Multiple functionalities of biochemical reaction networks

We consider a biological cell as a highly nterconnected network of chemical reactions, which is constituted of a large number of semi-autonomous functional modules. Depending on the global state of the network, the separate functional modules may display qualitatively different behavior. As an example, we study a conceptual network of phosphorylation cycles, for which the steady-state concentration of an output compound depends on the concentrations of two input enzymes. We show that the input-output relation depends on the expression of the proteins in the network. Hence changes in protein expression, due to changes in the global regulatory network of the cell, can change the functionality of the module. In this specific example, changed expression of two proteins is sufficient to switch between the functionalities of various logical gates.