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HMP shunt
The pentose phosphate pathway generates NADPH and pentose sugars. It has both an oxidative and non-oxidative phase. In the oxidative phase, glucose-6-phosphate is oxidized to produce NADPH. In the non-oxidative phase, 5-carbon sugars are converted into 3 and 6 carbon sugars through a series of isomerization, epimerization, and transketolase reactions. The pathway is important as it provides NADPH for biosynthetic reactions and pentose sugars for nucleotide synthesis. A defect in glucose-6-phosphate dehydrogenase can lead to insufficient NADPH and glutathione, resulting in hemolytic anemia due to oxidative damage of red blood cells.
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HMP shunt
- 1. Pentose Phosphate Pathway presentedby Abdulrahman H. Amer . Ph.D. Student , P.S. Medical College Sardar Patel University
- 2. pentose phosphate pathway •(also called Phosphogluconate Pathway, or Hexose Monophosphate Shunt) • This is an alternative pathway to glycolysis , it is shunted through this pathway, so it is known as the shunt pathway. • HMP shunt is more anabolic in nature.
- 3. pentose phosphate pathway •It is concerned with the biosynthesis of NADPH & pentose phosphate. • About 10% of glucose entering in this pathway/day. • The liver & RBC metabolise about 30% of glucose by this pathway.
- 4. Location of thepathway • It occurs in the cytosol. • The tissues such as liver, adipose tissue, adrenal gland, erythrocytes, testes & lactating mammary gland, are highly active in HMP shunt. • Minimal activity in muscle and brain . • Most of these tissues are involved in biosynthesis of fatty acids and steroids which are dependent on the supply of NADPH.
- 5. HMP shunt-unique multifunctional pathway •It starts with glucose 6-phosphate. • No ATP is directly utilized or produced in HMP shunt • It is multifunctional pathway, several inter convertible substances produced, which are proceed in different directions in the metabolic reactions
- 6. One fate ofG6P is the pentose pathway.
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- 10. the pentose phosphatepathway achieve • The pathway yields reducing potential in the form of NADPH to be used in anabolic reactions requiring electrons. • The pathway yields ribose 5-phosphate. –Nucleotide biosynthesis leading to: • DNA • RNA • Various cofactors.
- 11. Reactions of thepathway • Reactions of the pathway: • Divided into Two phases oxidative & non-oxidative. • Oxidative phase • Step:1 • Glucose 6- phosphate is oxidized by Glucose 6- phosphate dehydrogenase (G6PD), to formed 6-phosphogluconolactone • NADP+ act as co-enzyme is specific for this reaction. One molecule of NADPH is formed in this reaction and this is a rate limiting step. This reaction is irreversible
- 12. Glucose-6-phosphate Dehydrogenase catalyzes oxidationof the aldehyde at C1 of glucose-6- phosphate, to formed 6-phosphogluconolactone. NADP+ serves as electron acceptor. H O OH H OHH OH CH2OPO3 2 H H OH H O OH H OHH OH CH2OPO3 2 H O 23 4 5 6 1 1 6 5 4 3 2 C HC CH HC HC CH2OPO3 2 O O OH HO OH OH NADPH+H+ NADP+ H2O H+ 1 2 3 4 5 6 Glucose-6-phosphate Dehydrogenase 6-Phospho- glucono- lactonase glucose-6-phosphate 6-phoshogluconolactone 6-phosphogluconate
- 13. Regulatory enzyme This isthe rate limiting step, regulation is effected by Glucose 6- phosphate dehydrogenase Step:1
- 14. Oxidative phase • Step:2 •6-phosphogluconolactone is hydrolysed by glucono lactone hydrolase (gluconolactonase) to form 6- phosphogluconate, by addition of water molecule . • This reaction also is irreversible
