The effect of external oxygen conditions on the metabolic flux distribution of Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a facultative anaerobic organism able to switch the metabolism to fermentation when the aerobic respiration is limited or completely prevented. The availablity of oxygen is a major determinant of the metabolism of S. cerevisiae mainly because when the oxygen availability is restricted, cells need other acceptors to the electrons of NADH and FADH2. Furthermore, when the function of the respiratory system is limited additional means for ATP generation and cross-membrane transport of metabolites and ions are required. Growth under limited respiration thus requires major redistribution of metabolic fluxes compared to the aerobic metabolism. S. cerevisiae CEN.PK113-1A was grown in glucose-limited chemostat culture with 0%, 0.5%, 1.0%, 2.8% and 20.9% O2 in the inlet gas (D = 0.10 h-1, pH 5, 30 ?C) and labeled with 10% of [U-13C]-glucose to study the metabolic flux distribution response to oxygen. The labeled biomass was sampled for METAFoR (Metabolic Flux Ratio) analysis using 13C HSQC NMR spectroscopic detection (Szyperski et al., 1999). A stoichiometric model of the central carbon metabolism of S. cerevisiae was set up comprising of 29 reactions and transport fluxes and 20 metabolites in two intracellular compartments. Additional constraint equations for the stoichiometric model were obtained from the six metabolic flux ratios from METAFoR analysis (Fischer et al., 2004). The uptake and production rates were experimentally measured and the precursor requirements for biomass synthesis were taken from literature. The metabolic net fluxes were solved using an in-house program for weighted optimization based on the matlab function fmincon. A slight decrease in the oxygen uptake rate resulted in low ethanol production already in the 2.8% oxygen conditions. However, major changes in the intracellular metabolic flux distribution compared to aerobic conditions, in particular in the pyruvate branching point, were observed in the 1% oxygen conditions when the oxygen availability was severely reduced.