Pentose Phosphate Pathway and Nicotinamide Adenine Dinucleotide Phosphate

Question 1

NADPH can be used for:

1) Cellular respiration
2) Synthesis of fructose
3) Synthesis of lactose

Answer 1

ONLY 1 and 2

Question 2

Where does the pentose phosphate pathway (or hexose monophosphate shunt) occur?

Answer 2

In cytosol

Question 3

What does the PPP include?

Answer 3

3 irreversible oxidative reactions, followed by a series of reversible sugar interconversions

• No ATP consumed/produced
• C1 of G-6-P released as CO2

Question 4

Why would the PPP be taken?

Answer 4

1) You are in need of NADPH (2 NADPHs are produced)

2) You are in need of ribose (for nucleotides)

Question 5

What does the oxidative portion of the PPP consist of?

Answer 5

3 irreversible reactions that lead to formation of ribulose 5-phosphate, CO2, and 2 NADPH for each G-6-phosphate

• Important in the liver, lactating mammary glands, and adipose tissue for the NADPH-dependent biosynthesis of fatty acids
• Also important in the testes, ovaries, placenta, adrenal cortex for the NADPH-dependent biosynthesis of steroid hormones
• And important in RBCs for the NADPH to keep glutathione reduced

Question 6

What is the 1st irreversible oxidative reaction?

Answer 6

Dehydrogenation of glucose 6-phosphate by GLUCOSE 6-PHOSPHATE DEHYDROGENASE (G6PD)

• Irreversible, committed and main regulated step of PPP
• NADPH is a potent competitive inhibitor of G6PD
• Insulin upregulates G6PD expression

Question 7

What is the 2nd irreversible oxidative reaction?

Answer 7

Formation of ribulose 5-phosphate: 6-Phosphogluconolactone hydrolyzed by 6-phosphogluconolactone hydrolase
-Irreversible and not rate limiting

Question 8

What is the 3rd irreversible oxidative reaction?

Answer 8

Oxidative decarboxylation by 6-phosphogluconate dehydrogenase
-Irreversible reaction produces a pentose sugar-phosphate (ribulose 5-phosphate), CO2 (from C1 of glucose), and second molecule of NADPH

Question 9

What do the reversible nonoxidative reactions catalyze?

Answer 9

The interconversion of sugars containing 3-7 carbons

• Permit ribulose 5-P to be converted either to ribose 5-P (nucleotide synthesis) or intermediates of glycolysis (fructose 6-P -> G6P and glyceraldehyde 3-P)
• Occur in all cell types synthesizing nucleotides and nucleic acids

Question 10

How many carbon units does a transketolase transfer?

Answer 10

2

Question 11

How many carbon units does a transaldolase transfer?

Answer 11

3

Question 12

How does NADPH differ from NADH?

Answer 12

Only by phosphate on ribose

-High-energy molecule destined for reductive biosynthesis, rather than electron for transfer to oxygen

Question 13

What are 5 uses of NADPH?

Answer 13

1) Reductive biosynthesis
2) Reduction of H2O2
3) Cytochrome P450 monooxygenase system
4) Phagocytosis by white blood cells
5) Synthesis of nitric oxide

Question 14

How is NADPH used in reactive biosynthesis?

Answer 14

Fatty acids and chain elongation, cholesterol and steroids, NTs, nucleotides, superoxide

Question 15

How is NADPH used in cytochrome P450 monooxygenase system?

Answer 15

Biosynthesis of steroid hormones, detoxification of foreign compounds (xenobiotics), alcohol

Question 16

How is NADPH used in phagocytosis by WBCs?

Answer 16

• Receptor-mediated endocytosis of microorganisms, foreign particles, and cellular debris by cells such as neutrophils and macrophages (monocytes)
• Important defense mechanism, particularly in bacterial infections
• O2 independent (use pH changes in phagolysosomes and lysosomal enzymes to destroy pathogens)
• O2 dependent mechanisms for killing bacteria

Question 17

What is the O2-dependent mechanism for killing bacteria with NADPH?

Answer 17

• NADPH oxidase and myeloperoxidase (MPO) work together in killing bacteria
• MPO system: most potent of bactericidal mechanisms
• After internalization of bacteria, NADPH oxidase reduces O2 to O2 radical
• “Respiratory burst”: rapid consumption of O2
• O2 radical converted to H2O2 by superoxide dismutase
• Peroxide + chloride -> hypochlorous (HOCl) aka monster killer (what is inside bleach)

Question 18

In general is H202 bad for us?

Answer 18

Not a free radical but can turn into one; ROS are formed continuously as byproducts of aerobic metabolism, reactions with drugs and environmental toxins, or wen level of antioxidants is diminished -> oxidative stress

• Damage to DNA, proteins, lipids, cell membranes
• Implicated in cancer, inflammatory disease, and aging

Question 19

How does NADPH reduce H2O2?

Answer 19

Reduced glutathione (G-SH; a tripeptide-thiol present in most cells) is used by glutathione peroxidase to reduce the peroxide to water
-The oxidized glutathione (G-S-S-G) produced is no longer protective -> regenerated/reduced by glutathione reductase, using NADPH as the source of electrons

Question 20

What are some natural detoxifiers of ROS?

Answer 20

Ascorbate, vitamin E, beta-carotene

-if part of diet, not supplements

Question 21

What is nitric oxide (NO)?

Answer 21

Important free radical glass; Mediator in broad array of systems; its synthesis requires NADPH

• Endothelium-derived relaxing factor: vasodilation by relaxing vascular smooth muscle
• Acts as a NT
• Prevents platelet aggregation
• Helps mediate macrophage bactericidal activity
• Very short half-life
• Reacts with O2 and O2 radical -> nitrates, nitrites, RNS

Question 22

NO is made from what?

Answer 22

Made from arginine and O2 by 3 different NADPH-dependent NO synthases (NOS):

1) endothelial (eNOS) - constantly produce very low levels of NO for vasodilation
2) neuronal (nNOS) - constantly produce very low levels of NO for neurotransmission
3) inducible (iNOS) - produces large amount of NO for defense against pathogens

Question 23

What does G6PD deficiency cause?

Answer 23

Impairs the ability of the cell to form the NADPH that is essential for the maintenance of G-SH pool -> cells most affected are RBC because they do not have additional source of NADPH