Figure 1.

The 2-Kinase/1-Phosphatase loop. (A) Two independent kinases (K₁ and K₂) phosphorylate their respective substrates (S₁ and S₂). The solid circles indicate the phosphorylation of the substrates. A single shared phosphatase, P, dephosphorylates both substrates. (B) The fraction of phosphorylated S₁ (denoted as S₁^∗) as a function of response parameter r₁ when r₂ = 0 (black) and r₂ = 2 (red). r₁ and r₂ represent the ratio of the maximum velocity of the respective kinase to the maximum velocity of the phosphatase and are the dominant response parameters for the system. The initial concentrations of both S₁ and S₂ are set at 10 μM, K_M,P,1 at 1 mM, and K_M,P,2 at 1 μM. As such, S₁ does not saturate either the kinase K₁ or phosphatase, whereas S₂ saturates K₂ and P. Note the increase in phosphorylated S₁ in response to activation of the second loop, as described previously for this motif (2). (C) The fold change in S₁^∗ as a function of K_M,P,1. The fold change in S₁^∗ is calculated as the fraction S₁^∗ at r₂ = 2 divided by the fraction S₁^∗ at r₂ = 0. The dotted green line represents no change in S₁^∗. To see this figure in color, go online.