Illustration of hierarchical control. The lower part represents a metabolic supply–demand system, in which the supply is catalyzed by enzyme Es (or enzymes stemming from an operon). The upper part describes the synthesis of enzyme Es in process a and its degradation in process b. Full arrows represent chemical conversions. Dashed arrows represent allosteric influences or catalysis. ‘s’ and ‘d’ stand for ‘supply’ and ‘demand’, respectively.