WEEK 4 (HMP Shunt)

Question 1

What is the HMP shunt also known as?

Answer 1

Pentose phosphate pathway

Question 2

In which molecule in Glycolysis does the reaction shunt away?

Answer 2

Glucose-6-Phosphate

Question 3

What are the key things to remember about the HMP shunt?

Answer 3

• Occurs in the cytoplasm
• Main organs are LIVER, MAMMARY GLANDS, RBCs & ADRENAL CORTEX
• Goal is to produce NADPH
• 2 phases: Oxidative (irreversible, rate-limiting) & Reductive (reversible)

Question 4

What does the HMP shunt synthesise?

Answer 4

• 2 molecules of NADPH
• Ribose-5-Phosphate
• CO2

Question 5

Why is NADPH needed?

Answer 5

• Glutathione reduction (Free radical detoxification)
• Fatty acid synthesis
• Steroid hormone synthesis

Question 6

Which organs use the HMP shunt to synthesis fatty acids?

Answer 6

• Liver
• Lactating mammary glands
• Adipose tissue

Question 7

Which organs use the HMP shunt to synthesis Steroid hormones?

Answer 7

• Testes
• Ovaries
• Placenta
• Adrenal cortex

Question 8

What requires NADPH to keep glutathione reduced?

Answer 8

RBCs

Question 9

What are the irreversible oxidative reactions?

Answer 9

3 reactions that lead to formation of ribulose-5-phosphate, CO2 & 2 molecules of NADPH

Glucose-6-phosphate -> 6-Phosphoglucanolactone -> 6-Phosphogluconate -> Ribulose-5-phosphate

Question 10

What is the enzyme that converts Glucose-6-Phosphate to 6-Phosphoglucanolactone?

Answer 10

Glucose-6-Phosphate Dehydrogenase

Question 11

What is the enzyme that converts 6-Phosphoglucanolactone to 6-Phosphogluconate?

Answer 11

Glucanolactone Hydrolase

Question 12

What is the enzyme that converts 6-Phosphogluconate to Ribulose-5-Phosphate?

Answer 12

6-Phosphogluconate Dehydrogenase

Question 13

What are the Reversible non-oxidative reactions?

Answer 13

Reversible reactions that permit Ribulose-5-Phosphate to be converted to either RIBOSE-5-PHOSPHATE (needed for nucleotide synthesis) or to INTERMEDIATES OF GLYCOLYSIS (Fructose-6-Phosphate & Glyceraldehyde-3-Phosphate)

Question 14

What are the properties of Transketolase?

Answer 14

• Transfers a carbon unit to create Fructose-6-Phosphate
• Requires thiamine (B1) as a co-factor
• Associated with WERNICKE-KORSAKOFF SYNDROME (memory disorder that results from vitamin B1 deficiency and is associated with alcoholism)
• Abnormal transketolase may predispose
• Affected individuals may have abnormal binding to thiamine

Question 15

What are the uses of Hydrogen peroxide reduction?

Answer 15

• Co-factor in fatty acid & steroid synthesis
• Protection from oxidative damage
• Phagocytosis
• Used in “reductive” reactions (releases hydrogen to form NADP+)

Question 16

How are Reactive Oxygen Species (ROS) formed?

Answer 16

• Continuously as by-products of aerobic metabolism
• Through reactions with drugs & environmental toxins
• When the level of antioxidants is diminished

All creating conditions of OXIDATIVE STRESS

Question 17

What can highly reactive oxygen intermediates cause?

Answer 17

Serious chemical damage to DNA, Proteins and can lead to cell death

Question 18

What is the function of Reduced Glutathione (G-SH)?

Answer 18

Reduced Glutathione (G-SH) can chemically detoxify H2O2 why by GLUTATHIONE PEROXIDASE forms OXIDISED GLUTATHIONE (G-S-S-G) which no longer has protective properties

Question 19

How does the cell regenerate Reduced Glutathione (G-SH)?

Answer 19

In a reaction catalysed by GLUTATHIONE REDUCTASE using NADPH as a source of reducing equivalents. Additional enzymes (e.g SUPEROXIDE DISMUTASE & CATALASE) catalyse the conversion of other ROS (Reactive oxygen species) to harmless products.

Question 20

What is able to reduce and detoxify ROS in the lab?

Answer 20

Antioxidant chemicals & a number of intracellular reducing agents

(e.g ASCORBATE, VITAMIN E & B-CAROTENE)

Question 21

What is Phagocytosis and what are its properties?

Answer 21

PHAGOCYTOSIS = Ingestion by receptor-mediated endocytosis of microorganisms, foreign particles & cellular debris

• Neutrophils & Macrophages
• Generate H2O2 to kill bacteria

Question 22

What are the 2 mechanisms of Phagocytosis?

Answer 22

• OXYGEN-INDEPENDENT KILLING (uses pH changes in PHAGOLYSOSOMES & LYSOSOMAL ENZYMES to destroy pathogens)
• OXYGEN-DEPENDENT SYSTEM (uses NADPH OXIDASE, SUPEROXIDE DISMUTASE & MYELOPEROXIDASE)

Question 23

Describe CGD-Chronic Granulomatous Disease

Answer 23

• Loss of function of NADPH oxidase
• Phagocytes cannot generate H2O2
• Phagocytes use despite enzyme deficiency
• Catalase (-) bacteria generate their own H2O2
• Catalyse (+) bacteria breakdown H2O2 -> Host cells have no H2O2 to use -> Recurrent infections

Question 24

Which 5 organisms cause almost all CGD infections?

Answer 24

• Staph aureus
• Pseudomonas
• Serratia
• Nocardia
• Aspergillus

Question 25

What are the properties of Nitric Oxide?

Answer 25

- Mediator - Endothelium-derived relaxing factor that causes vasodilation by relaxing vascular smooth muscle - Neurotransmitter -> Prevents platelet aggregation -> Plays an essential role in macrophage function

Question 26

What are the properties of Nitric Oxide synthesis?

Answer 26

- 3 NOS isozymes - 2 are CONSTITUTIVE, CALCIUM-CALMODULIN-DEPENDENT enzymes that are found primarily in ENDOTHELIUM & NEURAL TISSUE and constantly produce very low levels of NO for vasodilation and neurotransmission - INDUCIBLE, CA2+ INDEPENDENT ENZYME can be expressed in many cells as an early defence against pathogens

Question 27

Describe the physiology behind Glucose-6-Phosphate

Answer 27

NADPH is necessary to keep GLUTATHIONE reduced which detoxifies free radicals and peroxides -> Decrease in NADPH in RBCs leads to HEMOLYTIC ANEMIA due to poor RBC defines against oxidising agents -> Infection can cause HEMOLYSIS -> Inflammatory response produces FREE RADICALS that diffuse into RBCs -> Oxidative damage

Question 28

What are the properties of Glucose-6-Phosphate Dehydrogenase deficiency?

Answer 28

- X-linked recessive disorder - Most common human enzyme deficiency - Increased malarial resistance - HEINZ BODIES (oxidised haemoglobin precipitated in RBCs) - BITE CELLs (Result from the Phagocytic removal of Heinz bodies by splenic macrophages)