Hexose Monophosphate (HMP) Shunt

1. HexoseMonophosphate (HMP) Shunt

2. PENTOSE PHOSPHATE PATHWAY (PPP); HEXOSE MONOPHOSPHATE (HMP) SHUNT Definition: • The hexosemonophosphate (HMP) shunt or pentose phosphate pathway (PPP) is an alternative pathway for the metabolism of glucose. It does not generate ATP but produces NADPH and pentose-5-phosphates and other sugar-phosphates.

3. Location HMP shunt is localized in the cytosol of the following:(1) RBC’s and WBC’s.(2) Adipose tissue.(3) Liver.(4) Adrenal glands.(5) Lens of the eye.(6) Retina.(7) Lactating mammary glands.(8) Testes & ovaries

4. Importance of HMP shunt 1. Formation of NADPH + H+:which is required for:a) Synthesis of fatty acids (lipogenesis).b) Synthesis of cholesterol and other steroids.c) Protecting RBCs wall against oxidation by keeping glutathione in the reduced form.d) Keeps Hb iron in the ferrous (Fe2+) state, not to be converted to met- hemoglobin. c) synthesis of NOe) Monoxygenases (hydroxylases), as it takes part in the process of mixed function oxidation (MFO).

5. 2. Formation of Pentoses:Xylulose, ribulose, and ribose. Pentoses (e.g., ribose & deoxyribose) are essential components of nucleic acids and nucleotides. 3. Energy production:HMP shunt is the source of energy in lens and retina; one glucose molecule gives 2 NADPH + H+; (equivalent to 6 ATP).

6. Pentose Phosphate Pathway (Hexose Shunt or Phosphogluconate) Part I Oxidative steps. The pentose phosphate pathway. The numerals in the blue circles indicate the steps discussed in the text.

7. 22.6 Pentose Phosphate Pathway (Hexose Shunt or Phosphogluconate) The pentose phosphate pathway. Part II Rearrangement of sugars.

8. The Oxidative Steps of the Pentose Phosphate Pathway The glucose-6-phosphate dehydrogenase reaction is the committed step in the pentose phosphate pathway.

9. Utilization of Glucose-6-P Depends on the Cell’s Need for ATP, NADPH, and Rib-5-P • Glucose can be a substrate either for glycolysis or for the pentose phosphate pathway. • The choice depends on the relative needs of the cell for biosynthesis and for energy from metabolism. • ATP can be made if G-6-P is sent to glycolysis • Or, if NADPH or ribose-5-P are needed for biosynthesis, G-6-P can be directed to the pentose phosphate pathway. • Depending on these relative needs, the reactions of glycolysis and the pentose phosphate pathway can be combined in four principal ways.

10. Four Ways to Combine the Reactions of Glycolysis and Pentose Phosphate • When both Ribose-5-P and NADPH are needed by the cell: • Enter at G-6-P. The first four reactions of the pentose phosphate pathway predominate. • NADPH is produced and ribose-5-P is the principal product of carbon metabolism. 2) When more Ribose-5-P than NADPH is needed by the cell: • Enter at F-6-P and use the nonoxidative steps in reverse to make ribose-5-P. Enter at G-6-P to make NADPH as needed and some R-5-P.

11. Four Ways to Combine the Reactions of Glycolysis and Pentose Phosphate Case 1: Both ribose-5-P and NADPH are needed When biosynthetic demands dictate, the first four reactions of the pentose phosphate pathway predominate and the principal products are ribose-5-P and NADPH.

12. Four Ways to Combine the Reactions of Glycolysis and Pentose Phosphate 3) More NADPH than ribose-5-P is needed by the cell: • Enter at G-6-P. The excess ribose-5-P produced in the HMS continues on to produce glycolytic intermediates. 4) Both NADPH and ATP are needed by the cell, but ribose-5-P is not. • This can be done by recycling ribose-5-P, as in case 3 above. Fructose-6-P and glyceraldehyde-3-P made in this way proceed through glycolysis to produce ATP and pyruvate, and pyruvate continues through the TCA cycle to make more ATP.

14. Importance of HMP shunt 1. Formation of NADPH + H+:which is required for:a) Synthesis of fatty acids (lipogenesis).b) Synthesis of cholesterol and other steroids.c) Protecting RBCs wall against oxidation by keeping glutathione in the reduced form. c) synthesis of NOe) Monoxygenases (hydroxylases), as it takes part in the process of mixed function oxidation (MFO).

