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Lesson - 14 Pentose Phosphate Pathway (PPP or HMP pathway) Dr. P. K. Agrawal M.Sc., Ph.D., CSIR NET (URF), SRF, GATE, FIAZ Youtube Channel with over 1 million views 20 Years of Pre-medical teaching experience Ex Faculty, Allen Career Institute, Kota Visit following link to follow my profile https://unacademy.com/user/biologybypkagrawal-6909
PENTOSE PHOSPHATE PATHWAY (PPP) or Hexose Monophosphate Shunt (HMP) or (Warburg Dickens Pathway) or (Direct oxidation pathway.) or (Phosphogluconate pathway) Krebs' cycle is not the only mechanism by which glucose is oxidised. Cells also have alternative pathways .The pentose phosphate pathway is such an alternative route for the metabolism of glucose. It does not generate ATP but has 2 major functions (1) The generation of NADPH for the synthesis of fatty acids and steroid biosynthesis, and (2)The formation of ribose residues for nucleotide and nucleic acid biosynthesis. found both nrplanthanc aninwliy The pentose phosphate pathway (also called, hexose monophosphate shunt or HMP) is found both in plants and animals. hexose . The enzymes of the pentose phosphate pathway, as in glycolysis, are found in the cytoplasm, therefore this process occurs in cytoplasm.
It is a multi-cyclic process in which glucose-6-ph phosphogluconate' osphate is oxidised to 6- In this pathway, glucose-6-phosphate' is oxidised directly without entering the glycolysis. Hence this cycle is also called 'direct oxidation pathway'. The cycle begins with 6 molecules of 'glucose-6-phosphate'. Each one is oxidised to produce one molecule of '6-phosphogluconate'. Thus a total of 6 molecules of 6- phosphogluconate' are produced. In this reaction 6 molecules of NADP are reduced to [NADPH H']. The reaction occurs in the presence of enzyme glucose-6-P dehydrogenase. Glucose-6-P dehydrogenase 6 moles (glucose-6-phosphate) 6 moles (6-phosphogluconate) (1) 6[NADP] 6INADPH+H+
Now these 6 molecules of '6-phosphogluconate', are decarboxylated to produce 6 molecules of CO2and 6 molecules of 5-carbon sugars, called ribulose-5-phosphate. Phosphogluconicdehy drogenase 6 moles (6-phosphogluconate) 6 moles (ribulose-5-phosphate) 6 Co2 (2) The 6 molecules of ribulose-5-phosphate now regenerate 5 molecules of 'glucose-6-phosphate' by a complex series of reactions. Regeneration 6 moles (ribulose-5-phosphate) 5 moles (glucose-6-phosphate) One new molecule of glucose-6-phosphate is now added to the regenerated 5 molecules of glucose-6-phosphate' making the number again to 6. This helps to initiate a fresh cycle (i.e., cycle is repeated). Therefore from each cycle 6 molecules of CO2 are produced (indicating the complete breakdown of glucose) In the process 12 molecules of [NADPH H+] are produced (6 from 1st reaction and 6 from 2nd reaction).
Flow chart! 6-phosphate- (6C02 Glucose gluconate (Six mols.) INADP Glucose-6 (Six mols) phosphate 6 (NADPH+H] Glucose -6- phosphate (One mols.) Glucose-6 Regeneration Ribulose 5 phosphate phosphate (Six mols.) (Five mols.)
Energy production and significance Energy production . These [NADPH H+] can be used for many purposes. But as far as energy production is concerned, they can give 36 ATPs (3 from each, when transferred to ETS). Thus this pathway is almost as efficient as the 'glycolysis + Krebs' cycle. Significance . It provides an alternative route for the oxidation of carbohydrates and generates energy. . It produces many key intermediates, e.g., erythrose-4-phosphate, produced in this cycle, is used to produce many phenolic compounds, such as anthocyanins, lignin and shikimic acid etc. . The pathway also produces ribose-5-phosphate. This also an important intermediate, which is used for the biosynthesis of nucleotides and nucleic acids
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