  The metabolic state of a cell, comprising fluxes, metabolite concentrations
and enzyme levels, is shaped by a compromise between metabolic benefit and
enzyme cost. This hypothesis and its consequences can be studied by
computational models and using a theory of metabolic value. In optimal
metabolic states, any increase of an enzyme level must improve the metabolic
performance to justify its own cost, so each active enzyme must contribute to
the cell's benefit by producing valuable products. This principle of value
production leads to variation rules that relate metabolic fluxes and reaction
elasticities to enzyme costs. Metabolic value theory provides a language to
describe this. It postulates a balance of local values, which I derive here
from concepts of metabolic control theory. Economic state variables, called
economic potentials and loads, describe how metabolites, reactions, and enzymes
contribute to metabolic performance. Economic potentials describe the indirect
value of metabolite production, while economic loads describe the indirect
value of metabolite concentrations. These economic variables, and others, are
linked by local balance equations. These laws for optimal metabolic states
define conditions for metabolic fluxes that hold for a wide range of rate laws.
To produce metabolic value, fluxes run from lower to higher economic
potentials, must be free of futile cycles, and satisfy a principle of minimal
weighted fluxes. Given an economical flux mode, one can systematically
construct kinetic models in which all enzymes have positive effects on
metabolic performance.
