Figure 3

Reminder of the complexity of the control of the activity of an enzyme. In the bottom metabolic pathway, the generic metabolite γ can be: (a) a positive- or negative-feedback effector of the generic enzyme E₅₀₀₀; (b) a positive- or negative-feedforward effector of the generic enzyme E_5000+k; (c) a product inhibitor or homotropic effector of the enzyme that catalyzes its production; (d) a positive or negative effector of an enzyme that catalyzes a chemically 'distant' (unrelated, non-precursor chemical structures) reaction in another pathway'; and (e) a product affecting the transcription of a gene and/or its translation to a mature enzyme that is properly transferred to its 'correct' cellular compartment. The generic enzyme E₁₀₀ affects other reactions: (f) by protein-protein interactions, as a macromolecular effector; and (g) through entry into the nucleus and affecting DNA transcription, or, in the cytoplasm, messenger RNA translation into protein. External effectors (H), such as H⁺ ions, hormones, or xenobiotics, can interact with one of more enzymes and metabolites to influence the flux through one or more metabolic pathways.