Oxidative Stress Flashcards
Explain how a superoxide is formed
- 0.1-2% of electrons do not reach the end of the ETC and prematurely reduce O2
- Formation of O2- which is a highly reactive free radical
List 3 reactive oxygen species found in cells
- Superoxide (O2🕳-)
- Hydrogen Peroxide (H2O2)
- Hydroxyl radicals (OH🕳)
Explain what is meant by a ‘superoxide’
- Superoxide (O2🕳-) is a highly reactive free radical
- Produced by addition of 1 electron to molecular O2
- O2 molecule contains an unpaired electron
Explain how lipids can be damaged by ROS
- LIPID PEROXIDATION
- ROS takes electron from fatty acid, producing a lipid radical
- Lipid radical can react with O2 to make LIPID PEROXIDE which can continue forming free radicals in a chain reaction
What are the consequences of ROS reacting with DNA?
- MUTATION
- Failure to repair mutation could lead to CANCER
Why is mitochondrial DNA (mtDNA) more susceptible to ROS?
- Located near inner mitochondrial membrane where ROS are formed
- mtDNA is not protected by histones
How can ROS cause protein damage and degradation?
- Side chains of amino acids can react with ROS
- MODIFICATION OF TERTIARY PROTEIN STRUCTURE
- Leads to loss of function
- Targets protein for degradation
Explain the consequences of formation of inappropriate disulphide bonds within proteins due to ROS
- ROS takes electron from CYSTEINE residue leading to formation of S-S bond
- Causes misfolding, cross linking and disruption to protein structure
- Can lead to formation of HEINZ BODIES in RBCs
What is a Heinz body and how is it formed?
- Heinz bodies are inclusions within RBCs caused by denaturation of haemoglobin
- Inappropriate disulphide bonds formed between haemoglobin molecules due to presence of ROS
- Haemoglobin becomes ‘cross linked’
Describe how the cell protects against damaging effects of Superoxide
- SUPEROXIDE DISMUTASE (SOD) converts superoxide to hydrogen peroxide and oxygen
- Hydrogen peroxide is rapidly broken down by CATALASE enzyme into water and oxygen in the reaction:
2H2O2 —-> 2H2O + O2
What is Glutathione (GSH)?
- Antioxidant which protects against oxidative damage
- Tripeptide (GLY-CYS-GLU)
- Gamma peptide linkage (between carboxyl group of GLU side chain and amine group of CYS)
Explain how Glutathione (GSH) protects against oxidative stress
- Thiol group of CYS residue donates an electron to ROS
- Disulphide bond forms between 2 CYS residues on 2 Glutathione molecules, forming OXIDISED GLUTATHIONE (GSSG)
- Catalysed by GLUTATHIONE PEROXIDASE
How is NADPH used in the recycling of Glutathione (GSH) from Oxidised Glutathione (GSSG)?
- GSSG is reduced back to GSH by the enzyme GLUTATHIONE REDUCTASE
- Glutathione reductase catalyses the transfer of electrons from NADPH to the S-S bond of GSSG, converting it back into 2GSH which can be reused
Give 5 examples of free radical scavengers. Where might these be obtained?
- Vitamins C and E (antioxidants)
- Carotenoids
- Flavonoids
- Melatonin
- Obtained from diet by eating fruits and vegetables
Why is vitamin E an essential component of the diet?
- Vitamin E is lipid soluble
- ANTIOXIDANT
- Important in protecting against lipid peroxidation
- Regenerated by vitamin A (water soluble)
What is a free radical?
- Any atom, molecule or ion that contains one or more UNPAIRED ELECTRONS
- Capable of ‘free’ independent existence
How are ROS used in the body’s immune response to infection?
- RESPIRATORY BURST
- Neutrophils and monocytes when stimulated can rapidly produce a release of ROS
- Cell is destroyed, and surround bacteria or fungal cells are also destroyed
Explain the role of NADPH oxidase in the respiratory burst
- Membrane bound enzyme in membrane of phagosomes
- Transfers electrons from NADPH across membrane
- E- coupled with O2 to produce superoxide free radicals
- Important in the development if atherosclerosis
How can galactosaemia lead to oxidative stress?
- Increased activity of ALDOSE REDUCTASE so consumes NADPH supply
- NADPH needed to recycle GSSG to GSH so leaves cells more susceptible to oxidative damage
How can G6PD deficiency lead to oxidative stress?
- G6PD is the first enzyme in the pentose phosphate pathway which produces NADPH
- Deficiency of enzyme leads to limited production of NADPH
- NADPH needed for the recycling of GSSG to GSH so leaves cells more susceptible to oxidative damage
Explain the cellular defences in place to protect against oxidative stress
- SUPEROXIDE DISMUTASE (SOD) converts superoxide to hydrogen peroxide (H2O2) and O2
- CATALASE converts hydrogen peroxide to H2O and O2
Describe how increased activity of aldose reductase can lead to cataracts
- NADPH stores depleted as galactose is converted to galacticol
- Less NADPH available to protect against ROS - can lead to protein denaturation of Crystallin in lens of eye
- Accumulation of galacticol decreases osmotic pressure - draws water into eye causing lens fibres to swell and rupture
Describe the formation of the reactive nitrogen species (RNS) Nitric Oxide
- Arginine is converted to Citrulline using nitric oxide synthase and NADPH
- Nitric oxide free radical also formed - in low levels it is used as a signalling molecule, but can be very toxic in high levels
What is the role of inducible Nitric Oxide Synthase (iNOS)?
Produce high concentration of NO free radicals in phagocytes that can have a direct toxic effect on pathogens if released
What are the effects of NAPQI?
- Direct toxic impact on hepatocytes (oxidative stress - causes damage to proteins, lipids and DNA)
- Depletes stores of glutathione so compromises cell defence mechanisms against OS
How is an overdose of paracetamol treated?
- ACETYLCYSTEINE
- Replenishes glutathione levels in liver cells so cell defence mechanisms are less compromised
Explain how ischaemic reperfusion injury can lead to oxidative damage
- Occurs when there is lack of oxygen to tissues due to blockage
- When blood supply is restored the reintroduction of oxygen can cause the formation of ROS from incompletely metabolised species
- Loss of antioxidants during ischaemia
Why might an overdose of paracetamol lead to oxidative damage?
- Toxic metabolite NAPQI accumulates with high levels of paracetamol
- Causes oxidative damage to liver cells (denaturation of proteins, DNA damage, lipid peroxidation)
Explain how DNA can be damaged by ROS
- ROS reacts with BASE which causes modified base - lead to mispairing or mutation
- ROS reacts with SUGAR which can cause strand break and mutation on repair
