Oxidative stress Flashcards
What is a ROS?
Reactive Oxygen Species
What is a RNS?
Reactive Nitrogen Species
What is a free radical?
A species with an unpaired electron
How is a free radical denoted?
Using a superscript dot
How are ROS produced?
O2 gains an electron forming Superoxide (O2•–)
What happens to Superoxide?
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
What enzyme converts superoxide into Hydrogen peroxide H2O2?
Superoxide dismutase (SOD)
How does Hydrogen peroxide go onto cause damage?
Fenton reaction
H2O2 reacts with Fe2+ producing ROS/free radicals
What can H2O2 by reduced to to cause damage?
Water and Hydroxyl radical OH .
What 2 ways can ROS/RNS damage DNA?
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).
What can the mutation on repair of DNA caused by ROS/RNS damage lead to?
Cancer
What 2 ways can ROS damage proteins?
Damage the:
-Backbone
-Sidechains
How does ROS interacting with the protein back bone cause problems?
Leads to backbone fragmenting
Leads to protein degradation
How does ROS reacting with the amino acid side chains of a protein cause problems?
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
How can changing the protein structure affect a protein?
Lead to loss of function
Degradation
Or gain of function
How do inappropriate disulphide bonds form when a ROS reacts with a protein?
ROS takes electrons from cysteines
Causes misfolding, cross linking and disruption of function
How do ROS cause damage to lipids?
(General process)
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….
Why are ROS dangerous to cellular membranes?
The hydrophobic environment of the bilayer gets disrupted by Lipid peroxidation and membrane integrity fails
What is Lipid peroxidation?
The chain reaction caused by the damage of free radical/ROS to lipids of the cell membrane
What are the 2 types of sources of oxidants?
Endogenous
Exogenous
List 3 main examples of endogenous oxidants
Electron transport chain
Nitric oxide synthases
NADPH oxidases
List 4 examples of Exogenous oxidants
Radiation (UV, X-rays and cosmic rays)
Pollutants
Drugs (primaquine =anti malarial)
Toxins (herbicides)
How is the Electron Transport chain a source of ROS?
Electrons sometimes escape and react with dissolved O2 forming Superoxide
What is Respiratory burst?
The rapid production of Superoxide and H2O2 from Phagocytes to destroy invading bacteria
Which enzyme provides the electron to oxygen to form superoxide in phagocytes?
NADPH Oxidase
What is Chronic granulomatous disease?
Genetic defect in NADPH oxidase
Why does Chronic granulomatous disease cause increased susceptibility to bacterial infections?
Superoxide can’t be produced by phagocyte so respiratory burst will not occur to destroy bacteria
What are the 2 key enzymes that are important in the cellular defences against ROS?
Superoxide dismutase (SOD)
Catalase
What is the role of Superoxide dismutase in cellular defences against ROS?
Converts Superoxide to H2O2 and oxygen
Superoxide is very good at initiating chain reactions
What is the role of catalase in the cellular defences against ROS?
Converts H2O2 to water and oxygen
What is GSH?
The reduced/active form of Glutathione
How does GSH protect from oxidative stress/ROS?
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
What is GSSG?
The oxidised/inactive form of Glutathione
How is GSSG recycled back into the active reduced form of Glutathione (GSH)?
NADPH transfers electron to disulphide bond
GSH reforms
Which pathway is a major source of NADPH?
Pentose Phosphate pathway
What is the significance of Vitamin E and Vitamin C in cellular defences against free radicals?
Vitamin E protects against lipid peroxidation
Vitamin C regenerates reduced/active form of Vitamin E
What is oxidative stress?
The imbalance in Oxidants and defences/antioxidants
How does cataracts form occur as a result of oxidative stress in Galactosaemia?
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
Why are individuals with G6PDH (Glucose 6 phosphate dehydrogenase) Deficiency susceptible to oxidative damage?
Pentose phosphate pathway ceases
Major source of NADPH
NADPH essential in regenerating GSH
What are Heinz bodies?
When inappropriate disulphide bonds result in aggregations of cross linked haemoglobin
What is a clinical sign of G6PDH deficiency?
Heinz bodies
Which part of the body removes Heinz bodies?
Spleen
How are normal levels of paracetamol normally metabolised?
Metabolised via pathways producing safe products
What is the toxic product produced when somebody overdoses on paracetamol and why is this produced?
NAPQI
Made since safe metabolic pathways of paracetamol fully saturated
How is NAPQI directly and indirectly toxic?
Directly causes oxidative damage to Hepatocyte
Indirectly: Depleted GSH making cell vulnerable to oxidative damage
How is an overdose of Paracetamol treated?
Drug Acetylcysteine
How does Acetylcysteine help treat a Paracetomol overdose?
Replenishes GSH (Active Glutathione) to protect against oxidative damage
What are the 3 ways by which a cell can be damaged by an oxidant?
Lipid peroxidation
Damage to proteins
Damage to DNA
