Various new experimental and theoretical techniques were devised and applied to quantify the metabolic rates of the yeast S. cerevisiae CEN.PK 113-7D grown under various conditions and substrates in carbon-limited aerobic chemostat cultures. The main tool applied for this purpose was the co-factor based metabolic flux analysis. The scope of the work reaches from the initial analysis of biomass, to the quantification of the concentration of glycolytic intermediates.
The biomass composition of the yeast S. cerevisiae was analysed under different growth conditions. The introduction of tightly bound water, 4%, in the biomass resolved systematic differences. Metabolic flux analysis was based this. Using the maximum yield principle to select between different network options, a P/O ratio of 1.44, a maintenance coefficient kATP of 0.646 mol ATP/C-mol biomass and a growth related ATP requirement 55.2 mmol ATP/C-mol/hr were derived. The metabolic network model with these parameters was then used to make forecasts for critical ethanol to glucose ratios. The maximal ratios, for which the ethanol was completely metabolised, were 47:53, 57:43, and 73:27 on C-mol basis for three mutants, while a fourth one was not well predicted. This established the first successful quantitative prediction of mutants based on the maximum yield principle.
To expand the possibilities of the metabolic network analysis, it was found necessary to include the subcellular compartmentation in the model. Shuttle systems were used for components for which no transporters were identified and an existence was deemed doubtful, e.g the amino group and the C2-carbon skeleton. The proposed model thus used a isooxovalerate/valine shuttle system for the mitochondrial import of the amino group. It employed the branched-chain amino acid transaminase (BATl & BAT2) and would operate with transporters for isooxovalerate and valine. Special attention was given to the mitochondrial import of the C2-carbon skeleton. Three possible metabolic networks are proposed, each containing a cytosolic and a mitochondrial compartment, for the aerobic growth of S. cerevisiae on glucose, ethanol, or acetate.