Hexose monophosphate shunt

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Hexose monophosphate shunt

  1. 1. Pentose Phosphate Pathway
  2. 2. Pentose Phosphate Pathway Other names: Phosphogluconate Pathway Hexose Monophosphate Shunt Direct oxidative pathway The linear part of the pathway carries out oxidation and decarboxylation of the 6-C sugar (glucose-6- P), producing the 5-C sugar (ribulose-5-P) and others
  3. 3. Glucose-6-phosphate 6-Phospho- O O− Dehydrogenase glucono- 1C 6CH2OPO32− 6 CH OPO 2− lactonase + 2 3 NADPH + H HC OHH 5 O OH NADP + H 5 O H2O H+ 2 H H HO 3CH 4 OH H 1 4 OH H O 1 HC OH 4OH H OH 3 2 3 2 HC OH 5 H OH H OH CH2OPO32− 6 glucose-6-phosphate 6-phoshogluconolactone 6-phosphogluconate Glucose-6-phosphate Dehydrogenase catalyzes oxidation at C1 of glucose-6-phosphate, to a carboxylic acid, in ester linkage (lactone) NADP+ serves as electron acceptor
  4. 4. Glucose-6-phosphate 6-Phospho- O O− Dehydrogenase glucono- 1C 6CH2OPO32− 6 CH OPO 2− lactonase + 2 3 NADPH + H HC OHH 5 O OH NADP + H 5 O H2O H+ 2 H H HO 3CH 4 OH H 1 4 OH H O 1 HC OH 4OH H OH 3 2 3 2 HC OH 5 H OH H OH CH2OPO32− 6 glucose-6-phosphate 6-phoshogluconolactone 6-phosphogluconate 6-Phosphogluconolactonase catalyzes hydrolysis of the ester linkage, resulting in ring opening The product is 6-phosphogluconate Although ring opening occurs in the absence of a catalyst, 6-Phosphogluconolactonase speeds up the reaction, decreasing the lifetime of the highly reactive and potentially toxic, 6-phosphogluconolactone
  5. 5. O O− Phosphogluconate 1C Dehydrogenase HC OH CH OH 2 NADP+ NADPH + H+ 1 2 HO 3CH C O 2 HC OH HC OH 4 3 HC OH CO2 HC OH 5 4 CH2OPO32− CH2OPO32− 6 5 6-phosphogluconate ribulose-5-phosphatePhosphogluconate Dehydrogenase catalyzesoxidative decarboxylation of 6-phosphogluconate, toyield the 5-C ketose ribulose-5-phosphateThe OH at C3 (C2 of product) is oxidized to a ketoneThis promotes loss of the carboxyl at C1 as CO2NADP+ serves as oxidant
  6. 6. CH2OHEpimerase inter- C Oconverts stereoisomers Epimerase HO C Hribulose-5-P andxylulose-5-P CH2OH H C OH CH2OPO32−Isomerase converts the C O xylulose-5-ketose ribulose-5-P to H C OH phosphatethe aldose ribose-5-P H C OH HC OBoth reactions are CH2OPO32− H C OHreversible ribulose-5- H C OH phosphate Isomerase H C OH CH2OPO32− ribose-5- phosphate
  7. 7. CH2OH Transketolase C O CH2OH HC O HO C H C O H C OH H C OH HO C H H C OH HC O H C OH H C OH + H C OH H C OH + H C OH CH2OPO32− CH2OPO32− CH2OPO32− CH2OPO32− xylulose- ribose- glyceraldehyde- sedoheptulose- 5-phosphate 5-phosphate 3-phosphate 7-phosphate Transketolase transfers a 2-C fragment from xylulose-5-P to either ribose-5-P or erythrose-4-P
  8. 8. CH2OH Transketolase C O CH2OH HC O HO C H C O H C OH H C OH HO C H H C OH HC O H C OH H C OH + H C OH H C OH + H C OH CH2OPO32− CH2OPO32− CH2OPO32− CH2OPO32− xylulose- ribose- glyceraldehyde- sedoheptulose- 5-phosphate 5-phosphate 3-phosphate 7-phosphate Transfer of the 2-C fragment to the 5-C aldose ribose-5-phosphate yields sedoheptulose-7-phosphate Transfer of the 2-C fragment instead to the 4-C aldose erythrose-4-phosphate yields fructose-6-phosphate
  9. 9. CH2OH Transaldolase C O H2C OH HO CH C O HC OH HC O HO CH HC OH HC O HC OH HC OH HC OH + HC OH HC OH + HC OH H2C OPO32− H2C OPO32− H2C OPO32− H2C OPO32− sedoheptulose- glyceraldehyde- erythrose- fructose- 7-phosphate 3-phosphate 4-phosphate 6-phosphateTransaldolase catalyzes transfer of a 3-Cdihydroxyacetone moiety, from sedoheptulose-7-phosphateto glyceraldehyde-3-phosphate

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