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ESA1 - Metabolism > Oxidative Stress > Flashcards

Oxidative Stress Flashcards

1
Q

Explain how a superoxide is formed

A
  • 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
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2
Q

List 3 reactive oxygen species found in cells

A
  • Superoxide (O2🕳-)
  • Hydrogen Peroxide (H2O2)
  • Hydroxyl radicals (OH🕳)
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3
Q

Explain what is meant by a ‘superoxide’

A
  • Superoxide (O2🕳-) is a highly reactive free radical
  • Produced by addition of 1 electron to molecular O2
  • O2 molecule contains an unpaired electron
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3
Q

Explain how lipids can be damaged by ROS

A
  • 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
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5
Q

What are the consequences of ROS reacting with DNA?

A
  • MUTATION

- Failure to repair mutation could lead to CANCER

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6
Q

Why is mitochondrial DNA (mtDNA) more susceptible to ROS?

A
  • Located near inner mitochondrial membrane where ROS are formed
  • mtDNA is not protected by histones
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7
Q

How can ROS cause protein damage and degradation?

A
  • Side chains of amino acids can react with ROS
  • MODIFICATION OF TERTIARY PROTEIN STRUCTURE
  • Leads to loss of function
  • Targets protein for degradation
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8
Q

Explain the consequences of formation of inappropriate disulphide bonds within proteins due to ROS

A
  • 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
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9
Q

What is a Heinz body and how is it formed?

A
  • 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’
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10
Q

Describe how the cell protects against damaging effects of Superoxide

A
  • 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
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11
Q

What is Glutathione (GSH)?

A
  • 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)
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12
Q

Explain how Glutathione (GSH) protects against oxidative stress

A
  • 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
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13
Q

How is NADPH used in the recycling of Glutathione (GSH) from Oxidised Glutathione (GSSG)?

A
  • 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
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14
Q

Give 5 examples of free radical scavengers. Where might these be obtained?

A
  • Vitamins C and E (antioxidants)
  • Carotenoids
  • Flavonoids
  • Melatonin
  • Obtained from diet by eating fruits and vegetables
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15
Q

Why is vitamin E an essential component of the diet?

A
  • Vitamin E is lipid soluble
  • ANTIOXIDANT
  • Important in protecting against lipid peroxidation
  • Regenerated by vitamin A (water soluble)
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16
Q

What is a free radical?

A
  • Any atom, molecule or ion that contains one or more UNPAIRED ELECTRONS
  • Capable of ‘free’ independent existence
17
Q

How are ROS used in the body’s immune response to infection?

A
  • 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
18
Q

Explain the role of NADPH oxidase in the respiratory burst

A
  • 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
19
Q

How can galactosaemia lead to oxidative stress?

A
  • Increased activity of ALDOSE REDUCTASE so consumes NADPH supply
  • NADPH needed to recycle GSSG to GSH so leaves cells more susceptible to oxidative damage
20
Q

How can G6PD deficiency lead to oxidative stress?

A
  • 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
21
Q

Explain the cellular defences in place to protect against oxidative stress

A
  • SUPEROXIDE DISMUTASE (SOD) converts superoxide to hydrogen peroxide (H2O2) and O2
  • CATALASE converts hydrogen peroxide to H2O and O2
22
Q

Describe how increased activity of aldose reductase can lead to cataracts

A
  • 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
23
Q

Describe the formation of the reactive nitrogen species (RNS) Nitric Oxide

A
  • 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
24
Q

What is the role of inducible Nitric Oxide Synthase (iNOS)?

A

Produce high concentration of NO free radicals in phagocytes that can have a direct toxic effect on pathogens if released

25
Q

What are the effects of NAPQI?

A
  • 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
26
Q

How is an overdose of paracetamol treated?

A
  • ACETYLCYSTEINE

- Replenishes glutathione levels in liver cells so cell defence mechanisms are less compromised

27
Q

Explain how ischaemic reperfusion injury can lead to oxidative damage

A
  • 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
28
Q

Why might an overdose of paracetamol lead to oxidative damage?

A
  • Toxic metabolite NAPQI accumulates with high levels of paracetamol
  • Causes oxidative damage to liver cells (denaturation of proteins, DNA damage, lipid peroxidation)
29
Q

Explain how DNA can be damaged by ROS

A
  • 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

ESA1 - Metabolism (20 decks)

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