The effect of amplifying EM6 flux on ethanol productivity is more clearly presented in batch fermentation profiles (Fig. 3.2).

Obviously, overexpression of enzymes has a limit due to the finite internal resources and other reasons. Although it is difficult to estimate the upper limit to overexpression level, we constrain the constitutive synthesis rates of enzymes to be less than a certain threshold, i. e., the total increase of «м/s is less than or equal to 0.4.

Fig. 3.2 shows that xylose consumption rate is accelerated, while glucose consumption rate is reduced. The decrease of glucose consumption rate can be attributed to a combined effect of metabolic burden and cellular regulation. Consequently, simultaneous consumption of glucose and xylose is facilitated, leading to the substantial increase of ethanol productivity from 1.5 to 2.07 g/L/h (i. e., increase of volumetric productivity by 38%), while the ethanol yield is slightly decreased from 0.402 to 0.392. The total conversion of mixed sugars is fixed to 0.99 in this calculation.