Cell signaling by oxygen: Reactive Oxygen Flashcards
Does high pO2 or low pO2 cause damage?
Both cause damage
Which free radicals cause the most damage
hydroxyl radical
nitrite radical
What are reactive oxygen species?
Oxygen-containing molecules that oxidize substrates
What are free radicals?
Molecule that contains an unpaired electron
What is the fenton reaction?
2 H2O2 atoms react with Iron to create water, a peroxide radical and a hydroxyl radical
2 H2O2 -Fe2+–> H2O + HOO* + *OH
Name 2 free radical precursors
Hydrogen peroxide (H2O2) hypochlorite anion (OCl-)
Describe the superoxide-nitric oxide reaction
H+ + O2* + NO* –> H+ + ONOO* –> *OH + *NO2
What reaction produces hypochlorite anion from hydrogen peroxide?
myeloperoxidase reaction
Name the 6 reactions that can generate free radicals
Fenton reaction Haber-Weiss reaction Superoxide-nitric oxide reaction Myeloperoxidase reaction Hypochlorite-iron reaction Hypochlorite-superoxide reaction
What is the free radical chain reaction?
A free radical donates its electron to a substrate (R2)
r2 becomes a free radical
What causes the free radical chain reaction to speed up or slow down?
Speeds up when highly reactive radicals are formed
Slows down when unreactive molecules are formed
What is usually the R1 in the free radical chain reaction? What is the resulting R2?
R1 is usually a macromolecule (lipid, protein)
R2 is usually an aggressive free radical (*OH or *NO2)
What is the result of oxidation or nitration?
Alters function similar to phosphorylation
What are the 5 sources of reactive oxygen species?
Ionizing radiation (X-ray, radon, cosmis rays)
Metal catalysts (Fe2+)
ROS reactions
Metabolism (ubiquinon)
Enzymatic catalysts (NADPH oxidases, macrophages)
What is the mitochondrial ubiquinon cycle?
Mitochondria generate superoxide as a by-product of electron transport by ubiquinon cycle to the electron transport chain
What causes the most damage during reperfusion after ischemia?
Reactive oxygen species burst occurs between ischemia and reperfusion
What produces the ROS burst after ischemia?
Mitochondria
What can the ROS burst after ischemia lead to?
Apoptosis & necrosis
What is an example of an ischemia?
Heart attack
stroke
gut torsion
What occurs to ATP, cytosolic pH and mitochondrial potential during ischemia?
What about cytosolic Na+ and Ca2+?
ATP, pH, and mitochondrial potential go down
Na+ and Ca2+ increase
Describe the production of ROS by neutrophils
Neutrohpil binds with NADPH oxidase (NOX)
NOX associates with RAC to produce a superoxide radical (*O2)
*O2 forms hydrogen peroxide spontaneously or by using enzyme superoxide dismutase (SOD)
Neutrophil binds myeloperoxidase vesicles
MPO makes hypochlorite from H2O2 and Cl-
- O2 reacts with OCl- to form *OH
- O2 and *OH kill bacteria
What other pathway can nitric oxide take aside from cell signaling?
Why is this reaction important?
- NO + *O2 –> *NO2
* NO2 is highly reactive and capable of substance nitration
Name some of the damaging features of oxidation or nitration caused by ROS
Damage to lipid membranes Failing enzymes Failing cell function Altered DNA expression Failure to repair DNA
What are the 2 defense cycles against ROS?
Superoxide dismutase and catalase
GLutathione cycle
Describe the superoxide dismutase & catalase defense cycle
2 *O2 –SOD–> H2O2
2 H2O2 –catalase–> O2 + H2O
Describe the glutathione cycle
H2O2 + 2 GSH –> GSSG + 2 H2O
2 NADPH + GSSG –> 2 NADP+ + 2 GSH
What is GSH?
Reduced glutathione
What is GSSG?
Oxidized glutathione
What are the 2 enzmes associated with the glutathione defense cycle?
glutathione peroxidase
glutathione reductase
How can the control of iron defend against ROS?
Iron is bound to transferrin and stored in ferritin
No fenton reaction
How can antioxidants combat ROS damage?
Ascrobic acid is radicalized by a free radical
2 *Asc –> dehydro-ascorbate (DH-Asc) + Asc
Name 5 antioxidants
Ascorbic acid, Uric acid Vitamin E Vitamin A Beta-carotene
What receptors are responsible for oxygen sensing?
Peripheral receptors
Carotid bodies
Aortic bodies
How do peripheral sensors control oxygen?
Send afferent signals to brain to increase respiration
What cranial nerve do carotid bodies send signals to?
Cranial nerve 9
Describe the depolarization of a glomus cell in the carotid body
Cell membrane heme closes K+ channels in response to low blood pO2
low blood pO2 initiates AMP kinase to create AMP which acts to keep K+ channels closed
NADPH oxidase produces *O2 to keep K+ channels closed
Depolarization
Depolarization opens Ca2+ voltage-gated channels
Ca2+ influx initiates release of neurotransmitters acetylcholine and dopamine
neurotransmitters activate afferent dendrites of CN IX
What are the 3 sensors in the glomus cell that react to low pO2?
Cell membrane heme protein
production of AMP via AMP-kinase
Superoxide production by NADPH-oxidase
What is HIF-1?
During hypoxia, Hypoxia inducible factor upregulates expression of genes that enable cells to cope or to ensure hypoxia returns to normoxia
Describe HIF-1 during normoxia
prolyl-hydroxylases hydroxylate HIF-1alpha
Hydroxylated HIF-1alpha is ubiquinated
Describe HIF-1 during hypoxia
In absence of O2, HIF-1alpha moves to nucleus and binds with subunit HIF-1beta and transcription factors p300/CBP
Transcription of lactate dehydrogenase, GLUT1, EPO, VEGF
