model of overflow metabolism in Saccharomyces cerevisiae.
The specific rate for glucose uptake (qG) is assumed to
follow Monod kinetics.
At glucose concentrations below a
critical value (typically about 30 g/L glucose) corresponding
to a critical metabolic rate for glucose uptake, oxygen
consumption (qO) and growth rate (my) are proportional to qG
and no ethanol is produced. At higher glucose concentrations
glucose uptake rate increases but no further increase in oxygen
consumption is seen and the additional glucose uptake is
metabolized fermentatively, which results in ethanol production
and additional growth but at lower yield. The simulation
program also calculates the respiratory quotient RQ, which is
about 1 when no ethanol is produced but increases towards 3 at
high glucose concentrations.
In a process where both ethanol and glucose
are present at glucose concentrations below the critical value,
ethanol is re-assimilated at rate (qEcmax*E/(E+Ke)) that is proportional to the
“free capacity”, i.e. the difference between qOmax and the
actual qO. Therefore qO is equal to qOmax as long as ethanol is
present in the process.
further illustration of the model check the simulation with
yeast_fb.fig, which uses this model for simulation of a
fed-batch process where ethanol is first produced but later on
in the process consumed.
simulation of this type requires stoichiometric analysis.
that the model can not handle oxygen limitation.