(2010) on cell growth and metabolite concentration profiles. Izumi et al. (1994) reported that R. erythopolis D-1 desulfurized DBT to 2-hydroxybiphenyl (HBP) successfully. They used 500 mL of PXD101 a glucose-based biosynthetic medium with 0.125 mM DBT as the sole sulfur source at 30 °C to examine the desulfurization activity of growing cells.

They measured pH, cell growth, DBT concentration, and HBP concentration at various times during their experiment. In another study, Davoodi-Dehaghani et al. (2010) isolated R. erythropolis SHT87. They used growing cells at 30 °C in a 50 mL solution of glycerol containing a synthetic medium with 0.25 mM of DBT as the sole sulfur source. They also measured cell growth, DBT concentration, and HBP concentration at different times over 120 h. The experimental data from the above two independent studies provided a sound basis for validating our proposed model. We used their cell growth data and DBT/HBP concentration profiles from the exponential Daporinad concentration phase to compute specific cell growth rates (1 h−1) and DBT (HBP) uptake (secretion) rates (mmol g−1 dcw h−1). Our reconstructed model consists of 87 intracellular metabolic reactions, 66 transport reactions, and 196 metabolites related to either sulfur or

central metabolism. The sulfur metabolism includes the 4S pathway; the CoA biosynthetic pathway; metabolism of inorganic sulfur, cysteine, and methionine; and biosynthesis of cysteine, methionine, mycothiol, biotin, and thiamine. The central metabolism includes gluconeogenesis, citric acid cycle, pentose phosphate pathway, and Embden Meyerhoff next Paranas pathway for glycolysis. Figure 1 shows a complete picture of the pathways and reactions in our model, with full details in the Supporting information. We simulated the experiments

of Izumi et al. (1994) and Davoodi-Dehaghani et al. (2010) and compared our predicted cell growth rates with their measured data. As the 4S pathway is aerobic, we assumed unlimited oxygen flux in all of our validation studies and analyses. Sulfur was a limiting substrate in the experiments of Izumi et al. (1994) and Davoodi-Dehaghani et al. (2010). We inferred this from the fact that the stationary phase in their experiments was triggered, when DBT concentration went to zero and HBP concentration reached its maximum. Therefore, we allowed unlimited glucose flux for simulating the experiment of Izumi et al. (1994) and unlimited glycerol flux for Davoodi-Dehaghani et al. (2010). Then, we fixed the DBT uptake and HBP production rates (mmol g−1 dcw h−1) to be at some values computed from their data, and predicted specific cell growth rates at those values. Figure 2 shows that our growth predictions are in close agreement with the two experimental data. The accuracy of our predictions is confirmed by the argument that the limiting sulfur solely determines the growth.