FIGURE 1.

A kinetic schematics for PdPC. E₁ and E₂ represent kinase and phosphatase. The substrate, protein W, is phosphorylated by the kinase to become W^*, which in turn is dephosphorylated by E₂. A complete reaction cycle hydrolyzes one Hence, where is the phosphorylation potential. Both enzymatic reactions are nearly irreversible: Let be the total substrate concentration neglecting the small amount of enzyme-substrate complexes, E_1T and E_2T the total enzyme concentrations for E₁ and E₂. Goldbeter and Koshland (1981) showed that the number of model parameters can be reduced, with nondimensionalization, to four key parameters: , are the Michaelis-Menten constants, and , are maximal velocities, for E₁ and E₂ respectively. is a measure of the strength of stimuli/inhibition. K₁ and K₂ are directly related to the affinity of the enzymes to their respective substrates.