Reactions diagrams. (a) Reaction diagram for the basic model of phosphorylation-dephosphorylation cycle. (b-e) Reaction diagrams for the extended model with kinase-phosphatase complex formation. The complex can have kinase and/or phosphatase activity. The grey letters in the enzymatic complex name correspond to the inactive enzymes. There are four variants of the extended model with enzymatic complex having both inactive K and P (b), active P and inactive K (c), active K and inactive P (d) and both active K and P (e).