EXAM #1: HEMOGLOBIN Flashcards
What is the most abundant form of Hb in adult? What subunits does it have?
HbA
- Tetramer
- 2x alpha globins
- 2x beta globins
How many oxygen binding sites does Hb have?
4, one per subunit
What happens to oxygen binding as oxygen levels increase?
Hb binds O2 more efficiency at higher O2 concentrations b/c of cooperative binding
How does myoglobin’s affinity for oxygen differ from Hb?
Myoglobin binds O2 with HIGH affinity at LOW oxygen concentrations
What is the effect of lower pH/ higher CO2/ lower O2 on oxygen binding?
Favors oxygen dissociation
What is the effect of higher pH/ lower CO2/ higher O2 on oxygen binding?
Favors oxygen binding
What is the effect of 2,3-BPG on oxygen binding?
Favors dissociation
What is Hb A2?
Hb with:
- 2 alpha globins
- 2 delta globins
What are the three Hb sub-types present in the early embryo? What are the subunits?
Hb Gower 1= 2-zeta and 2 epsilon
Hb Gower 2= 2 alpha and 2 epsilon
Hb Portland= 2 zeta and 2 gamma
What sub-type of Hb predominates as the embryo begins to age? What are the subunits?
Hb F= 2 alpha and 2 gamma
This is the major Hb of the fetus
When does an infant have a switch from predominantly Hb F to Hb A?
By 1 year old, the 98% of Hb is Hb A (vs. F in utero)
How does the oxygen affinity of Hb Gower 1,2, Portland, and F compare to Hb A?
V. high relative oxygen affinity
What is the purpose of increased oxygen affinity of the fetal and embryonic Hbs?
To “capture” oxygen diffusing from maternal circulation
What is the molecular basis of the higher affinity of Hb F for oxygen than Hb A?
- Histidine in B-chain
- Replaced with serine in gamma-chain
2,3-BPG binds to the Histidine in Hb A (Beta globin). Serine removes the positive charge that 2,3-BPG binds to, increasing the affinity of Hb F for oxygen.
What are the alpha-like globins? What chromosome are they encoded on?
Chromosome 16
- alpha
- zeta
Note that the pseudo genes are also encoded on chromosome 16–these cannot produce a functional protein product.
What are the beta-like globins? What chromosome are they encoded on?
Chromosome 11
- Beta
- Gamma
- Delta
- Epsilon
What is the control region located upstream to the alpha-like globins?
HS-40
Confers high level tissue specific expression of alpha-like globins
What is the control region located upstream of the beta-like globins?
LCR
Confers high level tissue specific expression of beta-like globins
What are hemoglobinopathies?
Disorders of hemoglobin
What are the two categories of hemoglobinopathies?
1) Structural variants–>Heinz bodies
2) Thalassemias
What is the definition of Thalassemia?
Imbalanced synthesis of alpha and beta globins i.e. reduced synthesis of one or more of the globin chains
What is a Heinz body?
- Unstable Hb forms unstable complexes and is oxidized to methemoglobin
- Methemoglobin form hemichrome
- Hemichrome aggregates form Heinz bodies
What are the three major Hb structural variants?
1) Hb S
2) Hb C
3) Hb E
What is Hb S?
This is the Hb variant found in Sickle cell disease
How does Hb S differ from Hb A?
Glutamate at position 6 in the Beta-globin gene is replaced with VALINE
- Valine is v. hydrophobic and polymerizes when deoxygenated
What is the difference between Sickle Cell Trait and Sickle Cell Disease?
Trait= heterozygous mutation
Disease= homozygous mutation
Explain how the valine substitution at position 6 contributes to the pathology of Sickle Cell Disease.
- Valine is EXPOSED in deoxygenated Hb S
- Exposed valine forms a hydrophobic patch
- Hb S has a tendency to accumulate and stack
Stacked Hb forms the “Sickle” distortion of RBCs
What are the negative consequences of having Sickle-shaped RBCs?
- Clog the microcirculation
- Reduced lifespan
Leads to hemolytic anemia
What is the agent of choice for the management of sickle cell disease?
Hydroxyurea
How does hydroxyura treat Sickle Cell Disease?
- Antineoplastic agent
- Increases Hb F
- Hb F promotes solubility of Hb and suppresses the sickling of cells
What is Hb C?
