Figure 2.

Reference models for three signaling categories. (A i) Feedback model with bistability. (A ii) Multisite phosphorylation model that can be bistable or negative slope depending on parameters. (B i) Simple reaction scheme with positive slope. (ii) Zero-order ultrasensitive reaction (19) also has positive slope. For the bistable, negative slope, and positive slope categories, I used models A i, A ii, and B i, respectively. (C) Typical responses of intracompartmental reactions for bistable, negative slope, and positive slope systems. (D) Same responses as in C, showing dependence of M^∗ on Mtot. M^∗ is the modified form of the trafficked molecule. (E) Effects of flux on M versus Mtot curves for bistable, negative slope, and positive slope systems, respectively. Solid curve is reference, curves above it have reverse flux terms added, and curves below it have forward flux terms added. The categorization of the model does not change with flux except for the bistable model in Ei, which converts into a negative slope system for forward flux > 0.02 or backward flux > 0.5.