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Hexose monophosphate shunt
The hexose monophosphate (HMP) shunt is an alternative pathway to glycolysis for oxidizing glucose. Unlike glycolysis, it has selective tissue distribution and produces reducing equivalents like NADPH and pentose phosphates that are used for anabolic reactions rather than ATP. NADPH produced is used for biosynthesis of fatty acids, cholesterol, and other compounds. Ribose-5-phosphate produced is used for nucleotide and nucleic acid synthesis. The HMP shunt provides reducing power and pentose phosphates essential for biosynthesis in various tissues.
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Introduction to HMP Shunt and its significance in glucose metabolism by R.C. Gupta.
The HMP shunt is a glucose oxidation pathway, producing NADPH and ribose but no ATP, differing by tissue.
Alternative names for HMP shunt and its function in reductive synthesis, particularly via NADPH.
Oxidation and decarboxylation of hexose monophosphate, leading to ribulose-5-phosphate and glucose recycling.
Glucose-6-phosphate oxidation produces CO2 and NADPH, summarizing the outputs for both single and multiple glucose molecules.
Key tissues where the HMP shunt is active, including liver and adipose tissues.
Overview of the three basic phases of HMP shunt reactions: oxidation, decarboxylation, and regeneration.
Oxidative reactions details, including enzyme specifics and NADP involvement.
Decarboxylation generates ribulose-5-phosphate and potential isomerization among products.
Summary of overall oxidation reactions involving glucose-6-phosphate, illustrating inputs and outputs.
Regulatory mechanisms of glucose-6-phosphate dehydrogenase, crucial for HMP shunt regulation.
Importance of NADPH and ribose-5-phosphate production, their roles in cellular metabolism.
HMP shunt significance in lymphocytes for detoxifying reactive oxygen species via NADPH.
Consequences of glucose-6-phosphate dehydrogenase deficiency and related oxidative stress effects.
Common intermediates between HMP shunt and glycolysis promote dietary pentoses' entry into glycolysis.
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Hexose monophosphate shunt
- 1. Hexose Monophosphate (HMP) Shunt R.C. Gupta M.D. (Biochemistry) Jaipur, India
- 2. The main pathwayfor oxidation of glucose is glycolysis Hexose monophosphate shunt is an alternate pathway for oxidation of glucose Unlike glycolysis, HMP shunt has a selective tissue distribution
- 3. The proportion ofglucose oxidized via HMP shunt differs in different tissues The proportion depends upon the need for NADPH and ribose The requirements keep on changing
- 4. HMP shunt issaid to be an oxidative pathway but its role is anabolic NADPH and ribose produced in HMP shunt are used in anabolic reactions HMP shunt doesn’t produce any ATP
- 5. HMP shunt isalso known as: Pentose phosphate pathway Phosphogluconate oxidative pathway Direct oxidative pathway
- 6. Intracellular location ofHMP shunt is cytosol Oxidation of glucose occurs in the absence of oxygen in HMP shunt The reducing equivalents are taken up by NADP
- 7. Unlike NADH, NADPHcannot be oxidized in the respiratory chain NADPH is used mainly for reductive synthesis e.g. synthesis of fatty acids It has a number of other functions as well
- 8. Hexose monophosphate isoxidized as well as decarboxylated in HMP shunt Decarboxylation produces a pentose phosphate The pentose phosphate is ribulose-5- phosphate Ribulose-5-phosphate can be isomerized to ribose-5-phosphate
- 9. Ribose-5-phosphate can beused to form ribonucleotides Ribonucleotides can be converted into deoxyribonucleotides Thus, HMP shunt provides ribo- and deoxyribo-nucleotides for the synthesis of nucleic acids
- 10. After meeting theneed for nucleotides, some ribulose-5-phosphate may remain unutilized The unutilized ribulose-5-phosphate is reconverted into glucose-6-phosphate
- 11. Oxidation of onemolecule of glucose-6- phosphate in HMP shunt produces: One molecule of CO2 Two molecules of NADPH One molecule of pentose phosphate
- 12. Oxidation of sixmolecules of glucose-6- phosphate in HMP shunt produces: Six molecule of CO2 Twelve molecules of NADPH Six molecule of pentose phosphate Six pentose phosphate molecules can form five glucose-6-phosphate molecules
- 13. HMP shunt ispresent in: • Liver • Adipose tissue • Lactating mammary glands • Adrenal cortex • Testes • Thyroid gland • Erythrocytes
- 14. The reactions ofHMP shunt pathway may be divided into three phases: Oxidation of hexose monophosphate Decarboxylation of hexose monophosphate Regeneration of hexose monophosphate The second and third phase together are also called the non-oxidative phase
- 15. In the firstphase, two pairs of reducing equivalents are removed from glucose-6- phosphate Glucose-6-phosphate is first reduced to 6-phosphogluconolactone by glucose-6- phosphate dehydrogenase (G-6-PD) The reducing equivalents are accepted by NADP
