Figure 1.

Metabolic loops increase pathway multiplicity. (A) Pathway converting 1 mol of glucose to 2 mols of lactate and producing 2 mols of ATP. Reaction fluxes are represented in parentheses. Extracellular metabolites are defined by [e]. All other metabolites are in the cytosolic space. (B) Network loops and reactions that can hydrolyze ATP and transport protons across the cell membrane. These reactions and loops can be used to balance the reactions in (A), yielding multiple balanced pathways with the same set of backbone reactions