Normal and Abnormal Hemoglobin Structure and Function Flashcards
Hemoglobin
- Oxygen Affinity at [High] vs. [Low]
- Binding Curve Shape
- Factors favoring O2 dissociation
- Factors favoring O2 binding
- Hb has higher affinity for O2 when there is high oxygen present
- Sigmoidal
- Dissociation: favored by L__ow pH, High CO2, Low O2
and the presence of 2,3 BPG favors dissociation
- Binding: High pH, Low CO2, High O2
What subunits does Hb A2 consist of?
two alpha and two delta subunits
(Delta are related to Beta, and are referred to as Beta-like Globins)
What are the main hemoglobins found in the early embryo?
Hb Gower 1 (Z2E2), Hb Gower 2 (a2E2), and Hb Portland (Z2y2)
What is the major hemoglobin type of the fetus?
HbF
What is the main modification that allows HbF to have higher oxygen affinity than HbA?
In the gamma (y) chain, HbF contains a serine residue, which is less positive (than the histidine reside on the B chain in HbA) and therefore reduces the affinity of HbF for 2,3-BPG –> thus increasing the oxygen affinity of HbF
(The gamma (y) chain is also found in Hb Portland)
What are the two globin chain familes, and which of the six globin chains falls into each? Which chromosome do each of the globin familes fall on?
alpha-like globins: alpha (a) and Z
***Chromosome 16***
Beta-like globins: Beta (B), gamma (y), Delta (D), and Epsilon (E)
***Chromosome 11***
What are the two “obvious groups” of hemoglobinopathies?
- Structural Variants
- Thalassemias (i.e. imbalanced synthesis of alpha- and Beta-globin chains)
What are the 3 categories of Structural Hemoglobin Variants?
- Insoluble Complexes –> are oxidized to methemoglobin (Fe3+) and form hemichrome –> Heinz Bodies and hemolytic anemia
- Increased/Decreased Oxygen Affinity
- Increased Tendency to form Methemoglobin
What rare Hb structural variant results in increased oxygen affinity?
HbHelsinki (Beta subunit mutation (Lys –> Met))
What rare Hb structural variant results in decreased oxygen affinity?
Hb<strong>Kansas</strong> (Beta subunit mutation (Asn –> Thr))
What rare Hb structural variants more readily form methemoglobin?
Hb<strong>Boston</strong> (alpha subunit mutation (distal His –> Tyr)
HbHyde Park (Beta subunit mutation (proximal His –> Tyr)
HbS
- Disease
- Mutation
- Consequences
- Hetero- vs. Homozygote
- Treatment
Disease: Sickle Cell Disease
Mutation: Glutamate is replaced by valine at position 6 of the Beta-globin chain
Consequences: ***Deoxygenated HbS polymerizes*** –> distortion of shape of RBC, misshapen cells block microcirculation, cells lyse readily (Chronic Hemolytic Anemia)
Heterozygotes = Sickle Cell Trait
Homozygoets = Sickle Cell Disease
Treatment: Hydroxyurea (HU) (antineoplastic)—> increased HbF –> increased solubility of hemoglobin and decreased sickling of cells
***Reduced: sickling, painful crises, hospitalizations***
HbC
- Disease
- Mutation
- Consequences
- Heterozygotes
- Treatment
Disease: HbC Disease (restricted to West African Origin)
Mutation: Glutamate replaced by lysine at position 6 of the Beta-globin chain
Consequences: HbC does not polymerize and cells do not sickle
***HbC is less soluble than HbA and precipitates –> hemolytic anemia
Heterozygotes: Compound with both HbC and HbS is not uncommon
***HbSC is a MILDER disease than HbS***
HbE
- Disease
- Mutation
- Consequences
- Hetero- vs. Homozygote
- Treatment
Disease: HbE Disease (Common in Southeast Asia)
Mutation: Glutamate is replaced by lysine at position 26 of the Beta-globin chain
Consequences: Beta globin chain is not synthesized effectiely leading to imbalanced alpha and Beta-globin chain synthesis –> mild Thalassemia develops
Heterozygotes (HbE Trait) = Asymptomatic
Homozygotes (HbE Disease) = Microcytosis, Hypochromia, Mild Anemia
What are the most common types of Thalassemias?