- 15. 6-Phosphogluconolactonase catalyzes hydrolysisof 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,. H O OH H OHH OH CH2OPO3 2 H H OH H O OH H OHH OH CH2OPO3 2 H O 23 4 5 6 1 1 6 5 4 3 2 C HC CH HC HC CH2OPO3 2 O O OH HO OH OH NADPH+H+ NADP+ H2O H+ 1 2 3 4 5 6 Glucose-6-phosphate Dehydrogenase 6-Phospho- glucono- lactonase glucose-6-phosphate 6-phoshogluconolactone 6-phosphogluconate
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- 17. Oxidative phase • Step: 3 • The next reaction involving the synthesis of NADPH . • Oxidative decarboxylation of 6 phosphogluconate by 6 –phosphogluconate dehydrogenase to produce ketose sugar ,ribulose 5 – phosphate. Plus a molecule of NADPH. • All the above reaction are irreversible
- 18. Phosphogluconate Dehydrogenase catalyzesoxidative decarboxylation of 6-phosphogluconate, to yield ribulose- 5-phosphate. (5-C ketose) The OH at C3 is oxidized to a ketone in (C2 of product). This promotes loss of the carboxyl at C1 as CO2. NADP+ serves as oxidant. C HC CH HC HC CH2OPO3 2 O O OH HO OH OH 1 2 3 4 5 6 CH2OH C HC HC CH2OPO3 2 OH OH 1 2 3 4 5 O NADP+ NADPH + H+ CO2 Phosphogluconate Dehydrogenase 6-phosphogluconate ribulose-5-phosphate
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- 20. HMP-Shunt pathway oxidative phase Glucose6-phosphate 6-phosphoglucanolactone NADP+ NADPH + H+ Glucose 6P- dehydrogenase Mg+2 6-phosphogluconate gluconolactonase Ribulose 5-phosphate NADP+ CO2, NADPH + H+ 6 –phosphogluconate dehydrogenaseMg+2 H2O H+
- 21. Non-Oxidative Phase • Thesereactions take place in all the cells. • All these reactions are reversible in nature. • Step: 4 • The ribulose -5-phosphate is then isomerized to ribose -5-phosphate or epimerised to xylulose -5-phosphate
- 22. Epimerase inter-converts stereoisomers ribulose-5-P into xylulose-5-P. Isomerase converts theketose ribulose-5-P to the aldose ribose-5-P. Both reactions are reversible. C C C CH2OPO3 2 O OHH OHH CH2OH C C C CH2OPO3 2 O HHO OHH CH2OH C C C CH2OPO3 2 OH OHH OHH HC O H ribulose-5- phosphate xylulose-5- phosphate ribose-5- phosphate Epimerase Isomerase
- 23. Non-Oxidative Phase • Step:5 • Transketolase reaction • This reaction is catalyze by transketolase, which transfers two-carbon unit from xylulose 5- phosphate to ribose 5-phosphate to form a 7-carbon sugar, sedoheptulose 7-phosphate and glyceraldehyde 3- phosphate. • Thiamine pyrophosphate (TPP) a coenzyme for transketolase, serves as transient carrier of two carbon unite in this reaction
- 24. Transketolase transfersa 2-C fragment from xylulose-5- P to either ribose-5-P or erythrose-4-P. Transketolase utilizes as prosthetic group thiamine pyrophosphate (TPP). Pyruvate Dehydrogenase of Krebs Cycle also utilizes TPP as prosthetic group. C C C CH2OPO3 2 O HHO OHH CH2OH C C C CH2OPO3 2 OH OHH OHH HC O H C C C CH2OPO3 2 OH OHH OHH C H H HC C CH2OPO3 2 O OHH C CH2OH O HO + + xylulose- ribose- glyceraldehyde- sedoheptulose- 5-phosphate 5-phosphate 3-phosphate 7-phosphate Transketolase
- 25. Non-Oxidative Phase • Step:6 • Transaldolase reaction • Transaldolase brings about the transfer of a 3- carbon fragment from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate to give fructose 6- phosphate & 4 -carbon erythrose 4-phosphate.
- 26. Transaldolase catalyzes transferof a 3-C, from sedoheptulose-7-phosphate to glyceraldehyde-3-phosphate to formed fructose 6- phosphate. CH2OH C CH HC HC HC H2C OH OH OPO3 2 OH HO O HC HC HC H2C O OH OPO3 2 OH HC HC H2C O OPO3 2 OH H2C C CH HC HC H2C OH OPO3 2 OH OH HO O sedoheptulose- glyceraldehyde- erythrose- fructose- 7-phosphate 3-phosphate 4-phosphate 6-phosphate Transaldolase + +
- 27. Non-Oxidative Phase • Step:7 • Second transketolase Reaction • In another transketolase reaction a 2 – carbon unit is transferred from xylulose 5 – phosphate to erythrose 4 – phosphate to form fructose 6 – phosphate & glyceraldehyde 3 – phosphate. • Fructose 6 – phosphate & glyceraldehyde 3 – phosphate are directly enter to glycolysis.
