Figure 4 . Effect of the flux enforcement of threonine deaminase (THRD) and succinate dehydrogenase (SUCD1m) reactions on the optimal flux distribution for SARS-CoV-2 production. The enforced flux boundaries were [8.94; 9.14] and [-0.58; 8.60] mmol∙gDW-1∙h-1 for THRD and SUCD1m respectively. (A) Distribution of the producing (‘in’) and consuming (‘out’) fluxes for arginine and isoleucine in the cytosol and succinate in the mitochondrial matrix. For each metabolite, the in and out fluxes have been computed in four different conditions; from left to right; unmodified model, flux enforcement of SUCD1m only, flux enforcement of THRD only, and flux enforcement of both SUCD1m and THRD. On the succinate panel, the dashed blue line corresponds to the upper boundaries to which SUCD1m is enforced (8.60 mmol∙gDW-1∙h-1). (B)Cartoon representation of the reaction network involving threonine and other metabolites, with reaction fluxes from normal and perturbation conditions colour-mapped onto reaction arrows. Metabolite notations used are: 2-oxobutanoate (2obut), 3-methyl-2-oxobutanoate (3mob), 4-methyl-2-oxopentanoate (4mop), and alpha-keto-glutarate (akg).