Fig. 2.

Generic reaction networks that result in a downstream signal (output). (A) A⇋A^∗, a reaction involving a state change of A to its activated homologue A^∗. The forward rates can be either allosterically enhanced by the presence of the output k_f(A^∗) or modulated by the concentration of an effector molecule or enzyme k_f(B). The reverse rate is k_b. (B) A⇋A²⇋…Aⁿ, where the species Aⁿ alone gives rise to a downstream signal. Such a higher-order reaction acts a signal integrator. (C) A + B → AB, where A forms a complex with B, and B⇋B²⇋…B^m, where B gets cross-linked to each other via a multivalent antibody. Once the cross-linked aggregate reaches a size m, it loses its ability to complex with A. Such a higher-order reaction leads to sorting. (D) B⇋B^∗, A + B^∗⇋AB^∗ the inactive cytoplasmic molecule B gets activated to B^∗, a membrane bound molecule whose interaction with the cell surface signaling receptor A produces a downstream signal. The activation is typically enzymatically catalyzed.