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

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

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 H – C – OH 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

+ lactonase Phosphopentose epimerase CO2 Ribose 5-phosphate isomerase

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 HO – C – H CH2OPO32- 6-Phosphogluconate CH2OPO32- O Lactonase H H O + H2O + H+ OH H OH H OH 6-Phosphoglucono-δ-lactone

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

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

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

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

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

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

Stoichiometry for non-oxidative phase of pentose phosphate pathway 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

- + Glucose 6-phosphate dehydrogenase CH2OPO32- CH2OPO32- O O 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) - +

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

In addition to the pentose pathway, NADPH can be generated by the 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.