Using METATOOL v5.0 (von Kamp & Schuster, 2006), the network is decomposed into 201 EMs, which are too many to be incorporated in the model. In general, as the network size increases, the number of EMs undergoes combinatorial explosion (Klamt & Stelling, 2002), leading to overparameterization (which implies an excessive number of parameters relative to the measurements available to determine them). This problem can be avoided using the Metabolic Yield Analysis (MYA) developed by Song and Ramkrishna (2009) by which an original set of EMs is condensed to a much smaller subset. As a result, 201 EMs are reduced to 12 EMs which can be classified into three groups depending on the substrate associated with them (Table 2.2).

EM Net reaction

1 GLC ^ 2 CO2 + 2 ETH + 2 MAINT

2 25.31 GLC ^ BIOM + 41.43 CO2 + 33.21 ETH

3 40.41 GLC ^_______________________________________________ BIOM + 56.52 CO2 + 48.31 ETH + 15.10 GOLx

4 XYL ^ 1.833 CO2 + 1.583 ETH + 1.583 MAINT

5 2 XYL ^ 2 CO2 + 1.5 ETH + 1.5 MAINT + XOLx

6 31.97 XYL ^ BIOM + 49.42 CO2 + 33.21 ETH

7 138.5 XYL ^ BIOM + 160.4 CO2 + 117.6 ETH + 84.37 GOLx_____________

8 GLC + 4 XYL ^ 2 ACTx + 2 CO2 + 2 MAINT + 4 XOLx

9 GLC + 4 XYL ^ 9.333 CO2 + 8.333 ETH + 8.333 MAINT

10 2.39 GLC + 25.99 XYL ^ 22.19 ACTx + BIOM + 37.82 CO2 + 9.037 ETH

11 5.333 GLC + 2 XYL ^ ACTx + 8.5 CO2 + 4.5 ETH + 7.5 GOLx

12 81.62 GLC + XYL ^ 12.03 ACTx + 1.754 BIOM + 105.9 CO2 + 85.25 ETH + 39.01 GOLx

Table 2.2. EMs represented in terms of extracellular metabolites. Acronyms for metabolites: ACTx = acetate, BIOM = biomass, CO2 = carbon dioxide, ETH = ethanol, GLC = glucose, GOL = glycerol, MAINT = Dissipated ATP for maintenance, XOLx = xylitol, XYL = xylose.