Correlation between TCA cycle flux and glucose uptake rate during respiro-fermentative growth of Saccharomyces cerevisiae

Abstract

Glucose repression of the tricarboxylic acid (TCA) cycle inSaccharomyces cerevisiaewas investigated under different environmental conditions using13C-tracer experiments. Real-time quantification of the volatile metabolites ethanol and CO2allowed accurate carbon balancing. In all experiments with the wild-type, a strong correlation between the rates of growth and glucose uptake was observed, indicating a constant yield of biomass. In contrast, glycerol and acetate production rates were less dependent on the rate of glucose uptake, but were affected by environmental conditions. The glycerol production rate was highest during growth in high-osmolarity medium (2.9 mmol g−1h−1), while the highest acetate production rate of 2.1 mmol g−1h−1was observed in alkaline medium of pH 6.9. Under standard growth conditions (25 g glucose l−1, pH 5.0, 30 °C)S. cerevisiaehad low fluxes through the pentose phosphate pathway and the TCA cycle. A significant increase in TCA cycle activity from 0.03 mmol g−1h−1to about 1.7 mmol g−1h−1was observed whenS. cerevisiaegrew more slowly as a result of environmental perturbations, including unfavourable pH values and sodium chloride stress. Compared to experiments with high glucose uptake rates, the ratio of CO2to ethanol increased more than 50 %, indicating an increase in flux through the TCA cycle. Although glycolysis and the ethanol production pathway still exhibited the highest fluxes, the net flux through the TCA cycle increased significantly with decreasing glucose uptake rates. Results from experiments with single gene deletion mutants partially impaired in glucose repression (hxk2,grr1) indicated that the rate of glucose uptake correlates with this increase in TCA cycle flux. These findings are discussed in the context of regulation of glucose repression.