Oxidative Stress

Question 1

What is a free radical?

Answer 1

Contain one or more unpaired electrons

Question 2

What does it mean that O2 is biradical?

Answer 2

Has two unpaired electrons in different orbitals

Question 3

Explain the two types of ROS damage to DNA

Answer 3

Reacts with base - can lead to mispairing and mutation

Reacts with sugar - can cause strand break and mutation

Question 4

What can be used as a measurement of oxidative damage in cells?

Answer 4

8-oxo-dg

Question 5

Describe the two ways ROS damage proteins

Answer 5

Backbone - fragmentation leads to protein degradation

Sidechain - modified amino acids, changes protein structure, leads to gain or loss of function

Question 6

What is the importance of disulphide bonds? (2)

Answer 6

Folding
Stability

Of SOME proteins

Question 7

Where do disulphide bonds form?

Answer 7

Between thiol groups of cysteine residues

Question 8

When can inappropriate disulphide bonds form?

Answer 8

If ROS takes electrons from cysteines

Question 9

What happens if inappropriate disulphide bonds form? (3)

Answer 9

Misfolding
Cross linking
Disruption of function

Question 10

Describe ROS damage to lipids

Answer 10

1. Free radical extracts H atom from polyunsaturated fatty acid in membrane lipid
2. Lipid radical formed - reacts with O2 to form a lipid peroxyl radical
3. Lipid peroxyl radical extracts H atom from a fatty acid (chain reaction)
4. Hydrophobic environment disrupted - membrane integrity fails

Question 11

Two categories of biological oxidants

Answer 11

Endogenous
Exogenous

Question 12

What does endogenous and exogenous mean?

Answer 12

Endogenous - inside the cell

Exogenous - outside the cell

Question 13

Examples of endogenous biological oxidants (3)

Answer 13

Electron transport chain

Nitric oxide synthases

NADPH oxidases

Question 14

Examples of exogenous biological oxidants (4)

Answer 14

Radiation
Pollutants
Drugs
Toxins

Question 15

How are nitric oxide synthases a source of oxidants

Answer 15

Nitric oxide synthases catalyse the conversion of arginine into an intermediate and then into nitric oxide

Question 16

What are the two ROS and RNS that are not free radicals?

Answer 16

ONOO- (peroxynitrite)
H2O2 (hydrogen peroxide) - can react to produce free radicals

Question 17

What is the most damaging ROS?

Answer 17

OH*

Question 18

Symptoms of galactosemia

Answer 18

Renal failure, vomiting, brain damage, cataracts, hypoglycaemia

Question 19

What causes galactosemia?

Answer 19

Deficiency in galactokinase, UDP galactose epimerase or transferase

Question 20

What are three clinical examples of ROS?

Answer 20

Galactosemia
G6PDH deficiency
Paracetamol overdose

Question 21

How is galactosemia related to oxidative stress?

Answer 21

Build up of galactose
Galactose converted to galactitol by aldose reductase
Requires NADPH - less for oxidative stress defence

Question 22

What causes cataracts?

Answer 22

In galactosemia
Galactose converted to galactitol by aldose reductase

Galactitol causes osmotic pressure
Reaction requires NADPH - depleting levels in lens
Inappropriate disulfide bonds form
Depleting lens of eye

Question 23

Describe how a paracetamol overdose is linked to ROS

Answer 23

NAPQI is produced
NAPQI conjugates with glutathione
Less glutathione for oxidative stress defences

Question 24

Describe metabolism of paracetamol at safe levels

Answer 24

Conjugates with glucuronide or sulphate

Broken down in hepatocytes

Question 25

NAPQI is toxic, describe how (3)

Answer 25

Damage to proteins
Damage to DNA
Lipid peroxidation

Question 26

What is the antidote to a paracetamol overdose?
Describe how it works

Answer 26

Acetyl cysteine
Replenishes level of glutathione

Question 27

What toxic metabolite accumulates in the liver in a paracetamol overdose?

Answer 27

NAPQI

Question 28

Which enzyme converts H2O2 to water and oxygen?

Answer 28

Catalase