Fig. 3

Structural transitions and regulations of the toy-model. Blue and red arrows are regulated enzymatic reactions and black arrows are constantly active enzymatic reactions. Light-gray arrows shows the remaining reactions that belong to the universe of reactions of Fig. 2, but not selected within the optimal metabolic network. Bold-lines illustrate the active reactions in each environmental condition. a Low switching parameters, r<0.25, result in an efficient network design that connects the actual present substrate to the biomass reaction using the most direct metabolic route. b Intermediate switching parameters, 0.25<r<0.5, shows a strong reduction of enzymes that are regulated and the emergence of a common permanently upregulated metabolic route. c High switching parameters, r>0.5, result in permanent upregulation of all enzymes and a network design of minimal size