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 . 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).