Metabolic pathway/metabolite flow regulation

Metabolite flow along a metabolic pathway is mainly determined by the activities of the enzymes involved (see p. 88). To regulate the pathway, it is suf cient to change the activity of the enzyme that catalyzes the slowest step in the reaction chain. Most metabolic pathways have key enzymes of this type on which the regulatory mechanisms operate. The activity of key enzymes is regulated at three independent levels ... 

Modulation by ligands. An important variable that regulates flow through a metabolic pathway is precursor availability (metabolite A in the case shown here). The availability of precursor A increases along with the activity of the metabolic pathways that form A (3) and it decreases with increasing activity of other pathways that also consume A (4). Transport from one cell compartment to another can also restrict the availability of A. 

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