Oxidative Stress

Question 1

What is a superoxide molecule?

Answer 1

Produced by adding an electron to molecular oxygen (O2).
An important source of ROS

Question 2

How can ROS damage DNA? What are the consequences?

Answer 2

Can react with backbone -> fragmentation -> protein degradation
OR
Can react with sidechain -> modified amino acids (see lecture slides for details) -> change in protein structure -> protein degradation/loss of function/gain of function

Question 3

How can ROS damage lipids?

Answer 3

Causes lipid peroxidation -> hydrophobic environment of bilayer disrupted and membrane integrity fails

Question 4

Give an example of an endogenous source of ROS.

Answer 4

Electron transport chain, Nitric oxide synthase, NADPH oxidases, Peroxidases, Lipooxygenases, Xanthine oxidase, Monoamine oxidase

Question 5

Give an example of an exogenous source of ROS.

Answer 5

• Radiation (cosmic rays, UV light, X-rays)
• Pollutants
• Drugs (primaquine (anti-malarial))
• Toxins (paraquat (herbicide))

Question 6

Describe respiratory burst and how it protects against invading bacteria

Answer 6

Respiratory burst is rapid production of superoxide and H2O2 from phagocytic cells.
ROS and peroxynitrite destroy invading bacteria (when they’re engulfed by phagocytic cells)

Question 7

How does superoxide dismutase protect against ROS?

Answer 7

Converts superoxide to H2O2 and oxygen (H2O2 then acted on by catalase)
It is the primary defence because superoxide is a strong initiator of chain reactions

Question 8

How does catalase protect against ROS?

Answer 8

Converts H2O2 to water and oxygen
- widespread enzyme
- important in immune cells to protect against oxidative burst

Question 9

Key points of the Pentose phosphate pathway:

Answer 9

• Source of NADPH (protects against oxidative damage)
• Produces C5-sugar, ribose
• Phosphate dehydrogenase is the key enzyme

Question 10

What are Heinz bodies and how are they caused?

Answer 10

They are aggregates of haemoglobin, caused by G6PDH deficiency
G6PDH deficiency leads to decreased levels of NADPH, which is used to convert GSSG to GSH. This leads to decreased GSH, which is oxidised via glutathione peroxidase, which also converts H2O2 to H2O at the same time

Question 11

What drug can be used to treat paracetamol overdose?

Answer 11

Acetylcysteine
- is a precursor to glutathione (GSH)
- GSH used to protect against NAPQI (a toxic metabolite which causes oxidative damage to liver cells)

Question 12

What are the 2 important enzymes in glutathione metabolism?

Answer 12

1. Glutathione Peroxidase - 2GSH -> GSSG
   - H2O2 -> 2H2O
   - requires selenium
2. Glutathione Reductase - GSSG -> 2GSH
   - NADPH -> NADP
   - NADPH from pentose phosphate pathway, therefore pathway essential
   for protection against free radical damage

Question 13

What is Glutathione (GSH)?

Answer 13

GSH is a tripeptide synthesised by the body to protect against oxidative damage.
Thiol group of Cysteine donates e- to ROS. GSH then reacts with another GSH to form disulphide bond (GSSG)