Mathematical model for aerobic culture of a recombinant yeast

A mathematical model was formulated to simulate cell growth, plasmid loss and recombinant protein production during the aerobic culture of a recombinant yeast S. cerevisiae. Model development was based on three simplified metabolic events in the yeast: glucose fermentation, glucose oxidation and ethanol oxidation. Cell growth was expressed as a composite of these metabolic events. Their contributions to the total specific growth rate depended on the activities of the pacemaker enzyme pools of the individual pathways. The pacemaker enzyme pools were regulated by the specific glucose uptake rate. The effect of substrate concentrations on the specific growth rate was described by a modified Monod equation. It was assumed that recombinant protein formation is only associated with oxidative pathways. Plasmid loss kinetics was formulated based on segregational instability during cell division by assuming constant probability of plasmid loss. Experiments on batch fermentation of recombinant S. cerevisiae C468/pGAC9 (ATCC 20690), which expresses Aspergillus awamori glucoamylase gene and secretes glucoamylase into the extracellular medium, were carried out in an airlift bioreactor in order to evaluate the proposed model. The model successfully predicted the dynamics of cell growth, glucose consumption, ethanol metabolism, glucoamylase production and plasmid instability. Excellent agreement between model simulations and our experimental data was achieved. Using published experimental data, model agreement was also found for other recombinant yeast strains. In general, the proposed model appears to be useful for the design, scale-up, control and optimization of recombinant yeast bioprocesses.