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PENTOSE PHOSPHATE PATHWAY (Hexose monophosphate pathway)

PENTOSE PHOSPHATE PATHWAY (Hexose monophosphate pathway). Learning objectives: List the two phases (oxidative and non-oxidative) of the pentose phosphate pathway List key enzymes involved in the pathway Glucose 6-phosphate dehydrogenase Transketolase Transaldolase

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PENTOSE PHOSPHATE PATHWAY (Hexose monophosphate pathway)

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  1. PENTOSE PHOSPHATE PATHWAY (Hexose monophosphate pathway) • Learning objectives: • List the two phases (oxidative and non-oxidative) of the • pentose phosphate pathway • List key enzymes involved in the pathway • Glucose 6-phosphate dehydrogenase • Transketolase • Transaldolase • List key compounds involved in the pathway: • Glucose 6-phosphate • NADPH • Ribose 5-phosphate • Glyceraldehyde 3-phosphate • Fructose 6-phosphate • Discuss functions of the pentose phosphate pathway in cell metabolism

  2. The cell needs “reducing power” for biosynthesis and protection against oxidative stress. The “reducing power” is NADPH :H- H H NAD+ NADP+ NADPH

  3. The cell needs ribose 5-phosphate for synthesis of nucleotides used in DNA, RNA, NAD+, NADP+, FAD, and Coenzyme A CH2OH Ι H - C – OH Ι H – C – OH Ι H – C – OH Ι CH2OPO32- Ribose 5-phosphate PO32--OH2 OH O H H H H OH OH Ribose 5-phosphate The pentose phosphate pathways provides both NADPH and ribose 5-phosphate

  4. + lactonase Phosphopentose epimerase CO2 Ribose 5-phosphate isomerase

  5. Glucose 6-phosphate dehydrogenase CH2OPO32- CH2OPO32- O O H H H H H + NADP+ O + NADPH + H+ OH H OH H OH OH OH H OH H OH Glucose 6-phosphate 6-Phosphoglucono-δ-lactone COO- Ι H – C – OH Ι HO – C – H Ι H – C – OH Ι H – C – OH Ι CH2OPO32- 6-Phosphogluconate CH2OPO32- O Lactonase H H O + H2O + H+ OH H OH H OH 6-Phosphoglucono-δ-lactone

  6. COO- Ι H – C – OH Ι HO – C – H Ι H – C – OH Ι H – C – OH Ι CH2OPO32- 6-Phosphogluconate CH2OH Ι C – O Ι H – C – OH + NADPH + CO2 Ι H – C – OH Ι CH2OPO32- Ribulose 5-phosphate 6-Phosphogluconate dehydrogenase + NADP+

  7. Stoichiometry for oxidative phase of pentose phosphate pathway Glucose 6-phosphate + 2 NADP+ + H2O → Ribulose 5-phosphate + 2 NADPH + 2 H+ + CO2

  8. CHO Ι H - C – OH Ι H – C – OH Ι H – C – OH Ι CH2OPO32- Ribose 5-phosphate Ribose 5-phosphate isomerase CH2OH Ι C – O Ι H – C – OH Ι H – C – OH Ι CH2OPO32- Ribulose 5-phosphate CH2OH Ι C – O Ι HO – C – H Ι H – C – OH Ι CH2OPO32- Xylulose 5-phosphate Phosphopentose epimerase

  9. CHO Ι H - C – OH Ι H – C – OH Ι H – C – OH Ι CH2OPO32- Ribose 5-phosphate CH2OH Ι C – O Ι HO – C – H Ι H - C – OH Ι H – C – OH Ι H – C – OH Ι CH2OPO32- Sedoheptulose 7-phosphate Transketolase + CH2OH Ι C – O Ι HO – C – H Ι H – C – OH Ι CH2OPO32- Xylulose 5-phosphate + CHO Ι H – C – OH Ι CH2OPO32- Glyceraldehyde 3-phosphate

  10. CHO Ι H – C – OH Ι H – C – OH Ι CH2OPO32- Erythrose 4-phosphate CH2OH Ι C – O Ι HO – C – H Ι H - C – OH Ι H – C – OH Ι H – C – OH Ι CH2OPO32- Sedoheptulose 7-phosphate + Transaldolase CH2OH Ι C – O Ι HO – C – H Ι H – C – OH Ι H – C – OH Ι CH2OPO32- Fructose 6-phosphate + CHO Ι H – C – OH Ι CH2OPO32- Glyceraldehyde 3-phosphate

  11. CHO Ι H – C – OH Ι H – C – OH Ι CH2OPO32- Erythrose 4-phosphate CH2OH Ι C – O Ι HO – C – H Ι H – C – OH Ι H – C – OH Ι CH2OPO32- Fructose 6-phosphate Transketolase + CH2OH Ι C – O Ι HO – C – H Ι H – C – OH Ι CH2OPO32- Xylulose 5-phosphate + CHO Ι H – C – OH Ι CH2OPO32- Glyceraldehyde 3-phosphate

  12. Xylulose 5-phosphate Fructose 6-phosphate Transketolase Xylulose 5-phosphate Sedoheptulose 7-phosphate Glyceraldehyde 3-phosphate Erythrose 4-phosphate Glyceraldehyde 3-phosphate Transaldolase Transketolase Ribose 5-phosphate Fructose 6-phosphate Stoichiometry for non-oxidative phase of pentose phosphate pathway 3 Pentose 5-phosphate 2 Fructose 6-phosphate + Glyceraldehyde 3-phosphate Stoichiometry for the whole pentose phosphate pathway 3 Glucose 6-phosphate + 6 NADP+ + 3 H2O → 2 Fructose 6-phosphate + Glyceraldehyde 3-phosphate + 6 NADPH + 6H+ + 3CO2

  13. Glucose 6-phosphate dehydrogenase CH2OPO32- CH2OPO32- O O H H H H H H + NADP+ O + NADPH + H+ OH H OH H OH OH OH H OH H OH Glucose 6-phosphate 6-Phosphoglucono-δ-lactone The first step in the pentose phosphate pathway is the main regulatory step of the pathway NADPH (Competitive inhibitor of the enzyme) Insulin (enhances expression of the enzyme leading to enhanced flux through pathway in the well-fed state) - +

  14. Different outcomes of the pentose phosphate pathway The needs for NADPH and ribose 5-phosphate are balanced: Glucose 6-phosphate + 2 NADP+ + H2O → Ribose 5-phosphate + 2 NADPH + 2 H+ + CO2 The cell needs more NADPH than ribose 5-phosphate: Glucose 6-phosphate + 12 NADP+ + 7H2O → 12 NADPH + 12 H+ + 6 CO2 + Pi The cell needs more ribose 5-phosphate than NADPH: 5 Glucose 6-phosphate + ATP → 6 Ribose 5-phosphate + ADP

  15. In addition to the pentose pathway, NADPH can be generated by the malic enzyme Malic enzyme + NADP+ + NADPH + H+ + CO2 Malate is generated in the mitochondria by the TCA (Krebs) cycle. Erythrocytes which lack mitochondria are thus particularly susceptible to oxidative damage Defective glucose 6-phosphate dehydrogenase is the most common enzymatic defect in humans affecting hundreds of millions of people. Hemolytic anemia can occur if the individual experience oxidative stress, e.g. when treated with oxidant drugs or contract a severe infection.

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