Differences between reaction deletions and gene deletions. (A) Gene to protein to reaction (GPR) association for transketolase reactions (TKT1 and TKT2) in E. coli. (B) Solution spaces for wild-type (blue) and TKT1-PTAr reaction deletion strain (green) predicted by the metabolic model. Ethanol production is coupled to cellular growth at an improved rate in the mutant strain. (C) When the corresponding genes (tktA, tktB, pta, and eutD) are deleted to eliminate TKT1 and PTAr, no growth is predicted for the mutant strain as indicated by the solution space only residing on the y-axis.