Figure 4

The PEP formation rate is robust to PPDK levels and sensitive to ADP levels. Numerical simulations of the PPDK system show that the robust solution has a range of validity. At very low ADP levels (blue dashed line), the photosynthetic rate is high and the system is fully active, all PPDK subunits are in their PPDK₁ form. Then, PEP formation rate is dependent on PPDK total amounts. At medium levels of ADP (gold, purple and green), PEP formation rate is robust to PPDK total amounts and depends solely on ADP levels (the signal). This rate is dependent on PPDK total amount only when PPDK levels are too low to provide the robust solution, F* (see text). Finally, at high ADP levels (darkness), the system shuts down and almost all PPDK subunits are inactivated (PPDK₂), which yields a very low basal formation rate level. Loss of robustness also occurs when PPDK levels are extremely high such that ADP levels are not sufficient to phosphorylate the added active PPDK units (see text and Figure 1b)