Hemoglobin Flashcards
(81 cards)
1
Q
What is hemoglobin’s structure?
A
- 4 polypeptides called globins
- 4 heme prosthetic groups
2
Q
Mutation of Sickle Cell Disease
A
Mutation in the Beta globin chain
3
Q
What is the therapy for sickle cell disease?
A
Increase the number of F cells which express HbF and has a higher affinity for oxygen than HbA
4
Q
When is Gamma globin expressed?
A
After 6 months of age
5
Q
Hb is slowly non-enzymatically…
A
glycated (measurement of diabetes - hemoglobin A1C)
6
Q
What is Heme’s structure
A
Consists of protoporphyrin IX (tetrapoyrole ring) + ferrous iron (Fe2+) in the center
7
Q
When heme is not associated with a protein it is called?
A
Pro-oxidant
8
Q
The bonds of hemoglobin and heme are ___ and ___.
A
Proximal histidine (Fe 2+ of heme and histidine interaction in the F helix)
Distal histidine (histidine in the E helix - helps stabilize the interaction)
9
Q
What is oxidized hemoglobin called?
A
Methemoglobin (Fe3+) and cannot bind oxygen
10
Q
What is distal histidine’s role?
A
To prevent the oxidation of Fe2+ to Fe3+ and reduce Hb ability to CO. To stabilize the interaction of heme to hemoglobin.
11
Q
When Fe livers are high the liver produces…
A
Hepcidin - inhibits transport of Fe from the mucosal cell by degrading ferriportin
12
Q
Transferrin (binds only Fe3+)
A
Transfer of iron from mucosal cell to capillary bed
Carries Fe in blood, delivers to cells by reception-mediated endocytosis
13
Q
Transfer of iron out of mucosal cell is regulated by
A
Ferriportin (coupled with ferroxidase, hephaestin or ceruloplasmin) to form Fe3+
14
Q
Hepcidin is synthesized by
A
IL-6 which contributes to anemia of chronic disease
15
Q
Hemochromastosis
A
abnormal increase in iron absorption (Iron overload)–> decrease in hepcidin
16
Q
TIBC
A
Transferrin Iron Binding Capacity –> increases in Fe deficient states
17
Q
Ferritin
A
Storage of iron
18
Q
Hemosiderin
A
aggregates of degraded ferritin (and iron) in lysosomes that are increased in iron overload
19
Q
Ferritin translation
A
Regulated by IRE (iron response element) in the 5’ UTR
-When iron levels are low translation is inhibited
20
Q
Transferrin Receptor
A
IRE in 3’ UTR
- High levels of iron, mRNA is degraded
- Low levels of iron, IRE bound by IRE-BP and mRNA is stabilized and translated
21
Q
Transferrin and Ferritin Correlation
A
High iron = increase in ferritin (storage) decrease in transferrin
Low iron = decrease in ferritin and increase in transferrin via IRE-BP to IRE
22
Q
DMT1
A
Divalent Metal Transporter 1 - receptor of iron that other divalent metals compete for
23
Q
Ferrireductase
A
Reduction of Fe3+ to Fe2+
24
Q
Heme Synthesis
A
First step is the rate limiting step and occurs in the mitochondria, last three steps occur in the mitochondria
The intermediate steps occur in the cytosol
25
ALA Synthase
- Rate limiting step
- 8 molecules of glycine
- 1 molecule of succinyl coA
- creates ALA (d-aminolevulinic acid )
- inhibited by heme
26
ALA Dehydrastase Reaction
- second step
- occurs in the cytosol
- inhibited by lead
- forms PBG - porphobilinogen
27
Ferrochetalase
- Final Reaction
- in mitochondria
- inhibited by lead
- Catalyzed Fe 2+ into protoporphyrin IX
28
Regulation of Heme Synthesis
- By rate limiting step ALA synthetase
- Inhibited by heme
- Induced by Fe in red blood cells
- Glucose inhibits
- Steroids increase synthesis
- Cytochrome P450 increase ALA synthetase
29
ALA Synthase Isoform 1 (ALAS1)
ALAS1 - in the liver, and other tissues
Regulated by heme (negative feedback)
- Inhibits transcription
- Increases mRNA degradation
- Blocks post-translational translocation of ALAS1 to the mitochondria
- Short-half life
- Induced by steroid hormones
- Inhibited by glucose
- Activity increased by certain drugs
30
ALA Synthaste Isoform 2 (ALAS2)
ALAS2 - in the erythroid
- Makes heme for hemoglobin(85% daily heme)
- Response to heme levels is more leisurely
Regulated in response to Iron levels
- IRE in 5'UTR translation
- When Iron levels are low translation is inhibited
- Heme regulates synthesis indirectly by regulating Fe from the transport protein transferritin
-Long half life
31
Heme Synthesis Coordination
Erythroprotein induces translation of ALAS2 and alpha and beta globins
Heme increases transription of globins and stabilizes the mRNAs to be translated
- If heme levels are low a kinase is activated that phosphorylates eiF-2-GDP and inhibits translation and globin chains