- 29. The diagram at rightsummarizes flow of 15 C atoms through Pentose Phosphate Pathway reactions by which 5-C sugars are converted to 3-C and 6-C sugars. IS = Isomerase EP = Epimerase TK = Transketolase TA = Transaldolase (3) ribulose-5-P ribose-5-P (2) xylulose-5-P glyceraldehyde-3-P sedoheptulose 7 P fructose-6- P erythrose-4-P fructose-6-P glyceraldehyde-3-P IS EP TK TK TA
- 30. The balance sheetbelow summarizes flow of 15 C atoms through Pentose Phosphate Pathway reactions by which 5-C sugars are converted to 3-C and 6-C sugars. C5 + C5 C3 + C7 (Transketolase) C3 + C7 C6 + C4 (Transaldolase) C5 + C4 C6 + C3 (Transketolase) ____________________________ 3 C5 2 C6 + C3 (Overall) Glucose-6-phosphate may be regenerated from either the 3-C glyceraldehyde-3-phosphate or the 6- C fructose-6-phosphate, by enzymes of Gluconeogenesis
- 31. Non-Oxidative Phase • Step:8 • Two molecules of glyceraldehyde-3-phosphate formed in step7 are condensed to form one fructose-6-phosphate than converted to glucose-6-phosphate. (this step reversal of step 2 of glycolysis)
- 34. Regulation of HMPshunt pathway • The regulatory enzymes of HMP shunt pathway way are glucose-6-P dehydrogenase and 6-phosphogluconate dehydrogenase. – Synthesis of both the enzymes is Induced by insulin. – In fed state their intercellular concentrations rise , lead to enhanced oxidation of glucose through this pathway.
- 35. Regulation of HMPshunt pathway • The entry of glucose 6-phosphate into the pentose phosphate pathway is controlled by the cellular concentration of NADPH. • NADPH is a strong inhibitor of glucose 6-phosphate dehydrogenase in first reaction from oxidative phase. • nonoxidative phasecontrolled by the requirementof pentose. • The synthesis of glucose 6-phosphate dehydrogenase is induced by the increased insulin after a high carbohydrate meal.
- 36. Significance of HMPShunt • HMP shunt is unique in generating two important products- NADPH and pentoses . • Importance of pentoses: In HMP shunt, hexoses are converted into pentoses, the most important being ribose 5 – phosphate. • This pentose or its derivatives are useful for the synthesis of nucleic acids (DNA & RNA) • Many nucleotides such as ATP, NAD+& CoA
- 37. Importance of NADPH •Demand for NADPH • Biosynthetic pathways • FA synthesis (liver, adipose, mammary) • Cholesterol synthesis (liver) • Steroid hormone synthesis (adrenal, ovaries, testes) • NADPH is used in the synthesis of certain amino acids involving the enzyme glutamate dehydrogenase.
- 38. Significance of HMPShunt • Free radical Scavenging • The free radicals (super oxide, hydrogen peroxide) are continuously produced in all cells. • These will destroy DNA, proteins, fatty acids & all biomolecules & in turn cells are destroyed. • The free radicals are inactivated by the enzyme systems containing SOD, POD & glutathione reductase. • Reduced GSH is regenerated with the help of NADPH
- 39. A):Formation of reactiveintermediates form molecular oxygen B): Active of antioxidant enzymes
- 40. Significance of HMPShunt • Erythrocyte Membrane integrity • NADPH is required by the RBC to keep the glutathione in the reduced state. • In turn, reduced glutathione will detoxify the peroxides & free radicals formed within the RBC. • NADPH, glutathione & glutathione reductase together will preserve the integrity of RBC membrane.
- 41. • what happensif glucose 6-phosphate dehydrogenase is defective? • Insufficient production of NADPH. • Which translates into insufficient glutathione. • this Is a medical problem
- 42. Glutathione and Erythrocytes •GSH is extremely important particularly in the highly oxidizing environment of the red blood cell. • Mature RBCs have no mitochondria and are totally dependent on NADPH from the pentose phosphate pathway to regenerate GSH from GSSG by glutathione reductase. • In fact, as much as 10% of glucose consumption, by erythrocytes, is mediated by the pentose pathway.
- 43. Glutathione and Erythrocytes •Reduced glutathione(GSH) also detoxifies peroxides. • Cells with low levels of GSH are susceptible hemolysis. • Individuals with reduced GSH are subject to hemolysis. • This is often clinically seen as black urine under certain conditions.
- 44. In D-6-PD deficiency,activity impairs of the cell to form the NADPH that is essential for maintenance of G-SH . This result in a decrease in cellular detoxification of free radical and peroxides formed within the cell
- 45. Prevention of Met-Hemoglobinemia •GSH is essential for normal RBC structure and keeping hemoglobin in Fe++ state. • NADPH is also required to keep the iron of hemoglobin in the reduced (ferrous) state & to prevent the accumulation of met-hemoglobin. • Met-hemoglobin cannot carry the oxygen. • G6PD deficient persons will show increased Met – hemoglobin in circulation.