17. Importance of HMP shunt 1. Formation of NADPH + H+:which is required for:a) Synthesis of fatty acids (lipogenesis).b) Synthesis of cholesterol and other steroids.c) Protecting RBCs wall against oxidation by keeping glutathione in the reduced form. c) synthesis of NOe) Monoxygenases (hydroxylases), as it takes part in the process of mixed function oxidation (MFO).

20. Phagocytosis and the oxygen dependent pathway of microbial killing. IgG = the antibody immunoglobulin G.

21. Importance of HMP shunt 1. Formation of NADPH + H+:which is required for:a) Synthesis of fatty acids (lipogenesis).b) Synthesis of cholesterol and other steroids.c) Protecting RBCs wall against oxidation by keeping glutathione in the reduced form. c) synthesis of NOe) Monoxygenases (hydroxylases), as it takes part in the process of mixed function oxidation (MFO).

24. Importance of HMP shunt 1. Formation of NADPH + H+:which is required for:a) Synthesis of fatty acids (lipogenesis).b) Synthesis of cholesterol and other steroids.c) Protecting RBCs wall against oxidation by keeping glutathione in the reduced form. c) synthesis of NOe) Monoxygenases (hydroxylases), as it takes part in the process of mixed function oxidation (MFO).

25. General reaction

26. Cytochrome P450 monooxygenase system • Xenobiotics are chemical compounds that do not belong to the normal composition of the human body. These compounds enter the body via the diet, air and medication. The principal route of elimination of xenobiotics from the body is biotransformation. They are eliminated by microsomal phase I and microsomal and cytosolic phase II drugmetabolising enzymes. These enzymes add functional groups to make lipophilic molecules more hydrophilic and hence easier to eliminate. The oxidative reactions are mainly catalysed by cytochrome P450 (CYP or P450) enzymes. The CYP superfamily of microsomal hemoproteins catalyses the monooxygenation of a large number of endogenous and exogenous compounds. They play a key role in the metabolism of a wide variety of xenobiotics, such as drugs, pesticides and (pre)carcinogens.

27. Glucose 6-P dehydrogenase deficiency • G6PD deficiency is an inherited disease characterized by hemolytic anemia caused by the inability to detoxify oxidizing agent • G6PD deficiency is the most common disease-producing enz abnormality in humans, affecting > 200 million individuals worldwide. • This deficiency has the highest prevalence in the Middle East, tropical Africa & Asia, & parts of the Mediterranean • G6PD deficiency is X-linked, & is in fact, a family of deficiencies caused by > 400 different mutations in the gene coding for G6PD. Only some of these mutations cause clinical symptoms

28. Properties of the variant enzymes • Almost all G6PD variants are caused by point mutations in the G6PD gene. • Some mutations do not disrupt the structure of the enz’s active site &, hence, do not affect enzymic activity • However, many mutant enz’s show altered kinetic properties. E.g., variant enz’s may show decreased catalytic activity, decreased stability, or an alteration of binding affinity for NADP+, NADPH, or G-6-P • Severity of disease usually correlates with amount of residual enz activity in patients’ RBCs. E.g., variants can be classified as:

29. Manifestations • The defect is manifested as red cell hemolysis (hemolytic anemia) when susceptible individuals are subjected to oxidants, such as: (a) the antimalarial primaquine (b) Aspirin (c) Sulphonamides (d) eating fava beans (Vicia fava – hence the term favism).

30. Figure 13.12 Classification of G6PD deficiency variants.

31. Mechanism Normally, RBCs HMP shunt provides NADPH + H+ for reduction of oxidized glutathione (GSS-G) to reduced glutathione (2 GSH) catalyzed by glutathione reductase:Glutathione G-SS-G + NADPH + H+ --- 2 G-SH + NADP+ Reductase

32. Reduced glutathione (G-SH) removes H2O2 from RBC’s in a reaction catalyzed by glutathione peroxidase: Glutathione peroxidase 2 GSH + H2O2 ------------------- G-SS-G + + Selenium H2O

33. Heinz bodies in erythrocytes of patient with G6PD deficiency.

34. 5 yr old boy presents to the emergency room: febrile, pale, tachycardic, tachypneic and minimally responsive AM: good health PM: abdominal pain, headache, fever by late evening: tachypneic and incoherent Lab tests: massive nonimmuneintravasuclarhemolysis and hemoglobinurea The patient is of Greek ethnicity. Mother notes that although there is no family history of hemolysis, she has some European cousins with a ‘blood problem’ She later recalls that her son had been eating fava beans in the garden while she worked in the yard