Pentose Phosphate Pathway (PPP) | Respiration

In this article we will discuss about:- 1. Meaning of Pentose Phosphate Pathway (PPP) 2. Reactions of Pentose Phosphate Pathway (PPP) 3. Significance.

Meaning of Pentose Phosphate Pathway (PPP):

Major pathway for aerobic respiration of glucose is through glycolysis and Krebs cycle, however, an alternate pathway exists in many organisms. This pathway, which requires the presence of oxygen, is called pentose phosphate pathway (PPP) or hexose monophosphate shunt (HMS).

As shown in figure that reduced NADP is formed in the reactions forming 6-phosphogluconic acid and ribulose-5-R If the equivalent of a molecule of glucose is oxidised to CO₂ and Hp via this cyclic pathway (six turns of cycle), then 12 molecules of reduced NADP would be formed. In the presence of enzyme transhydrogenase the hydrogens of NADPH can be transferred to NAD to form NADH.

Hence the formation of 12 molecules of reduced NADP via hexose monophosphate shunt ultimately can lead to synthesis of 36 molecules of ATP. Thus, capture of energy released in oxidation of glucose via this pathway (hexose monophosphate shunt) is as effective as that of glycolytic-Krebs cycle pathway.

This pathway (PPP) is also known as Warburg-Limpam-Dlckens cycle and phosphogluconate shunt. This was first studied by Warburg (1935) and Dickens (1938). This pathway occurs within the cytosol, where all enzymes of pentose phosphate pathway (PPP) are present.

Reactions of Pentose Phosphate Pathway (PPP):

Starting from 6 molecules of glucose 6-phosphate, the various reactions of PPP are as follows:

a. 6 molecules of glucose-6-phosphate in the presence of coenzyme NADP are 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.

b. 6-phosphogluconolactone is hydrolysed by the enzyme lactonase to produce 6 molecules of 6-phosphogluconic acid.

c. Phosphogluconic acid is oxidatively decarboxylated by the enzyme 6-phosphogluconic acid dehydrogenase. 6 molecule of NADP are reduced, 6 molecule of CO₂ are released, and 6 molecule of ribulose-5-phosphate are produced.

d. 6 molecule ob ribulose-5-phosphate isomerise into 4 molecule of xylulose-5-phosphate and 2 molecule of ribose-5-phosphate in the presence of enzymes ribulose phosphate-3-epimerase and pentose phosphate isomeras, respectively.

e. 2 molecule of xylulose-5-phosphate and 2 molecule of ribose-5-phosphate combine In the presence of the enzyme transketolase to form 2 molecule of sedoheptulose-7-phosphate and 2 molecule of phosphoglyceraldehyde.

f. 2 molecule of sedoheptulose-7-phosphate and 2 molecule of 3-phosphoglyceraldehyde combine in the presence of enzyme transketolase to form 2 molecules of fructose-6-phosphate and 2 molecule of erythrose-4-phoshate.

g. 2 molecules of erythrose-4-phoshate reacts with remaining 2 molecules of xylulose-5-phosphate ( reaction 4 and 5 ) In the presence of transketolase to form 2 molecules of fructose-6-phosphate and 2 molecule of 3-phosphoglyceraldehyde.

h. One molecule of phosphoglyceraldehyde isomerise into dihydroxyacetone phosphate, in the presence of enzyme phosphotriose isomerase.

i. Remaining one molecule of 3-phosphoglyceraldehyde unites with dihydroxyacetone phosphate In of enzyme aldolase to form one molecule of fructose 1, 6-diphosphate, which in the presence of phosphatase forms one molecule of fructose-6-phosphate.

j. 5molecule of fructose-6-phosphate produced in reaction 6, 7 and 9, isomerise into molecules of glucose-6-phosphate in presence of enzyme phosphohexose isomerase.

To summarise, 6 molecules of glucose-6-phosphate which enter into this pathway, produce 6 molecules of CO₂, after oxidation, and 12 molecules of reduced coenzyme NADPH₂, while 5 molecules of glucose-6-phosphate are regenerated.

6 glucose-6-phosphate + 12 NADP⁺ → 5-glucose-6-phosphate + 12 NADPH₂ + 6 CO₂

Complete oxidation of a molecule of glucose produces 12 molecules of NADPH₂, which is equal to 36 ATP molecules. This capture of energy released in oxidation of glucose via this pathway (PPP) is as effective as that of glycolytic-Krebs cycle pathway, where 38 ATP molecules per glucose molecule are produced.

Significance of Pentose Phosphate Pathway (PPP):

a. This pathway provides alternative route for carbohydrate degradation.

b. This pathway (PPP) generates NADPH² molecules which are used as reductants in biosynthetic processes under conditions when NADPH molecules are not generated by Fructose-6-p photosynthesis. It is, therefore, important in non-photosynthetic tissues, such as differentiating tissues, germinating seeds and during period of darkness.

Production of NADPH is not linked to ATP generation in pentose phosphate pathway (PPP).

c. It produces ribose sugars for synthesis of nucleic acids.

d. It plays an important role in fixation of CO₂ in photosynthesis through ribulose-5- phosphate; ribulose 1, 5-phosphate derived from ribulose-5-phosphate is the primary acceptor of CO₂ in photosynthesis.

e. It provides erythrose-4-phosphate, which is required for synthesis of shikimic acid. The latter is precursor of aromatic ring compounds.

f. It produces a number of tetroses and pentoses for synthesis of nucleosides, nucleotides, nucleic acids, anthocyanins and other compounds.