- 16. G-6-PD is inhibitedby: Fava beans and Sulphonamides and quinacrine This can limit the availability of NADPH
- 17. A molecule ofwater is added to 6-phospho- gluconolactone by gluconolactone hydrolase The product is 6-phosphogluconate, a straight chain phosphorylated compound A pair of reducing equivalents is trans- ferred from 6-phosphogluconate to NADP The product is 3-keto-6-phosphogluconate
- 19. 3-Keto-6-phosphogluconate is then decarboxylated Thereaction is catalysed by 6- phosphogluconate dehydrogenase The product is a pentose sugar, ribulose-5-phosphate
- 20. Some of theribulose-5-phosphate molecules are isomerized to ribose-5- phosphate The reaction is catalysed by ribose-5- phosphate ketoisomerase Some ribulose-5-phosphate molecules are epimerized by ribulose-5-phosphate epimerase to xylulose-5-phosphate
- 22. The final phasebegins now The purpose of this phase is to reconvert unutilized ribulose-5- phosphate into glucose-6-phosphate
- 23. For reconversion, onemolecule of ribose-5- phosphate and two molecules of xylulose-5- phosphate react with each other By the end, two molecules of fructose-6- phosphate and one molecule of glyceraldehyde-3-phosphate are formed
- 24. To begin with,xylulose-5-phosphate reacts with ribose-5-phosphate A glycol aldehyde moiety is transferred from the former to the latter The former is converted into glyceraldehyde-3-phosphate and the latter into sedoheptulose-7-phosphate
- 26. A dihydroxyacetone moietyis then transferred from sedoheptulose-7-phos- phate to glyceraldehyde-3-phosphate The former is converted into erythrose-4- phosphate and the latter into fructose-6- phosphate
- 28. Erythrose-4-phosphate now receivesa glycol aldehyde moiety from another molecule of xylulose-5-phosphate The former is converted into fructose-6- phosphate and the latter into glyceraldehyde-3-phosphate
- 30. Thus, three moleculesof glucose-6- phosphate are first converted into three molecules of pentose-5-phosphate These three are converted into two mole- cules of fructose-6-phosphate and one molecule of glyceraldehyde-3-phosphate
- 31. Fructose-6-phosphate molecules areisomer- ised to two glucose-6-phosphate molecules Thus, out of three molecules of glucose-6- phosphate that entered the pathway: Two molecules of glucose-6-phosphate are regenerated The third is converted into glyceralde- hyde-3-phosphate
- 33. Three other moleculesof glucose-6- phosphate undergo the same sequence of reactions Two molecules of glucose-6-phosphate are regenerated; the third is converted into glyceraldehyde-3-phosphate
- 34. Thus, when sixmolecules of glucose-6- phosphate are oxidized in the HMP shunt: Four molecules of glucose-6-phosphate are regenerated Two molecules of glyceraldehyde-3- phosphate are formed
- 35. Out of twoglyceraldehyde-3-phosphate molecules, one is converted into dihydroxyacetone phosphate (DHAP) One glyceraldehyde-3-phosphate molecule and one DHAP molecule can form one glucose-6-phosphate molecule Thus, fifth glucose-6-phosphate molecule is regenerated
- 37. The overall reactionleading to the oxidation of six molecules of glucose-6- phosphate may be summed up as: 6 Glucose-6- +12 NADP+ + 7 H2O → 5 Glucose-6- + Pi + 6 CO2 + 12 NADPH + 12 H+
- 38. Glucose-6-phosphate dehydrogenase is theregulatory enzyme of HMP shunt It is regulated by induction and allosteric mechanism Its synthesis is induced by insulin NADP is its allosteric activator Regulation
- 39. Importance of HMPshunt Provision NADPH Production of ribose-5-phosphate Restoring the level of reduced glutathione in erythrocytes
- 40. An important functionof HMP shunt is to provide NADPH Complete oxidation of one molecule of glucose in HMP shunt produces twelve molecules of NADPH NADPH
- 41. NADPH is usedmainly for the synthesis of: Fatty acids Cholesterol Steroid hormones Some amino acids Nucleotides HMP shunt is highly active in tissues synthe- sizing these compounds
- 42. Ribose-5-phosphate is anintermediate of HMP shunt It is used for synthesizing nucleotides and nucleic acids Ribose-5-phosphate
- 43. In erythrocytes, glutathioneis oxidized to detoxify hydrogen peroxide and free radicals NADPH is required for restoring reduced glutathione This prevents premature destruction of erythrocytes Reduced glutathione
- 45. In hereditary glucose-6-phosphatedehydro- genase deficiency, haemolysis occurs due to decreased production of NADPH Drugs like quinacrine and primaquin, which increase oxidative stress, can precipitate haemolysis
- 46. Two intermediates arecommon to HMP shunt and glycolysis These are glyceraldehyde-3-phosphate and fructose-6-phosphate HMP shunt and glycolysis can interconnect through these two Due to this interconnection, dietary pentoses can enter glycolysis via HMP shunt