and they are also useful as a quantitative tool for metabolic pathway design. Quantitative predicti ons of cell growth and metabolic by-product secretion that are experim entally testable can be obtained from these models. In the present rep ort, we used independent measurements to determine the model parameter s for the wild-type Escherichia coli strain W3110. We experimentally d etermined the maximum oxygen utilization rate (15 mmol of O-2 per g [d ry weight] per h), the maximum aerobic glucose utilization rate (10.5 mmol of Glc per g [dry weight] per h), the maximum anaerobic glucose u tilization rate (18.5 mmol of Glc per g [dry weight] per h), the non-g rowth-associated maintenance requirements (7.6 mmol of ATP per g [dry weight] per h), and tbe growth-associated maintenance requirements (13 mmol of ATP per g of biomass). The flux balance model specified by th ese parameters was found to quantitatively predict glucose and oxygen uptake rates as well as acetate secretion rates observed in chemostat experiments. We have formulated a predictive algorithm in order to app ly the flux balance model to describe unsteady-state growth and by-pro duct secretion in aerobic batch, fed-batch, and anaerobic batch cultur es. In aerobic experiments we observed acetate secretion, accumulation in the culture medium, and reutilization from the culture medium. In fed-batch cultures acetate is cometabolized with glucose during the la ter part of the culture period. Anaerobic batch culture is observed to primarily secrete the by-products acetate, ethanol, and formate. The flux balance model was found to quantitatively predict the time profil es of cell density and glucose and by-product concentrations in the ab ove-described experiments. Taken together, the experimental data and m odel predictions presented show that observed growth and by-product se cretion of wild-type E. coli are consistent with stoichiometrically op timal pathway utilization. Flux balance models can thus be used to des cribe prokaryotic metabolic physiology, and they can be applied to bio process design and control.