Lecture 22: PPP, DLA 10: Reactive oxygen, DLA 11: G6PD Def Flashcards
List the tissues in which the PPP is active, and identify its subcellular location
- Occurs in the Cytosol of the cells
1) liver, mammary glands, adipose tissue (NADPH for fatty acid biosynthesis)
2) Adrenal cortex, testes, ovaries (NADPH for steroid hormone synthesis
3) ERYTHROCYTES-RBC’s (NADPH for reduction of glutathione)
- MOST IMPORTANT, they are dependent on it to get their NADPH!
4) WBC and macrophages (NADPH for phagocytosis)
Summarize the function of the PPP (metabolic roles of pentoses and NADPH)
-Alternate pathway for the oxidation of glucose
Function:
1) Provides tissues NADPH+ H+ that it produces
2) Provides (PRPP) –> purines and pyrimidines synthesis
Outline the overall reactions of the PPP
Glucose —(Enzyme: G6PD)——> Glucose 6-phosphate
Possible outcomes next:
1) Pyruvate
2) Glycogen
3) Ribulose 5-phosphate (Oxidative
- Needs NADP+
- Used for purines and pyrimidines synthesis
State the reactions of the oxidative portion of the PPP (emphasize the significance of G6PD)
- Oxidative phase:
- 2 NADPH formed
- Irreversible
- Enzyme: G6PD –> Regulator Enzyme/Step (Will increase or decrease based on NADPH needs)
- Form 1st pentose
Path: Glucose 6-phosphate --(G6PD + NADP+)-> 6-Phosphogluconolactone + NADPH + H+ --> 6-Phosphogluconate --> (6-phosphogluconate dehydrogenase + NADP+)--> Ribulose 5-Phosphate + NADPH + H+
State the enzymes and provide an overview of the intermediates of the non-oxidative part of the PPP.
Highlight the importance of transketolase
- Nonoxidative phase:
- Happens in cells that don’t require pentose phosphates –> converts extra Ribulose 5-Phosphate from oxidative phase –> Forms intermediates for glycolysis
- Reversible
Path:
Inversion of Pentose:
1) Glucose 6-P –> (Isomerase) –> Ribose 5-P
2) Glucose 6-P–>(Epimerase) -> Xylulose 5-P
3) Glucose 6-P–>(Epimerase) -> Xylulose 5-P
Forming Intermediates:
1) Xyulose 5-P -> (TPP) ->Glyceraldehyde 3-P –> (Transaldolase) –> Fructose 6-P = intermed. (Enters into glycolysis)
2) Ribose 5-P -> (TPP) -> Sedoheptulose 7-P
- -> Erythrose 4-P –> Fructose 6-P = intermed (enters into glycolysis)
3) Xyulose 5-P -> (TPP) -> Glyceraldehyde 3-P = intermed (enters into glycolysis)
(Comes in between erythrose and fructose)
-Transketolase =(TPP) catalyzes the reaction making it key (Requires Vit B1= thyiamine) –> Most important enzyme
Explain how the PPP is regulated and how different parts of the pathway are used, depending on the needs of the cell
1) Requires both NADPH and Pentose:
(Ex: Liver)
-Will only go through oxidative phase bc it makes everything it needs
-there will also be no excess of products so it doesn’t need the non-oxidative phase
2) Cells requiring only NADPH (RBC’s):
- Goes through oxidative phase to obtain NADPH
- But doesn’t require pentose so it undergo non-oxidative phase
3) Cells requiring only pentose:
- Synthesis of nucleotides
- Non-oxidative phase goes in reverse (Intermediates –> pentoses needed)
- Doesn’t require oxidative phase
Evaluate the use of NADPH in RBC and WBC: Detoxification of hydrogen peroxide,
Phagocytosis, Nitric oxide synthesis, Reductive biosynthesis
1) Phagocytosis
- Increase in oxygen consumption by phagocyte = Respiratory or oxidative burst
- NADPH (Enzyme: NADPH Oxidase) used to form superoxide (O2) which kills bacteria or fungi, Superoxide dismutase (SOD) –> Hydrogen Peroxide (H2O2) which kills bacteria –> Forms HOCl which kills bacteria and fungi
2) Detoxification of hydrogen peroxide
-Goal: maintain reduced gluathione (GSH)
(Detoxifies H2O2)