This is a Hb variant with the glutamate at position 6 substituted with LyCine and forms Crystals
What is the pathology associated with Hb C?
- Hb C has reduced solubility and Crystallizes in RBCs
- Makes RBCs INFLEXIBLE, which leads to extravascular hemolysis/ anemia
What is Hb SC?
Disease caused by compound heterozygote of Hb S and Hb C; the disease course is milder than Hb S
What is Hb E?
- Hb variant with the glutamate at position 26 replaced by lysine
- This mutant B-chain is NOT synthesized as efficiently as Hb A
–>Imbalanced chain synthesis; found in SE asia
What is the clinical presentation of Hb E?
Mild thalassemia i.e. imbalance in the synthesis of alpha and beta globin chains causing:
- Microcytosis
- Hypochromia
- MILD anemia
What types of mutations lead to Thalassemias?
1) Deletion
2) Point mutation
Generally, what is an alpha or beta thalassemia?
Disorder where either an alpha or beta globin chain is not being produced
Generally, what is an alpha+ or beta+ thalassemia? What about alpha or beta0?
- Disorder where an alpha or beta chain is being produced in REDUCED QUANTITIES
- vs. no quantities
Why are alpha-thalassemias manifest in both fetal and adult life?
Hb requiring alpha-globins are found throughout life:
- Embryonic= Gower 2 (a2e2)
- Fetal= (a2g2)
- Adult= HbA and HbA2
Describe the phenotype of a patient with one defective alpha gene and three functional alpha genes.
No clinical signs
Describe the phenotype of a patient with two defective alpha genes and two functional alpha genes.
Mild anemia
Describe the phenotype of a patient with three defective alpha genes and one functional alpha gene.
Severe deficiency that leads to formation of:
1) Gamma tetramers i.e. Hb-Barts in the fetus
2) Beta tetramers i.e. Hb H in the adult
Both are POOR oxygen carriers**
Describe the phenotype of a patient with four defective alpha genes and one functional alpha gene.
This is LETHAL and results in Hb Barts hydrops fetalis syndrome
How is Hb-constant spring formed?
This is a POINT MUTATION that leads to alpha-thalassemia
- stop codon converted to codon for glutamine
- read-through occurs and increases alpha-globin chain length
Specifically, T is replaced by a C
What would you expect to see in patients expressing Hb-constant spring?
mRNA is very UNSTABLE; thus, only 1-2% of the Hb is Hb-constant spring; thus, this behaves like alpha+
What is Hb-lepore?
Hb with part beta and part delta globin in one globin chain
How is Hb-lepore formed?
- Recombination events delete part of Beta and Delta globin
- A fusion protein is produced that has both parts
Describe the case where a point mutations gives rise to a Beta+ thalassemia.
- Point mutations ( A sub for T) generates an inappropriate splice site that is SOMETIMES recognized
- Result is reduced message and too little Beta-globin
What are the advantages to the use of electrophoresis in the analysis of Hb variants?
Useful for neonatal screening
- Cheap
- Easy
What are the disadvantages to the use of electrophoresis in the analysis of Hb variants?
- Insensitive
- Difficult to use in premature infants
How is restriction fragment length polymorphism used to identify individuals who carry particular Hb mutations?
1) Take blood sample
2) Expose to PCR primers
3) Amplify B-globin gene
4) Expose to MstII restriction enzyme
- Normal= 550 bp
- Mutant HbS= 700 bp
What is the structural cause for the positive cooperatively seen with oxygen binding?
- O2 binds alpha subunits first
- Binding of O2 to the second alpha globin causes a conformational change that rotates valine from the oxygen binding site on beta-globin
What are the structural variants of Hb that alter oxygen affinity? What is the mutation and effect?
Hb-Helsinki=
- B-subunit mutation of Lysine for Methionine at 2,3 BPG binding site
- INCREASED oxygen affinity
Hb-Kansas
- B-subunit mutation of asparagine for thereonine at a1B1 contact site
- DECREASED oxygen affinity
What are the structural variants of Hb that readily form Methemoglobin?
Hb M-Boston
- Alpha mutation (Distal histidine for tyrosine)
Hb M-Hyde Park
- B mutation (Proximal histidine for tyrosine)
