Economic principles in cell metabolism
Metabolic systems are governed by compromises between metabolic benefit and enzyme cost. This hypothesis and its consequences can be studied by kinetic models in which enzyme profiles are predicted by optimality principles. In enzyme-optimised states, active enzymes must provide a benefit, that is, higher enzyme levels must improve the metabolic objective to justify the additional enzyme cost. This entails general relations between metabolic fluxes, reaction elasticities, and enzyme costs.
Metabolic economics in kinetic models
The laws of metabolic economics can be formulated using economic potentials and loads, economic state variables that quantify how the metabolic performance depends on metabolites, reactions, and enzymes in a steady state. Enzyme-optimised states can be characterised by balance equations that link fluxes, reaction elasticities, and enzyme levels to the adjacent economic potentials, loads, and enzyme costs in the network. Economical fluxes must be free of futile cycles and lead from lower to higher economic potentials, just like they need to lead towards lower chemical potentials according to by thermodynamics. In particular, flux distributions obtained from the principle of minimal fluxes are proven to be economical. Metabolic economics provides algebraic conditions for economical fluxes, independent of the underlying kinetic model. It justifies and extends the principle of minimal fluxes and shows how economical metabolic fluxes can be realised by kinetic models in which all enzymes exert a positive control on the metabolic objective.