(A) A regulatory structure of a metabolic reaction with a metabolic flux. Metabolic flux through a reaction can be influenced by enzymes and metabolites as substrates, products, and cofactors. Post-translational modifications and allosteric regulations of enzymes can also regulate enzyme activities, which in turn influence the metabolic flux. (B) A trans-omic network across the transcript, enzyme, reaction, and metabolite layers. Metabolic flux can be represented as the size of nodes in the reaction layer. Solid lines represent the conversion of metabolites through reactions, while dashed lines represent regulations.