Figure 4

Essential pathways in pentose phosphate metabolism. The figure shows the two essential pathways (orange and green) in pentose phosphate metabolism that are necessary for the synthesis of biomass precursors e4p and r5p (in square boxes). Reactions in black are essential regardless of the metabolism in which they occur. Reactions catalyzed by G6PDH, PGL and GND form an essential pathway (orange), while reactions catalyzed by TALA and TKT1 (green) form another essential pathway. If the reactions in orange are absent, the reactions in green become essential and vice versa. This is because removal of TALA and TKT1 requires the synthesis of r5p through the reactions catalyzed by G6PDH, PGL and GND, while removal of these reactions forces the synthesis of r5p through TAL and TKT1. Note that metabolites g6p, f6p and g3p also participate in glycolysis and therefore can be produced there and supplied to the pentose phosphate pathway. Enzymes catalyzing each of the reactions are shown in uppercase italic typeface. Abbreviations - g6p, D-glucose-6-phosphate; r5p, D-ribose-5-phosphate; e4p, D-erythrose-4-phosphate; f6p, D-fructose-6-phosphate; fdp, fructose-diphosphate; g3p, glyceraldehyde-3-phosphate; G6PDH, glucose-6-phosphate dehydrogenase; PGL, 6-phosphogluconolactonase; GND, 6-phosphogluconate dehydrogenase; RPI, ribose-5-phosphate isomerase; RPE, ribose-5-phosphate 3-epimerase; TKT1, transketolase 1; TALA, transaldolase; TKT2, transketolase 2.