Discuss the clinical relevance of a deficiency of glucose 6-phosphate dehydrogenase:
Identify the precipitating factors, Predict the symptoms and laboratory findings in the disorder, Analyze biochemical basis of symptoms and laboratory findings
(hemolysis and Heinz bodies in RBC), Correlate role of PPP in hydrogen peroxide metabolism
- X-Linked recessive
- Old age increases risk
- Allelic Heterogeneity
- G6PD Mediterranean deficiency = severe
- G6PD A- = moderate
Symp:
- Jaundiced (increased serum bilirubin)
- Acute hemoglobin
- Reddish brown urine (hemoglobin)
Inducing Factors (Oxidants) Resulting in hemolysis:
1) Infection
2) Oxidant drugs (Sulfa drugs, primaquine)
3) Fava (Broad) Beans
Can Result in:
1) Hemolysis: Increased precipitation of proteins bc decrease in detoxification of free radicals and peroxides
2) Heinz bodies:
- denatured proteins as insoluble masses
Predict the effect of NADPH oxidase deficiency and myeloperoxidase deficiency.
-Both from defective phagocyte
NADPH oxidase deficiency: -Causes: Chronic granulomatous disease (CGD) --> Bacterial and fungal infection -Lack of oxygen consumption -Test = -
Myeloperoxidase Deficiency:
- Can get a reccurent fungal infection bc can’t form HOCl (Hypochlorous acid) which kills fungi
- Test = + or normal burst and decreased HOCl
(Can do respiratory burst test to figure out which is deficient)
Discuss the function of glucose-6-phosphate dehydrogenase
and its role in protecting the red blood cell membrane from
oxidative stress
Function
- Catalyzes the rate limiting step for the pentose phosphate pathway
- Supplies NADPH and Ribose
Role:
-Protects the RBC from oxidative state –> Helps w detoxifying H2O2 and turning it into H2O
Discuss the function of glucose-6-phosphate dehydrogenase
and its role in protecting the red blood cell membrane from
oxidative stress
Function
- Catalyzes the rate limiting step for the pentose phosphate pathway
- Supplies NADPH and Ribose
Role:
-Protects the RBC from oxidative state –> Helps w detoxifying H2O2 and turning it into H2O
-W/o this RBC have shorter half-life, less tolerant to oxidative agents, impairs formation of NADPH = RBC decrease levels of glutithione in its reduced form and will break apart quicker
Discuss the X-linked recessive inheritance pattern of G6PD
- Never a male to male transmission
- All males are Hemizygous for the x chromosome
Explain how malaria may be a factor in the relatively high
rate of G6PD deficiency alleles in some populations
-Protects them against malaria
Define the terms radical, free radical and reactive oxygen species (ROS), RNOS.
Discuss cellular damage and effects of ROS
Definition:
1) ROS = free radials
2) RNOS= free radical
3) Free radical = molecule w/ a single unpaired electron in orbital and is highly reactive
Cellular damage:
1) High oxygen tension
(COPD pts on supplement oxygen)
2) Hypoxia, carbon monoxide poisoning, ischemic reperfusion
3) Acetaldehyde which is formed in the liver
after large intake of ethanol
Describe formation of superoxide anion radical related to the ETC or cytochromes
- Takes place in the Electron transport chain
- Molecular oxygen after uptake of one electron
-It is formed by the CoQH.
radical (Accidentally donates electron to oxygen)
Describe the reaction catalyzed by superoxide dismutase
Function:
-Responsible for scavenging of superoxide
Action:
-Uses two superoxides as substrates and forms
hydrogen peroxide (H2O2) and oxygen (O2) as products
Location of Superoxide dimutases:
1) Extracellular space (copper)
2) Cytosol (copper and zinc)
3) Mitochondria (manganese)
Indicate the biochemical defect in amyotrophic lateral
sclerosis (ALS).
