Fig. 1.

Logic models approximate molecular interactions. The motivation behind logic models comes from the sigmoidal relationship observed between regulatory molecules and their target molecules. This relationship approximates a Boolean switch and can be thought of as having two states: saturated (ON) and non-saturated (OFF). (A) An inhibitor regulates a target molecule in a logic model: the target remains ON until the inhibitor’s activity surpasses a threshold of activation. (B) An activator regulates a target molecule in a logic model: the target remains OFF until the activator’s activity surpasses a threshold of activation. In these models, ON means the molecule has reached a threshold of functional activation that is high enough to affect the state of other molecule(s) it directly regulates, while OFF means the molecule is not present at a high enough level to affect the molecules it directly regulates. Critically, OFF does not necessarily mean the molecule has a zero concentration. (C) A standard hyperbolic saturation curve measuring reaction velocity, v, as substrate concentration, S, increases. K_0.5 is the substrate concentration needed to reach half the maximum velocity, V_max. (D) A plot of v/V_max vs. S/K_0.5 (the specific substrate concentration) takes the form of a sigmoidal saturation curve approximating a Boolean- ≪1 like switch. When S/K_0.5 1, the reaction is effectively OFF. When S/K_0.5is ≫1, the reaction is effectively ON.