3. Oxidative Stress Flashcards
Name examples of diseases involving cellular damage caused by oxidative stress.
CVD, COPD, Alzheimer’s, rheumatoid arthritis, Crohn’s disease, multiple sclerosis, Parkinson’s, pancreatitis, cancer, ischaemia/reperfusion injury
What is a free radical?
Any atom. molecule or ion that contains 1 or more unpaired electrons (usually move in pairs within an orbital) and is capable of independent (“free”) existence.
How do free radicals cause cellular damage?
- Are very reactive within cells and tend to “steal” electrons from other molecules. causing damage (e.g. to protein, lipid or DNA).
- Reaction can also generate a second radical - propagates damage.
Explain how Reactive Oxygen Species are produced by the ETC.
- Molecular oxygen = biradical - has 2 unpaired electrons in different orbitals (so stable).
- In the mitochondrial ETC, the final destination for an e- is an O2 molecule (which is then combined with protons to produce water).
> But 0.1-2% of e- don’t reach end of chain and prematurely reduce O2 to form SUPEROXIDE (O2•-) - important source of other ROS.
> Superoxide + 2H+ and e- = HYDROGEN PEROXIDE (H2O2) - not a free radical but can react with Fe2+ to produce free radicals.
> Hydrogen peroxide can catalyse formation of HYDROXYL RADICAL (OH•), eg. from glutathione - most reactive and damaging free radical.
Describe the formation of Reactive Nitrogen Species.
Superoxide can react with signalling molecule NITRIC OXIDE (NO•) to produce PEROXYNITRITE (ONOO-) - not a free radical but powerful oxidant that can damage cells.
Name different sources of biological oxidants.
Endogenous:
- ETC
- Nitric oxide synthases
- NADPH oxidases
Exogenous:
- Ionising radiation (UV light, X-rays, cosmic rays)
- Pollutants
- Drugs (eg. Primaquine anti-malarial)
- Toxins (eg. Paraquat herbicide)
Describe the enzymatic production of nitric oxide.
Oxidation of L-arginine by nitric oxide synthase (requries NADPH) to produce citrulline + NO•
3 types of NOS:
- iNOS - inducible NOS; produces high NO concentrations in phagocytes for direct toxic effect
- eNOS - endothelial NOS; signalling
- nNOS - neuronal NOS; signalling
Describe the roles of NO• as a signalling molecule.
- vasodilation
- neurotransmission
- s-nitrosylation
Explain the 2 ways ROS causes damage to DNA.
- ROS reacts with nitrogenous base… modified base can lead to mispairing and mutation.
- ROS reacts with ribose/deoxyribose sugar… can cause strand break and mutation on repair.
How can one measure the amount of oxidative damage in a cell?
Amount of 8-oxo-dG present in cells (form of deoxyguanosine oxidised by ROS).
Why is mtDNA particularly sensitive to ROS damage?
- mtDNA is situated near inner MT membrane where ROS are formed.
- Unlike nuclear DNA, mtDNA is not protected by histones.
Explain how ROS damage proteins?
- React with protein backbone… fragmentation… protein degradation.
- React with protein side chain… modification of amino acid… chain in protein structure… protein degradation, loss of function or gain of function.
Describe an example of amino acid modification caused by ROS.
ROS sequesters e- from cysteine residue… formation of disulphide bond between thiol groups of cysteine residues… protein misfolding, cross-linking and/or function disruption.
Explain how ROS damage lipids.
Via lipid peroxidation:
- Free radical (eg. OH•) sequesters hydrogen atom from a polyunsaturated fatty acid in lipid membrane… forms a lipid radical.
- Lipid radical can react with O2… forms a lipid peroxide radical.
- Chain reaction is formed as lipid peroxyl radicals extract hydrogen from nearby fatty acids.
- Hydrophobic environment of bilayer is disrupted and membrane integrity fails.
In which condition does lipid peroxidation by ROS play an important part?
atherosclerosis
