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Characterization of problem metabolites inmetabolic networks. Metabolite A is defined as a root no-production metabolite because there is no-production or transport mechanism for it in the network. Metabolite C is a downstream no-production metabolite because, despite there being a reaction that produces it, it can carry no flux as A cannot be produced in the network. Equivalently, B and D are defined as root and downstream no-consumption metabolites.
