Coordinated switching. This figure illustrates another aspect related to the coupling of two switches through common upstream regulation. One of the switches (A) is a multiphosphorylation switch while the other (B) is a monophosphorylation switch. The response of the multiphosphorylation and monophosphorylation switches are shown in a) and b), respectively. The solid curves show their response to [X
] in isolation, while dashed curves show their response to [X
] when together. We see that the sharp switching of the multiphosphorylation switch can be "transmitted" to the monophosphorylation switch, resulting in coordinated switching at a well defined location. The reason for this is the sharp change in the uptake of enzyme by the multiphosphorylation switch, as the threshold is crossed, as shown in c), resulting in an increase in free enzyme. The response of the single phosphorylation switch does not exhibit this behaviour, as shown in d).