Thermodynamic evaluation of energy metabolism in mixed substrate catabolism: Modeling studies of stationary and oscillatory states

Publisher Website
Google Scholar
Abstract
Thermodynamic and kinetic calculations were performed in a model of mixed substrate metabolism. The model simulates the catabolic breakdown of a first substrate, glucose (S1), in the presence of a second substrate, formate (S2), which acts as an additional source of free energy. The principal results obtained with different relative rates of uptake of S2 allow to predict and interpret the following experimental observations: (1) the existence of increased ATP yields by mixed substrate utilization with a maximum ATP yield and optimum input (or molar) ratio for both substrates; (2) a greater assimilation of S1 which may be interpreted as a decreasing fraction of energy required for assimilation; (3) a decrease in ATP yields due to increasing energy demand for transport; (4) an increased assimilation of the carbon source (S1) as a function of increasing inputs of the additional energy source; (5) thermodynamic efficiency (η) defined as the ratio between the output power of ATP synthesis and the input catabolic power, increases for S2/S1 ratios ranging between 0.08 and 2 while for ratios higher than two a slight decrease of η was noticed; and (6) the observed maximum in ATP yield for optimum molar ratio of the two substrates corresponds to high η predicting that higher biomass yields may be obtained through a variable, high, η by chanelling fluxes through catabolic pathways with different ATP yields. During oscillatory behavior, maxima in fluxes were almost coincident with maxima in forces. Thus, the pattern of dissipation was not so advantageous as in the single substrate model under starvation conditions.