A set of linear pathways often does not capture the full range of behaviors
of a metabolic network. The concept of 'elementary flux modes'
provides a mathematical tool to define and comprehensively describe all metabolic
routes that are both stoichiometrically and thermodynamically feasible for
a group of enzymes. We have used this concept to analyze the interplay between
the pentose phosphate pathway (PPP) and glycolysis. The set of elementary
modes for this system involves conventional glycolysis, a futile cycle, all
the modes of PPP function described in biochemistry textbooks, and additional
modes that are a priori equally entitled to pathway status. Applications include
maximizing product yield in amino acid and antibiotic synthesis, reconstruction
and consistency checks of metabolism from genome data, analysis of enzyme
deficiencies, and drug target identification in metabolic networks.
