Figure 1

The PPDK enzyme produces PEP and is regulated by the bifunctional enzyme RP. Pyruvate, orthophosphate dikinase (PPDK₀) uses ATP and Pi to produce phosphoenolpyruvate (PEP) from pyruvate. It does so in two stages: first, it auto-phosphorylate itself to its active form (PPDK₁). Second, it transfers the phosphoryl group to pyruvate and returns to its natural form (PPDK₀). Another regulatory cycle can phosphorylate the active form PPDK₁ at a different residue to form PPDK₂, the inactive form of PPDK. The second phosphorylation and de-phosphorylation are done by the bifunctional enzyme RP. This enzyme is regulated by ADP levels (an indication to photosynthetic rate). The products of the kinase/phosphatase activity of RP, AMP and PPi, are also the products of the auto-phosphorylation reaction of PPDK₀ with ATP and Pi (b) Enzyme activity of PPDK is constant across a range of PPDK expression levels - as measured in a mutated strain of Maize (Zea Mays) by Ohta et al [18]. Mean levels of PPDK expression in non-transformants is measured to be 1.6 mg/g fresh weight). Enzyme activity changes only at extreme expression levels of PPDK (see text).