Reactions of Pentose Phosphate Pathway

Starting from 6-molecules of glucose-6-phosphate, the various reactions of this pathway (Fig. 16.18) can be summarised as follows:

(1) 6 molecules of glucose-6-phosphate in the presence of coenzyme NADP are converted (oxidised) into 6 molecules of 6-phosphogluconolactone by the enzyme glucose-6-phosphate dehydrogenase. 6 molecules of NADP are reduced in the reaction which is reversible.

(2) 6-Phosphogluconolactone is hydrolysed by the enzyme Lactonase to produced 6 molecule of 6-phosphogluconic acid.

(3) 6-Phosphogluconic acid is oxidatively decarboxylated by the enzyme 6-Phosphogluconic acid dehydrogenase. 6 molecules of NADP are reduced, 6 molecules of CO2 are released and 6 mols, of Ribulose-5-Phosphate are produced.

(4) 6 mols. of Ribulose-5-P isomerise into 4 mols. of XyIuIose-5-Phosphate and 2 mols. of Ribose-5-Phosphate in the presence of Ribulose phosphate-3-epimerase and Pentose phosphate isomerase respectively.

(5) 2 mols. of xylulose-5-Phosphate and 2 mols. of Ribose-5-phosphate combine in the presence of Transketolase to form 2 mols. of Sedoheptulose-7-Phosphate and 2 mols. of 3- Phosphoglyceraldehyde.

(6) 2 mols. of Sedoheptulose-7-Phosphate and 2 mols. of 3-Phosphoglyceraldehyde combine in the presence of Transaldolase to form 2 mols. of Fructose-6-Phosphate and 2 mols. of Erythrose-4-Phosphate (4-carbon atoms sugar).

(7) 2 mols. of Erythose-4-Phosphate react with remaining two mols. of xylulose-5-Phosphate (see reaction No. 4 and 5) in the presence of Transketolase to form 2 mols. of Fructose- 6-Phosphate and 2 mols of 3-Phosphoglyceraldehyde.

(8) One mol. of 3-phosphoglyceraldehyde isomerises into dihydroxyacetone phosphate. The enzyme is Phosphotriose isomerase.

(9) Remaining one mole, of 3-Phosphoglyceraldehyde unites with Dihydroxyacetone phos­phate in presence of Aldolase to form one mol. of Fructose 1, 6-bisphosphate. The latter, in the presence of Phosphatase forms one mol. of Fructose 6-Phosphate.

(10) 5 molecules of Fructose-6-phosphate produced in reactions 6, 7 and 9, isomerise into 5 mols. of Glucose-6-P in presence of Phosphohexose isomerase.

To summarise, 6 mols. of Glucose-6-P which enter into this pathway after oxidation produce 6 mols. of CO2 (Reaction No. 3. CO2 comes from C-No. 1 of the glucose molecule) and 12 mols. of reduced coenzymes NADPH2 (reaction 1 and 3) while 5 mols of Glucose-6-Phosphate are regenerated (Reaction No. 10).

6 (Glucose-6-P) + 12 NADP+ → 5 (Glucose-6-P) + 12 (NADPH + H+) + 6 CO2

In other words one mol. of Glucose-6-P after oxidation produces 6-mols. of CO2 and 12 (NADPH + H+) molecules.

All the enzymes of pentose phosphate pathway are present in cytosol