- If heme levels are high, kinase is inhibited and translation is allowed
32
Defects in Heme Synthesis
- Lead poisoning
| - Porphyrias
33
Lead Poisoning
Inhibits ALA dehydrastase
Inhibits Ferrochetalase
Anemia because decreased Hb
Competes with Calcium and causes calcium disruption in homeostatis
Symptoms are age and dose related
34
Porphyria
- Genetic defect in heme synthesis
- Symptoms due to accumulation of toxic intermediates
- Depletion of essential cofactors, substrates
- Dominantly inherited
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-Precipitating factors:
Drugs metabolized by Cytochrome P450
Alcohol
Fasting/Severe dieting
Hormones
Stress
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35
Treatment of porphyria
Hematin (stable derivative of heme)
Carb rich diet
Withdrawal of any precipitating diet
36
Porphyria cutanea Tarda
```
Most common porphyria
Dominant
In patients with alcoholism
No neurological symptoms
Treatment is abstaining from alcohol, avoiding sunlight, and phlebotomy to reduce iron
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37
Acute Intermittent Porphyria
Intermittent attacks
Dominant
Neurological dysfunction
Colored urine or stool
38
Hemoglobin Deoxygenated State
(T form or taut)
Fe is puckered out from center of heme
Heme plane has a slight dome shape
F helix of the globin chain is at a slight angle
two alpha beta dimers are held by ionic and hydrogen bonds = constrain the movement of the globin chains
39
Hemoglobin Oxygenated State
(R form or relaxed)
Globin movement relieves strain
Oxygen binding ruptures some of the bonds between the two dimers = freedom of movement
Affinity for Oxygen
40
Allosteric inhibitors of Hb affinity for O2
2,3 BPG
H+
CO2
Increasing temperature
41
2,3 BPG
2,3 bisphosphoglycerate
intermediate of glycolysis
binds to a positively charged pocket of 2 Beta globin chains
Stabilizes the T form
Decreases O2 affinity in tissues
42
Why does HbF have a high affinity for O2
HbF - a2gamma2
gamma globin chains have fewer positive charges in center pocket so 2,3 BPG cannot bind as well as it does to the Be a globin chain in adult hemoglobin
HbF low affinity for 2,3 BPG
43
2,3 BPG Importance in...
Adaptation to altitude
Hypoxia
Anemia
COPD - Cardiopulmonary insufficiency
44
Bohr Effect
lower pH = hemoglobin has lower affinity for oxygen
oxygen dissociation curve is shifted to the right
H+ stabilized the T form
45
What converts 70-80% of CO2 produced during aerobic metabolism to bicarbonate in RBC?
carbonic anhydrase
46
What happens to the remaining 15-20% of CO2
Transported to the lungs on hemoglobin as carbamino-hemoglobin
CO2 binds to N terminal of hemoglobin and produces H+
This reaction promotes the release of O2 from Hb in tissues
47
Hemoglobinopathies
Disorders affecting structure, function, or production of globin chains
Co-dominant in inheritance
Common where malaria is present
Severity ranges
Examples:
Thalassemias
Sickle Cell Disease
Acquired:
Methemoglobinemia
CO poisoning
48
Thalassemia
Most common single gene genetic disorder
Partial of complete absence of one or more globin chains
Alpha and Beta
49
Alpha Thalassemia
Deletion of 1 or more of the alpha globin chains
50
Hemoglobin Barts Syndrome
Deletion of all 4 alpha globin chains
51
Types of Hemoglobin Barts Syndrome
Hydrops fetalis - abnormal accumulation of interstitial fluid in a fetus or newborn
Hb Barts - Gamma 4
Increased oxygen affinity makes it a poor transporter
Fatal at or before birth
52
Hemoglobin H disease
A type of alpha thalassemia -
3 alpha genes deleted
HbH - Beta 4
Unstable tetramer in patients with 3 alpha chains deleted
53
Beta Thalassemia
B Thalassemia Minor
| B Thalassemia Major
54
B Thalassemia Minor
```
Heterozygous
Asymptomatic (except mild anemia)
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55
B Thalassemia Major (Cooley Anemia)
Homozygous
Transfusions for severe anemia
Suffer iron overload
56
Sickle Cell Disease
Autosomal recessive
Hemoglobin S == missense
Mutation of glu to val in Beta globin
Decreased solubility but only in deoxy form (insolubility predicates RBC and causes sickling)
Homozygous only have symptoms
57
Sickle Cell Trait
Heterozygous
| No symptoms
58
Carbon Monoxide Poisoning
Acquired Hemoglobinopathies
-Most common fatal poisoning in US
