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RESEARCH ARTICLE Open Access Robust Control of PEP Formation Rate in the

Summary: RESEARCH ARTICLE Open Access
Robust Control of PEP Formation Rate in the
Carbon Fixation Pathway of C4 Plants by a Bi-
functional Enzyme
Yuval Hart1
, Avraham E Mayo1
, Ron Milo2
and Uri Alon1*
Background: C4 plants such as corn and sugarcane assimilate atmospheric CO2 into biomass by means of the C4
carbon fixation pathway. We asked how PEP formation rate, a key step in the carbon fixation pathway, might work
at a precise rate, regulated by light, despite fluctuations in substrate and enzyme levels constituting and regulating
this process.
Results: We present a putative mechanism for robustness in C4 carbon fixation, involving a key enzyme in the
pathway, pyruvate orthophosphate dikinase (PPDK), which is regulated by a bifunctional enzyme, Regulatory
Protein (RP). The robust mechanism is based on avidity of the bifunctional enzyme RP to its multimeric substrate
PPDK, and on a product-inhibition feedback loop that couples the system output to the activity of the bifunctional
regulator. The model provides an explanation for several unusual biochemical characteristics of the system and
predicts that the system's output, phosphoenolpyruvate (PEP) formation rate, is insensitive to fluctuations in
enzyme levels (PPDK and RP), substrate levels (ATP and pyruvate) and the catalytic rate of PPDK, while remaining


Source: Alon, Uri - Departments of Molecular Cell Biology & Physics of Complex Systems, Weizmann Institute of Science


Collections: Biology and Medicine