alpha-Thalassemias
- Genes
- Types of Hb affected
- Carrier States
Genes: 2 genes for a-globin on chromosome 16 (so 4 total copies)
Types of Hb affected: Embryonic (Hb Gower 2), Fetal, and Adult
***a-thalassemias manifest during development and adult life
Carrier States:
- 3 functional genes/1 defective –> No clinical signs
- 2 functional/2 defective –> Mild Thalassemic anemia
- 1 functional/3 defective –> Severe a-globin chain deficiency (Bart’s hemoglobin or HbH)
- 4 defective –> a-thalassemia –> only embryonic hemoglobins (Gower 1 and Portaland) can be produced –> Hemoglobin Bart’s Hydrops Fetalis _***Lethal Condition***_
What is Bart’s hemoglobin?
When gamma (y)4 tetramers form in fetus
-Poor oxygen carriers (due to High oxygen affinity)
What is HbH?
When Beta (B)4 tetramers form later in development
- Poor oxygen carriers (due to High oxygen affinity)
- Precipitates, shortening RBC life
What is the most common molecular defect that gives rise to a-thalassemia?
Deletions
-Duplicated a-globin genes are found in a region of homology, and gene deletion appears to occur during homologous recombination events
What happens when both a-globin genes are deleted?
Where is this most common?
Hemoglobin Bart’s Hydrops Fetalis Syndrome (a LETHAL condition)
Most common in Southeast Asia
HbConstant Spring Mutation
- Mutation Type
- Results
- Symptoms
Point Mutation
Results: T is replaced by C, so the normal TAA stop codon is bypassed, leading to a read through and increased a-globin chain length from 141 to 172
-Expected Hbconstant spring to comprise 2025% total Hb, however, only contributes 1-2%, as mRNA fro Hbconstant spring is unstalbe
Symptoms: like a+-thalassemia (moderate hemolytic anemia, splenomegaly) even though it has 2 of 4 functional globin genes and would be expected to only show mild thalassemic anemia
Beta-thalassemias
- Heterozygote vs Homozygote
***Only one gene for Beta-globin on chromosome 11***
Heterozygote defect = asymptomatic (lower MCV, lower MCH, increased HbA2)
Homozygote defect = SEVERE phenotype
-Abnormal cell shape, Target cells, Hypochromia
B-thalassemias: Generation by Deletion
- Homologous Recombination Events
May delete B-globin gene or both B and Delta
May cause B+ thalassemia, B thalassemia, or DB-thalassemia
B-thalassemias: Lepore hemoglobin
Recombinatnio events delete part of both B and D gloin
Generates Lepore fusion globin
- Functions poorly as a globin chain
- HbLepore trait asymptomatic
- HbLepore diseases rare and severe
B-thalassemias: generation by Point Mutation
***Best characterized point mutations affect splicing***
T normally present is replaced with A, generating a decent match site to the 5’ splice donor site. Splicing occurs here, as well as the true splice site** –> **REDUCES the amount of correct message produced
Screening Techniques: Electrophoresis
Typically used in neonatal screening for hemoglobinopathies
- Relatively cheap and easy
- Somewhat insensitive –> Difficulty testing premature infants
***Also issues with co-migration of some Hb variants***
Screening Techniques: Isoelectric Focusing
- When is it used?
Used during initial screen
Also can confirm with a second electrophoretic technique using agarose electrophoresis under acidic conditions
What is the main screening technique that uses Molecular Biology?
When is it best used?
Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP)
Best suited for screening for common mutations (HbS) or for confirming the identity of a mutant hemoglobin
