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Ultrasensitivity in Phosphorylation-Dephosphorylation Cycles with Little Substrate

Figure 5

Allostery is not required for ultrasensitivity and models with steric hindrance where the binding of one enzyme inhibits binding of the other can be highly ultrasensitive.

(A) A model with steric hindrance. The kinase (red) and the phosphatase (blue) bind either to the same docking site or to different docking sites and block the access of the other enzyme to its docking site. The two sinks, and , are shown with shadows. The non-distributive catalytic rates (dashed grey arrows) were not allowed in this model. (B) Simulation of the response curve (in black) and the concentration of the states of the system (coloured bars) as a function of the signal, i.e., the ratio of phosphatase to kinase for a system, with , , s⁻¹, s⁻¹ (in units of the inverse total concentration of the substrate), s⁻¹, s⁻¹, s⁻¹. The Hill number is approximately 10.

doi: https://doi.org/10.1371/journal.pcbi.1003175.g005