Application of FOCAL to couple SSALx with biomass production. (a) The putrescine degradation pathway. FOCAL predicts that the NAD-dependent succinic semialdehyde dehydrogenase (SSALx) reaction is coupled with biomass by removing gabD (whose gene product catalyzes SSALy) and growing the mutant in minimal media with putrescine and NH4+. (b) The feasible region for the ΔgabD mutant under this condition excludes points that lie on the x-axis with the exception of the origin (right), while the wild-type can grow on glucose without flux through SSALx (left). Metabolite abbreviations not reported in the text: α-kg, α-ketoglutarate; glu-L, L-glutamate.