Figure 4.

Retrobiosynthesis approaches for synthetic metabolic pathway design. Reaction rules (colored boxes) are applied to the target to obtain a set of intermediates to which the reaction rules are iteratively applied until a compound from the native metabolic network is reached. Metabolite X can be synthesized from F via two pathways, F→ Y₂ → X₁ → X and F→ Y₂ → X₂ → X. The two pathways are ranked based on different criteria, such as thermodynamics, concentration of metabolites, enzyme costs, and effects on native metabolism (e.g. production of metabolite C). The reactions marked in red are not feasible due to unfavorable thermodynamics.