Fig. 1.

A sample of regulatory network architectures for a linear metabolic pathway. Metabolism can be regulated by simple mass action (A), by allosteric regulation of enzyme activity (B), or, typically, by a combination of allosteric regulation and transcriptional regulation of enzyme levels (C and D). Two classic examples of transcriptional regulatory architectures are the end product inhibition (EPI) network (C) and the intermediate metabolite activation (IMA) network (D). The two networks create different feedback structures.