Figure 1.
The schematic model of the nitrogen assimilation network.
Arrows denote the direction of the reactions. GDH, GS and GOGAT denote the enzymes catalyzing the reactions. For GDH, one αKG and one NH4+ are converted to one Glu. And for each turn of GS-GOGAT cycle, one more ATP is needed to form one Glu. X and Y denote all other nitrogen-containing metabolites obtaining their nitrogen atoms via Glu- or Gln-dependent aminotransferases, respectively.
Table 1.
Nitrogen contribution from Glu and Gln at doubling time = 80 minutes.
Table 2.
Prediction of Vmax at 3 hours grown in 2mM ammonium.
Figure 2.
Isotope labeling kinetics of the central intermediates in nitrogen assimilation.
(A) Curves represent the model simulations of the decay kinetics for the unlabeled glutamate (solid line) and the unlabeled glutamine (dashed line), and of the ammonia diffusion kinetics (dot line). Symbols represent the experimental data from (Yuan et al, 2006). (B) Curves represent the model simulations of the labeling kinetics for the formation of 15N-labeled glutamate (dot line), single-labeled glutamine (dashed line) and double-labeled glutamine (solid line). Symbols represent the experimental data from (Yuan et al, 2006).
Figure 3.
Isotope labeling kinetics of the wild type, ΔGDH and ΔGOGAT strains.
(A) Decay kinetics of unlabeled glutamine. (B) Decay kinetics of unlabeled glutamate. In both A and B, curves represent the model simulation for the wild type (dot line), ΔGDH (solid line) and ΔGOGAT (dashed line). Symbols represent the experimental data from (Yuan et al, 2009).
Figure 4.
Response curves for pairs of enzymes to achieve the objective function against changes in the external ammonium concentration.
(A) Vmax of GDH and GS. (B) Vmax of GDH and GOGAT. (C) Vmax of GS and GOGAT.
Figure 5.
Minimal solutions of the 5 variables in response to NH4 changes.
The x-axis is the NH4 concentration inside of the cell. (A) The sum of squared relative changes of the 5 variables. (B) αKG. (C) Gln. (D) Vmax of GDH. (E) Vmax of GS. (F) Vmax of GOGAT.