- 46. Detoxification of Drugs •Most of the drugs and other foreign substances are detoxified by the liver microsomal P450 enzymes, with the help of NADPH. • Detoxification of H2O2 is inhibited, and cellular damage results - lipid peroxidation leads to erythrocyte membrane breakdown and hemolytic anemia. • Most G6PD-deficient individuals are asymptomatic - only in combination with certain environmental factors (sulfa antibiotics, aspirin, antimalarial.
- 47. • Lens ofEye: • Maximum concentration of NADPH is seen in lens of eye. • NADPH is required for preserving the transparency of lens.
- 48. Macrophage bactericidal activity: •Macrophages and neutrophils are armed with both oxygen-independent and oxygen –dependent mechanisms for killing bacteria. oxygen-independent mechanism: in this mechanism use PH changes in phagolysosomes and lysosomal enzymes to destroy pathogens.
- 49. Macrophage bactericidal activity: •Oxygen- dependent system : NADPH is required for the production of reactive oxygen species (ROS) by macrophases to kill bacteria. • The NADPH oxidase and myeloperoxidase present in macrophages degrade micro-organisms. combined action as bactericidal.
- 50. reactive oxygen species(ROS) • The ingestion of oxidative agents that generate peroxides or reactive oxygen species (ROS). • Molecular oxygen and partially reduced, reactive forms of oxygen. • Reduction of molecular O2 in a series of one- electron steps yields superoxide, hydrogen peroxide, hydroxyl radical, and water. • The intermediate, activated forms of oxygen are known as reactive oxygen species (ROS)
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- 52. Role of NADPHand glutathione in protecting cells against ROS Role of NADPH and glutathione in protecting cells against highly reactive oxygen derivatives. Reduced glutathione (GSH) protects the cell by destroying hydrogen peroxide and hydroxyl free radicals. Regeneration of GSH from it oxidized form (GS-SG) requires the NADPH produced in the glucose 6-phosphate dehydrogenase reaction.
- 53. hemolytic anemia relatedG6PD deficiency. • The ingestion of oxidative agents that generate peroxides or reactive oxygen species (ROS). Antimalarials-primaquine purine glycoside from fava beans. • Individules with G6PD deficiency can not produce sufficient GSH to cope with the ROS. • Proteins become cross linked leading to Heinz body formation and cell lysis.
- 54. • Hb moleculesthen cross-link with one another to form aggregates called Heinz bodies on membranes. • Membranes damaged by the Heinz bodies & ROS become deformed & the cell undergos LYSIS Hemolytic anemia
- 55. • Availability ofRibose: Ribose & Deoxy -ribose are required for DNA & RNA synthesis. • Ribose is also necessary for nucleotide co-enzymes. • Reversal of non -oxidative phase is present in all tissues, by which ribose could be made available. • ATP ATP is neither utilized nor produced by the HMP shunt. • Cells do not use the shunt pathway for energy production.
- 56. • Another functionof NADPH is in the formation of nitric oxide from arginine, which causes vasodilation by smooth muscle relaxation, thereby reducing blood pressure. • Further, nitric oxide prevents the platelet aggregation and also acts as neurotransmitter.
- 57. formation of nitricoxide from arginine,and some of the actions of nitric oxide (NO)
- 58. FAVISM • Individuals withG6PD deficiency must not eat Fava beans. • The haemolytic effect of ingesting fava beans, is not observed in all individuals with G6PD deficiency , but all patients with favism have G6PD deficiency. • Erythrocytes lyse=dark or black urine.
- 59. G6PD deficiency &malaria • G6PD deficiency is associated with resistance to malaria (caused by plasmodium infection) • The parasite requires reduced glutathione for its survival, which will not be available in adequate amounts in deficiency of G6PD. • When exposed to certain drugs or toxins ,e.g, primaquin stimulates peroxide formation inside RBC. • In G-6-PD deficient the level of NADPH is low, hence further production of peroxides will lead to cell lysis.
- 60. Thiamine Deficiency • Thetransketolase activity is measured in RBCs is an index of the thiamine status of an individual. • The occurrence & manifestation of Wernickes korsakoffs syndrome (encephalopathy) which is seen in alcoholics & those with thiamine deficiency is due to a genetic defect in the enzyme transketolase. • The symptoms include mental disorder, loss of memory & partial paralysis.
- 61. Differences between glycolysisand HMP shunt glycolysis • Occurs in all tissues. • NAD+ is H+ acceptor. • ATP production. • CO2 is never formed HMP shunt • Occurs in certain special tissues. • NADP+ is H+ acceptor. • ATP is not production. • CO2 is produced
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