The effect of oxygen level on metabolic flux distribution of Saccharomyces cerevisoae

The metabolic flux responses of the yeast Saccharomyces cerevisiae to different oxygen levels were studied by growing the yeast in chemostat conditions under five different oxygen levels, corresponding to 21 (fully aerobic), 2.5, 1.0, 0.5 and 0% of the influx gas of the fermentor. The growth rate was maintained at 0.10 h-1. Intra-cellular metabolic flux ratios of the central carbon metabolism were determined by 13C tracer experiments. Approximately 10% of the carbon source glucose was replaced by its uniformly 13C labelled variant for 1.5 residence times and the flux ratios were determined from the fine structure of the 13C NMR signals of the proteinogenic amino acids in [1H, 13C] HSQC spectra. The distribution of metabolic net fluxes was estimated by 13C MFA, i.e. metabolic flux analysis with the flux ratios of the 13C tracer experiments as additional constraints. The metabolic reaction network used in the analysis consists of 25 metabolites and 28 reactions. In addition to the fluxes to the biomass, four extra-cellular fluxes and six intra-cellular flux ratios from the 13C tracer experiments were used in the flux balancing. Due to the additional constraints from the flux ratio analysis, it was not necessary to include the co-factors in the flux balancing. The result from both the flux ratio analysis and from the 13C-MFA showed that the most dramatic changes in the flux distribution take place at low oxygen levels, while the flux ratios of the 2.5% oxygen cultivation resembled the fully aerobic case. However, the fraction of glucose processed through the pentose phosphate pathway responded to the oxygen level already at high oxygen levels, while the relative anaplerotic flux to the TCA cycle was virtually unaffected until very low oxygen levels are reached.