Cause:
-Deficiency of superoxide dismutase (SOD)
Results:
- Death upper and lower motor neurons
- Difficulty breathing
- Head drop
- Muscle weakness
Discuss the formation of hydroxyl radicals related to ionizing radiation, Haber-Weiss reaction and Fenton reaction.
Haber-weiss reaction:
-Hydrogen peroxide and superoxide combine
Fenton reaction:
-Hydrogen peroxide and ferrous iron combine
-Both form a hydroxide radical
Describe the reactions catalyzed by superoxide dismutase, catalase
and glutathione peroxidase and indicate the ROS scavenged.
Discuss the importance of selenium and indicate the enzyme containing
selenium
-All scavenge Hydroxyl peroxides
Super dimutase:
-Uses two superoxides and forms hydrogen peroxide and oxygen as products
Catalase:
- Scavenges Hydroxy radicals
- Found in peroxisomes
- Uses hydroxide peroxide for detoxification (Uses 2 Hydrogen peroxides as substrate)
Glutathione Peroxidase:
- Needs selenium as a cofactor
- Uses 2 molecules of reduced GSH
- Changes lipid peroxides back to normal poly-unsaturated fatty acids (PUFA) in membranes
- W/o repair= “leaky” membranes
Discuss the formation of hydroxyl radicals related to ionizing radiation, Haber-Weiss reaction and Fenton reaction.
Haber-weiss reaction:
-Hydrogen peroxide and superoxide combine
Fenton reaction:
-Hydrogen peroxide and ferrous iron combine
-Both form a hydroxyl radical
Discuss the compartmentation of free radical defense in the cell
- Act to protect the mitochondrial DNA
- Act as molecules to scavenge the free radicals and avoid making free radicals
Lines of defense:
1) Superoxide dimutase
2) Glutathione peroxidase
3) Vit E
4) Sequesterin
Indicate non-enzymatic radical scavengers.
Include vitamin C and E (found in fruits and vegetables).
- Nonenzymatic endogenous molecules:
1) Uric acid (High levels can lead to gout)
2) Bilirubin (High levels can lead to jaundice)
3) Glutathione (Released by the liver into the blood)
4) Vitamin C
5) Vitamin E
Discuss the beneficial role of ROS formation as an
immunological response in phagocytosis
Describe the reactions catalyzed by NADPH oxidase and myeloperoxidase
Benefits ROS:
-Destroys bacteria
NADPH oxidase:
-Generates superoxides using molecular oxygen and NADPH in phagolysomes
Myeloperoxidase:
- Secreted into the phagolysosome
- Forms hypochlorous acid (Bleach)
- Destroys bacteria and fungi
Discuss the direct and potential effects of excess
nitric oxide formation.
Discuss the importance of
inducible nitric oxide synthase for formation of RNOS
Excess NO:
-Sepsis –> Excess NO = vasodilation, hypotension
NO Importance:
- Formed in macrophages by inducible NO synthase
- Needed for RNOS formation as cell defense
- Destruction of invading bacteria
Predict the effect of NADPH oxidase deficiency and myeloperoxidase deficiency
Indicate the biochemical defect in chronic granulomatous disease.
Myeloperoxidase (MPO)
deficiency:
-Myeloperoxidase deficiency (respiratory burst test positive)
-Lack of HOCL in phagolysosomes leads to less destruction of fungi.
-Recurrent fungal infections with candida albicans –> Oral and genital infections
-Destruction of bacteria is normal
Chronic granulomatous disease:
- Hereditary NADPH Oxidase Deficiency (Respiratory burst test= -)
- Recurrent severe infections, bacteria, fungi
- X-linked
- Lack of superoxide formation in phagocytes leads to decreased ROS, RNOS, HOCL.
