bioreactor studies of heterologous protein production by ...

lower but ethanol concentrations were higher. conespondhg to a lowu enzyme
concentration. The relationship between dilution rate, œil mass and ethanol concentration
is shown in Figure 4.20. Ceii concentrations decreased with increasing dilution rates but
ethanol concentrations hcreased with increasing dilution rates. It has been reported that
the Crabtree effect could be induced by a high glucose concentration or by a high influx of
glucose even when the concentration of glucose was low ( 10-100 mgL) (van Dijken and
Scheffers, 1984). Experimentai data showed that the glucose concentrations in the
bioreactor were very low (consistentiy below 50 mgIl) at aU dilution rates (0.05.0.10 and
0.20 h") used in the present continuous cultum. The Crabtree effect was caused by a
high infiux of glucose.
The relationship between the specinc enzyme activity (or enzyme yield) and the
dilution rate during continuous culture is shown in Figure 4.21. It cm be seen that the
specific enzyme activity decreased with increasing dilution rate. This is also due to the
shift in metabolism h m glucose oxidation to glucose fermentation at higher dilution rates;
more glucose was femented to ethanol via glucose fermentation pathway. The estimated
fraction of the ah produced by anaembic pathway increased with increasing dilution rate
(See Table 4.5). But the fermentative pathway is not associated with enzyme production.
Thus, the specinc enzyme activity or enzyme yidd declined to a lower ievel at higher
dilution rates. Furthennon, enzyme yield &O decreased with increasùig specinc growth
rate (dilution rate) as discussed in Section 4.2.3.