CO competes with O2 for binding to Hb, O2 binding affinity increased in other sites
59
Methemoglobinemia
Acquired Hemoglobinopathies
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(Fe3+)
Oxidation exceeds capacity of reduction
Mutation limits ability to reduce
"chocolate" blood, cyanosis
certain drugs increase metHb
Infants are vulnerable
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Treatment: methane blue reduced metHb
60
Congenital forms of metHb
Hemoglobin M
NADH cytochrome b5 reductase deficiency
61
Hemoglobin M
Mutation in heme binding pocket
| Dominant inheritance
62
NADH cytochrome b5 reductase deficiency
Recessive inheritance
Type I - only erythrocytes
Type II - all cells
63
Heme degradation
Goal: Perserve Fe and convert protoporphyrin to product that can be safely excreted
Regulated to prevent toxic build up of heme and its breakdown products
First step is rate limiting - Heme oxygenase
Second step produces bilirubin - Biliverdin Reductase
64
Heme oxygenase
Cleaves the pyrrole ring and produces biliverdin
Requires: O2 and NADPH
Releases: Cardon Monoxide (CO) and Fe3+
65
Biliverdin Reductase
Requires: NADPH
Produces: bilirubin
66
Physiological Source of CO
ONLY through Heme oxygenase
-Amount of heme being broken down is correlated to the amount of CO produced
- CO acts as a signaling molecule in neural tissue
- CO Role - vasodilatory, anti-inflammatory, autoprotectant properties
67
Bilirubin Conjugation Pathway
Bilirubin carried to liver by albumin
Billirubin (BR) + 2 UDP-glucuronate (by bilirubin glucouronyl transferase) = Bilirubin diglucurnide
Br diglucuronide ACTIVELY transported to bile by Multidrug resistance protein 2 (MRP2)
68
Bilirubin Conjugation
Increases solubility
| Prevents reabsorption from the intestinal lumen and promotes excretion
69
Deconjugation of Bilirubin
Excreted from Bile to the intestine and deconjugated by bacterial hydrolyses and converted to urobilinogens
Most --> Urobilinogens are oxidized to urobilins and secreted in feces
Rest --> A small amount is reabsorbed to the blood and excreted in urine
70
Heme Oxygenase HO-1
Regulation of Heme degradation -->
Expressed in liver and spleen
71
Jaundice
Caused by deposits of bilirubin in skin and sclerae when levels in blood are increased
Increased production or decreased excretion of Br
- Prehepatic (Hemolysis)
- Hepatic (Neonatal, hepatitis, genetic)
- Post-hepatic (Bile duct obstruction)
72
Measurement of Bilirubin
Direct = Conjugated
-Coupled to diazonium salts (azo dyes) in a direct van den Bergh reaction
Indirect - unconjugated
-Bilirubin in non-colvanent complex with albumin won't react with the dyes until albumin is released by an organic solvent
Total - direct = indirect
73
Prehepatic (Hemolysis)
EXCESS PRODUCTION OF BILIRUBIN
Indirect hyperbilirubinemia
Cause: Hemolysis
- Increased production of Br, exceeding capacity of liver to conjugate
- Increase urobilinogen in blood and urine due to increased conjugated Br reaching intestine
74
Intrahepatic (hepatocellular) jaundice
PROBLEM WITH UPTAKE, CONJUGATION OR SECRETION FROM HEPATOCYTES
direct or indirect hyberbilirubinemia
75
Hepatitis
No bile duct obstruction
Conjugated bilirubin in urine
Liver enzymes often increased
76
Neonatal Jaundice
Most common in pre-mature babies
Bilirubin glucuronyl transferase is low
Bilirubin serum levels exceed levels of albumin it may rise to toxic levels in basal ganglia causing a toxin encephalopathy --> kernicterus
77
Intrahepatic Jaundice
Crigler-Najjar Syndrome
Gilbert Syndrome
Dubin-Johnson syndrome
78
Crigler-Najjar Syndrome
Rare deficiency bilirubin glucuronyl transferase
Type 1 = total deficiency
Type 2 = less severe
79
Gilbert Syndrome
Benign mild elevation indirect bilirubin
| Mutation in promoter of bilirubin glucuronyl transferase gene, decreased expression
80
Dubin-Johnson syndrome
Defective transport of conjugated bilirubin out of hepatocytes (or liver)
Rare mutation in MRP2 (multidrug resistant protein 2)
81
Post Hepatic (Obstructive) Jaundice
BILE DUCT OBSTRUCTION THAT PREVENTS EXCRETION OF CONJUGATED BILIRUBIN
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Direct hyperbilirubinemia
Bile acids may also accumulate in plasma
Conjugated bilirubin in urine
No urobilinogens in stool
Prolonged obstruction can lead to liver damage and increased unconjugated (indirect) Br
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