Oxidative stress

Question 1

What is a ROS?

Answer 1

Reactive Oxygen Species

Question 2

What is a RNS?

Answer 2

Reactive Nitrogen Species

Question 3

What is a free radical?

Answer 3

A species with an unpaired electron

Question 4

How is a free radical denoted?

Answer 4

Using a superscript dot

Question 5

How are ROS produced?

Answer 5

O2 gains an electron forming Superoxide (O2•–)

Question 6

What happens to Superoxide?

Answer 6

It is a ROS so can cause direct damage
Can react with Nitric oxide to form Peroxynitrite (ONOO-) a powerful oxidant

Or converted into Hydrogen peroxide H2O2 a powerful oxidant

Question 7

What enzyme converts superoxide into Hydrogen peroxide H2O2?

Answer 7

Superoxide dismutase (SOD)

Question 8

How does Hydrogen peroxide go onto cause damage?

Answer 8

Fenton reaction
H2O2 reacts with Fe2+ producing ROS/free radicals

Question 9

What can H2O2 by reduced to to cause damage?

Answer 9

Water and Hydroxyl radical OH .

Question 10

What 2 ways can ROS/RNS damage DNA?

Answer 10

ROS reacts with base (takes e-) changing the base leading to misplacing and mutation

ROS reacts with sugar (causes strand break and mutation on repair).

Question 11

What can the mutation on repair of DNA caused by ROS/RNS damage lead to?

Answer 11

Cancer

Question 12

What 2 ways can ROS damage proteins?

Answer 12

Damage the:
-Backbone
-Sidechains

Question 13

How does ROS interacting with the protein back bone cause problems?

Answer 13

Leads to backbone fragmenting
Leads to protein degradation

Question 14

How does ROS reacting with the amino acid side chains of a protein cause problems?

Answer 14

Modifies the amino acid and it’s properties
Can lead to the inappropriate formation of disulphide bonds (between thiol groups) thus changing the structure of the proteins at the secondary level

Question 15

How can changing the protein structure affect a protein?

Answer 15

Lead to loss of function
Degradation
Or gain of function

Question 16

How do inappropriate disulphide bonds form when a ROS reacts with a protein?

Answer 16

ROS takes electrons from cysteines
Causes misfolding, cross linking and disruption of function

Question 17

How do ROS cause damage to lipids?
(General process)

Answer 17

Free radical reacts with polyunsaturated fatty acid removing a H
Lipid radical forms
Lipid radical reacts with oxygen forming lipid peroxyl radical
CHAIN REACTION OCCURS
Lipid peroxyl radical reacts with a polyunsaturated fatty acid removing H ETC….

Question 18

Why are ROS dangerous to cellular membranes?

Answer 18

The hydrophobic environment of the bilayer gets disrupted by Lipid peroxidation and membrane integrity fails

Question 19

What is Lipid peroxidation?

Answer 19

The chain reaction caused by the damage of free radical/ROS to lipids of the cell membrane

Question 20

What are the 2 types of sources of oxidants?

Answer 20

Endogenous
Exogenous

Question 21

List 3 main examples of endogenous oxidants

Answer 21

Electron transport chain
Nitric oxide synthases
NADPH oxidases

Question 22

List 4 examples of Exogenous oxidants

Answer 22

Radiation (UV, X-rays and cosmic rays)
Pollutants
Drugs (primaquine =anti malarial)
Toxins (herbicides)

Question 23

How is the Electron Transport chain a source of ROS?

Answer 23

Electrons sometimes escape and react with dissolved O2 forming Superoxide

Question 24

What is Respiratory burst?

Answer 24

The rapid production of Superoxide and H2O2 from Phagocytes to destroy invading bacteria

Question 25

Which enzyme provides the electron to oxygen to form superoxide in phagocytes?

Answer 25

NADPH Oxidase

Question 26

What is Chronic granulomatous disease?

Answer 26

Genetic defect in NADPH oxidase

Question 27

Why does Chronic granulomatous disease cause increased susceptibility to bacterial infections?

Answer 27

Superoxide can’t be produced by phagocyte so respiratory burst will not occur to destroy bacteria

Question 28

What are the 2 key enzymes that are important in the cellular defences against ROS?

Answer 28

Superoxide dismutase (SOD) Catalase

Question 29

What is the role of Superoxide dismutase in cellular defences against ROS?

Answer 29

Converts Superoxide to H2O2 and oxygen Superoxide is very good at initiating chain reactions

Question 30

What is the role of catalase in the cellular defences against ROS?

Answer 30

Converts H2O2 to water and oxygen

Question 31

What is GSH?

Answer 31

The reduced/active form of Glutathione

Question 32

How does GSH protect from oxidative stress/ROS?

Answer 32

Electron from thiol group of cysteine in GSH is donated to the ROS This causes GSH to form a disulphide bond with another GSH forming GSSG

Question 33

What is GSSG?

Answer 33

The oxidised/inactive form of Glutathione

Question 34

How is GSSG recycled back into the active reduced form of Glutathione (GSH)?

Answer 34

NADPH transfers electron to disulphide bond GSH reforms

Question 35

Which pathway is a major source of NADPH?

Answer 35

Pentose Phosphate pathway

Question 36

What is the significance of Vitamin E and Vitamin C in cellular defences against free radicals?

Answer 36

Vitamin E protects against lipid peroxidation Vitamin C regenerates reduced/active form of Vitamin E

Question 37

What is oxidative stress?

Answer 37

The imbalance in Oxidants and defences/antioxidants

Question 38

How does cataracts form occur as a result of oxidative stress in Galactosaemia?

Answer 38

Galactose accumulates , enters ALDOSE reductase pathway depleting NADPH forming galactitiol Depleted NADPH means Glutathione can’t be converted back to its active reduced form Damage as a result of ROS occurs in eye Inappropriate disulphide bonds in crystallin protein in eye form causing protein getting denatured

Question 39

Why are individuals with G6PDH (Glucose 6 phosphate dehydrogenase) Deficiency susceptible to oxidative damage?

Answer 39

Pentose phosphate pathway ceases Major source of NADPH NADPH essential in regenerating GSH

Question 40

What are Heinz bodies?

Answer 40

When inappropriate disulphide bonds result in aggregations of cross linked haemoglobin

Question 41

What is a clinical sign of G6PDH deficiency?

Answer 41

Heinz bodies

Question 42

Which part of the body removes Heinz bodies?

Answer 42

Spleen

Question 43

How are normal levels of paracetamol normally metabolised?

Answer 43

Metabolised via pathways producing safe products

Question 44

What is the toxic product produced when somebody overdoses on paracetamol and why is this produced?

Answer 44

NAPQI Made since safe metabolic pathways of paracetamol fully saturated

Question 45

How is NAPQI directly and indirectly toxic?

Answer 45

Directly causes oxidative damage to Hepatocyte Indirectly: Depleted GSH making cell vulnerable to oxidative damage

Question 46

How is an overdose of Paracetamol treated?

Answer 46

Drug Acetylcysteine

Question 47

How does Acetylcysteine help treat a Paracetomol overdose?

Answer 47

Replenishes GSH (Active Glutathione) to protect against oxidative damage

Question 48

What are the 3 ways by which a cell can be damaged by an oxidant?

Answer 48

Lipid peroxidation Damage to proteins Damage to DNA