Oxidative Stress

Question 1

Cellular damage cause by ROS (reactive oxygen species) & RNS (reactive nitrogen species) is a significant component in a wide range of disease states. Name some diseases?

Answer 1

CVS disease 
MS
Parkinson's 
Pancreatitis 
Cancer
COPD 
Crohn's 
Rheumatoid arthritis

Question 2

Name some ROS & RNS

Answer 2

Ros; superoxide, hydrogen peroxide (when reacted with fe2+), hydroxyl radical (V. Damaging and reactive)

Rns; nitric oxide

Question 3

What are the two ways in which ROS damage DNA?

Answer 3

React with base; modified base can lead to mispairing and mutation

React with sugar (ribose or deoxyribose); can cause strand break and mutation on repair

Question 4

How can ROS damage lead to cancer?

Answer 4

ROS reacts with DNA

DNA damage

Failure to repair can lead to mutation

Can lead to cancer

Question 5

In what ways do ROS damage proteins? What are the consequences?

Answer 5

Effect side chain or back bone

If Backbone= fragmentation which leads to protein degradation

If sidechain= risk of modified amino acids which leads to changes in the protein structure. Which in turn may lead to gain of function/ loss of function (and protein degradation)/ protein degradation

Question 6

What is the role of disulphides bonds in proteins?

Answer 6

Folding and stability

Question 7

How are disulphide bonds formed?

Answer 7

Between thiol groups of cysteine residues

Question 8

When can inappropate disulphide bond form? What are the consequences?

Answer 8

If ROS effects the side chains on a protein, taking an electron from cysteines causing misfolding, cross-linking and disruption of function of protein (enzyme/hormone ie insulin)

Question 9

What damage can ROS cause to lipids? What are the consequences?

Answer 9

They can react to form lipid peroxyl radicals which set off chain reactions as the nearly formed radical steals an electron from the adjacent lipid.

This leads to the hydrophobic environment of bilayer to be disrupted and the membrane integrity fails.

Question 10

What are source of biological oxidants, both exogenous and endogenous?

Answer 10

Endogenous;

• ETC
• peroxidases
• nitric oxide synthesis
• NADPH oxidises

Exogenous;

• radiation (cosmos rays, UV light, x-rays)
• pollutants
• drugs (anti-malarial)
• toxins (herbicides)

Question 11

How are ROS formed in the electron transport chain?

Answer 11

NADH & FADH2 supply electrons

Electrons are passed through ETC and reduce oxygen to form water at complex IV

Occasionally electron can accidentally escape chain and react with dissolved oxygen to form superoxide

Question 12

How is nitric oxide synthase (NOS) a source of free radicals?

Answer 12

Arginine is converted by NOS to citrulline and nitric oxide

Question 13

When is respiratory burst?

Answer 13

Rapid release of superoxide and hydrogen peroxide from phagocytic cells ( ie monocytes and neutrophils) to kill pathogens (beneficial)

Question 14

There are some diseases in which a genetic defect in the NADPH oxidase complex causes enhanced susceptibility to bacterial infection. Name some of these infections

Answer 14

Pneumonia
Abscesses
Impetigo
Cellulitis

Question 15

What are the defences that we have against free radicals?

Answer 15

Superoxide dismutase
Catalase
Glutathione
Free radical scavengers

Question 16

How does superoxide dismutase (SOD)defend against free radicals?

Answer 16

Converts superoxide to hydrogen peroxide and oxygen

The h2o2 can then be converted further by catalase and isn’t itself actually a free radical

Question 17

How does catalase defend against free radicals?

Answer 17

It works after superoxide dismutase and converts the hydrogen peroxide (that the superoxide dismutase has produced from superoxide) into water and oxygen.

It it important in immune cells to protects against oxidative burst

**declining levels in hair follicles with age may explain grey hair, is the hair bleaching itself?

Question 18

How does glutathione defend against free radicals? What enzyme is needed and what does that enzyme require?

Answer 18

It donates an electron from a cysteine in its structure and then forms a disulphide bond with another glutathione structure

[GSH + GSH = GSSG] and free radical gets its electron.

Reaction needs glutathione peroxidase and selenium is required

Question 19

After glutathione has done its job it needs to be recycled back into its reduced form (GSH). How does this work?

Answer 19

Glutathione reductase uses a NADPH (from the pentose phosphate pathway) and produced 2x GSH from GSSG

Question 20

What is the pentose phosphate pathway an important source of?

Answer 20

NADPH

Required for;
Reducing power for biosynthesis
Maintenance of GSH levels
Detoxification reactions

Question 21

Name some free radical scavengers. How do they work?

Answer 21

Vitamins E
Vitamin C

Carotenoids
Uric acid
Flavonoids
Melatonin

Reduce free radical damage by donating hydrogen atoms to free radicals in non-enzymatic reactions

Question 22

How do vitamin e and c work together to defend against free radicals?

Answer 22

Vit E- lipid soluble antioxidant - important for protection against lipid peroxidation (takes a lipid radical and turns it into a lipid again)

Vit C-water soluble antioxidant- important role in regenerating reduced form of vit E so it can continue its work.

Question 23

What membrane bound enzyme complex produced the respiratory burst? How does it work?

Answer 23

NADPH oxidase

It transfers electrons from NADPH across the membrane to couple these to molecule oxygen to generate superoxide radicals