MCP Lecture 8 Flashcards
What are ROS? Give 3 examples
chemically reactive species formed upon incomplete reduction of oxygen - unpaired electron = free radicals
Superoxide (.O2-) = moderate activity Hydrogen peroxide (H2O2) = low activity (oxidant - not free radical) Hydroxyl Radical (.OH) = Highest reactivity
Where are superoxides generated?
mitochondria (byproduct of ATP synthesis)
complex III: superoxide - injected into intermembrane space
complex I: exclusively released into the matrix
Under what conditions is O2- production increased?
- high membrane potential = slower transfer of electrons leading to a higher reduction level of electron carriers at the Q site and increases electron leak to O2
- High NADH/NAD+ ratio: over reduction of the electron transport chain
- Electron transport chain damage - alters electron flow accuracy
- Xenobiotics: compounds can block electron transport which increases reduction level of electron carriers located upstream
- electron backflow in complex I - over-accumulated succinate during ischemia rapidly oxidized by complex II - drives electrons back through complex I and increases superoxide production
What is MPTP and what is an adverse effect?
an inducer of Parkinson’s disease - xenobiotic
blocks electron flow to the Q site and increases electron leak in complex I
What is the Fenton reaction?
.O2- + H2O2 —> O2 + H2O + .OH
presence of iron is catalytic - converts hydrogen peroxide into hydroxyl radical and water (so overaccumulation of free iron causes oxidative stress)
What does ionizing radiation cause?
homolytic fission of the O-O bond in H2O2
H2O2 —> 2 .OH
How is nitric oxide generated in the cell?
nitric oxide synthase which metabolizes arginine to citrullin
How is peroxynitrite formed?
reaction of NO. with superoxide to form peroxynitrate - very reactive (this can give rise to hydroxyl)
What are the forms of oxidative damage?
- DNA damage - nucleic acids bind iron well - target of the .OH from the fenton reaction
- lipid peroxidation - produce lipid peroxide which causes membrane damage
- hydroxyl radicals directly oxidize amino-acid side chains which cause protein damage
What is used as an indicator for the extend of DNA damage in the cell? (used for diagnosis of Friedreich’s ataxia)
8-hydroxy-2’-deoxyguanosine - due to oxidative leasion
Which FA are most susceptible to peroxidation?
arachidonic acid and linoleic acid due to hydrogens close to the double bonds being highly reactive and prone to lose electron to .OH
What are the most reactive/common carbonyl groups generated from lipid peroxidation? Example? What does it react with?
aldehydes = 4-hydroxy-(2E)-nonenal which reacts with the side chain of cysteine, histidine, and lysine residues
How can cells convert primary ROS (superoxide) into hydrogen peroxide? From hydrogen peroxide to water?
superoxide dismutase
catalase
What is the mechanism of superoxide dismutase?
converts 2 molecules of superoxide into one molecule of O2 and one H2O2 (less toxic)
What does the cytosolic SOD (superoxide dismutase) use as a ligand? Mutation of this enzyme causes what?
Cu and Zn
mutation in Sod1 causes amyotrophic lateral sclerosis (ALS/Lou Gehrig’s disease)
What does mitochondrial SOD use as a ligand?
Mn
What are the three different enzymes/pathways to decompose hydrogen peroxide?
- glutathione peroxidase - H2O2 to water, consuming two molecules of reduced glutathione
- peroxiredoxin pathway - has production of oxidized peroxiredoxin with a disulfide bond - must be reduced 2X more
- catalase - heme containing - located in peroxisome
What does deficiency in glucose-6 phosphate dehydrogenase cause? Why?
oxidative stress and hemolytic anemia because NADPH (created by PPP by glucose-6 phosphate dehydrogenase) is needed by glutathione reductase (to regenerate reduced glutathione for the glutathione peroxidase)
What are non-enzymatic ROS defenses?
Anti-oxidant therapy:
- coenzyme Q10 = dietary supplemetn, scavenge RO2. radicals and inhibit lipid peroxidation
- glutathione = also co-factor for glutathione peroxidase - keeps sulfhydryls of proteins reduced and maintains their biological activity
- vitamin E = protects membrane lipids and lipoproteins
- vitamin C = reacts with wide spectrum of radicals
- plant phenols = inhibits LDL oxidation
- Flavonoids = from tea, fruit skins, veggies - reduce coronary artery diseases and stroke
What are some physiological roles of ROS/RNOS?
moderate levels are require for redox signaling that regulate cell growth, differentiation, and apoptosis
-also: innate immunity, anti-cancer, body weight control, wound healing
How is redox homeostasis maintained?
glutathione (GSH) and thioredoxin (TRX)
How are ROS used in innate immunity?
neutrophils undergo respiratory burst - potent antimicrobial agents and also participate as redox signaling molecules (modulate transcription factors such as NF-kappa B which regulates expression of key cytokines and chemokines that further regulate the inflammatory response)
How do ROS cause apoptosis?
catalyze cardiolipin peroxidation which facilitates the detachment of cytochrome c from mitochondrial - release into cytoplasm
Why is lowering ROS by antioxidant therapy bad in cancer patients?
might stimulate growth of tumors by suppressing apoptosis (needs ROS)
How do ROS control food intake and body weight control?
ROS is signal for activating POMC neurons to release anorexigenic hormones and decreasing AgRP’s activity in secreting orexigenic hormones
= Reduced food intake
