Sensitivity analysis reveals the dependence of the ethanol productivity on the overexpression of enzymes catalyzing EM fluxes. The sensitivity of the ethanol productivity is calculated as follows:

Sensitivity of Peth — M, j dPETH, j є {1,2,…,12} (8)

Peth daM, j

The sensitivity plot (Fig. 3.1(a)) shows that all xylose-consuming EMs (EM4 to EM7) are effective in increasing the ethanol productivity but the highest sensitivity is found among glucose-consuming modes (i. e., EM2). Both can contribute to increasing the productivity but in different ways. The former (i. e., amplifying fluxes of EM4 to EM7) promotes the simultaneous consumption of mixed sugars as illustrated in Fig. 1.1(b). On the other hand, the latter (i. e., amplifying EM2 flux) effectively increases the biomass formation as the growth rate of EM2 is the highest among others.

It should be noted that information provided from the sensitivity analysis is local because it shows only the change of productivity with respect to the "infinitesimal" change of enzyme
expression level. It is more important to know how the productivity will change with respect to the "appreciable" change of enzyme levels. This information on nonlinear cellular behaviors can be acquired from dynamic simulations. The results are shown in Fig. 3.1(b) where mixed-sugar-consuming modes (EM8 to EM12) are excluded due to their negligible level of activation (Song et al., 2009). From this investigation, EM6 (red line) is chosen as the "best" mode, while EM2 is the second.

Non-monotonic profiles are observed in Fig. 3.1(b). For example, as the overexpression level of mode 5 increases, the productivity goes up initially but comes down afterwards. This may be seen as the outcome of competition between amplification of throughput flux of EM4 (i. e., benefit) and metabolic burden (i. e., cost).