L12: Pentose Phosphate Shunt

Question 1

LO1: List three functions of the PPP

Answer 1

1. major source of NADPH for biosynthetic reactions
2. major source of ribose-5-phosphate for nucleotide synthesis
3. recycles excess pentose phosphates back to glyceraldehyde-3-phosphate and glycolysis (VIP in RBCs as they have no biosynthetic reactions)

Question 2

LO2: Name/describe the enzymes+reactions that generate NADPH

Answer 2

oxidative branch of PPP: glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase

G6PDH: converts glucose-6-phosphate into 6P-gluconolactone and yields 1 NADPH

6PGDH: converts 6P-gluconate to ribulose-5P and yields 1 NADPH

non-oxidative branch of PPP: (indirect) can allow excess pentose phosphates to be converted to glycolytic intermediates to then generate NADPH

Question 3

LO3: What is the regulated enzyme in the PPP and how is it regulated?

Answer 3

G6PD

• inhibited by NADPH (negative feedback inhibition)
• inhibited by fatty acyl-CoA
• inhibition reversed by oxidized glutathione (since NADPH reduces glutathione)

Question 4

LO4: List four tissues that are enriched in enzymes of the PPP and why this is related to their other pathways

Answer 4

liver, adrenal glands, RBCs, phagocytic cells

• where fatty acids, cholesterole, bile acids, and steroid hormone synthesis occurs (liver/adrenal glands)
• where other biosynthetic pathways can’t occur and protection from ROS is needed (RBCs)
• where ROS species are made (phagocytic cells)
• where NADPH is needed (all)

Question 5

LO5: How can an understanding of the PPP be helpful in diagnosing a thiamin deficiency?

Answer 5

-transketolase activity in RBCs used to test for thiamin deficiency, as transketolase requires thiamine pyrophosphate, which also requires thiamine

Question 6

LO6: Why do individuals with G6PD deficiency have ADRs to antimalarial drugs?

Answer 6

• antimalarial drugs increase production of oxygen metabolites, increasing oxidative stress
• in G6PD deficient individuals, not enough NADPH will result in not enough reduction of ROS, resulting in anemia due to RBC damage

Question 7

LO7: Describe three mechanisms by which the PPP protects RBCs against oxygen toxicity

Answer 7

-the function of RBCs predisposes them to oxidative damage via heme ozidation and subsequent superoxide anion production via reduction of oxygen (since oxygen can’t be used by hemoglobin when heme iron is oxidized)

PROTECTIVE MECHANISMS
1. MetHB reductase: reduces iron of metHB back to its normal state (since it spontaneously oxidizes) to restore hemoglobin function

2. Superoxide anion disposal:
   a. superoxide dismutase converts two molecules of superoxide anion to hydrogen peroxide and molecular oxygen
   b. glutathione peroxidase reduces hydrogen peroxide to water, using glutathione as a reducing agent
   c. glutathione reductase uses NADPH to reduce oxidized glutathione back to reduced form
   d. catalase converts hydrogen peroxide to water and molecular oxygen
3. Antioxidant vitamins like Vit. E, B-carotene (Vit. A precursor) and Vit. C (ascorbic acid) can reduce toxic oxygen species to non-toxic products

Question 8

LO8: How is the PPP related to the cytotoxic activity of phagocytic cells?

Answer 8

• phagocytic cells generate superoxide anions/other ROS to kill ingested species, and require NADPH produced by PPP as a substrate
  - NADPH oxidase helps cell take up oxygen rapidly to generate superoxide (NADPH +2O2—-NADPH oxidase—>NADP+ +2O2-)