Enzyme economy in metabolic networks

Metabolic systems are governed by a compromise between metabolic benefit and enzyme cost.This hypothesis and its consequences can be studied by kinetic models in which enzyme profiles arechosen by optimality principles. In enzyme-optimal states, active enzymes must provide benefits: ahigher enzyme level must provide a metabolic benefit to justify the additional enzyme cost. Thisentails general relations between metabolic fluxes, reaction elasticities, and enzyme costs, the lawsof metabolic economics. The laws can be formulated using economic potentials and loads, statevariables that quantify how metabolites, reactions, and enzymes affect the metabolic performance in asteady state. Economic balance equations link them to fluxes, reaction elasticities, and enzyme levelslocally in the network. Economically feasible fluxes must be free of futile cycles and must lead fromlower to higher economic potentials, just like thermodynamics makes them lead from higher to lowerchemical potentials. Metabolic economics provides algebraic conditions for economical fluxes, whichare independent of the underlying kinetic models. It justifies and extends the principle of minimalfluxes and shows how to construct kinetic models in enzyme-optimal states, where all enzymes have apositive influence on the metabolic performance.