Hemoglobin 3 - molecular pathology of Hb 09/12 Traish Flashcards
where did sickle-cell hemoglobin get its name?
it causes RBCs to adopt elongated sickle shape at low O2 concentrations due to mutated hemoglobin aggregates
when do sickled RBCs cause pain?
block capillaries and lead to ischemia, infarction, and inflammation
why does sickle cell lead to anemia?
sickled cells are fragile and broken down more often, leading to low RBC count
why has the sickle-cell mutation become prevalent in certain populations?
heterozygous individuals have less severe symptoms and higher survival rate than homozygotes, and are resistant to malaria (fragility of sickled cells disrupts parasite lifecycle)
how do exercise and high altitudes affect sickle-cell patients?
exercise and high altitudes (hypoxic conditions) are more likely to cause cell sickling and symptoms
how does sickle-cell trait disrupt the malarial parasite lifecycle?
fragility and breakdown of RBCs alters ionic environment (lower K+ concentration, higher Ca++ concentration) in which malarial parasite normally incubates and matures within RBC
on electrophoresis, how can you tell the difference between…
- normal Hb
- sickle-cell trait
- sickle-cell anemia
- normal = one mark closer to (+) anode
- heterozygous = two marks
- homozygous = one mark closer to (-) cathode
in order to study sickled-cells, why is it important not to expose a blood sample to atmospheric air?
high pO2 can restore sickled cells to normal
how can sickle-cell lead to stroke?
sickled cells aggregate and clot
how can sickle-cell lead to susceptibility to infection?
SCD can lead to splenic disfunction (spleen is like a large lymph node that filters blood, degrades old RBCs, stores blood reserve, recycles iron, produces antibodies, and removes antibody-coated bacteria and pathogens from blood)
how can sickle-cell lead to priapism?
priapism = non-sexual sustained penile erection.
sickle cells can cause by blocking venous outflow from penis
what do you have to know about HbA2?
not much.
- a2d2 (2 alpha subunits, 2 delta subunits)
- expressed at low levels
- purpose unknown
- not really studied
why is it important for normal RBCs to have dispersed and non-aggregated Hb?
- the flexible biconcave disk shape of normal RBCs allow them to bend an squeeze through capillaries that are smaller than RBC diameter
- sickled cells do not have this flexibility and are more likely to get stuck
how do HbS form aggregates?
the sickle mutation is Glu6(A6) –> Val
in deoxy HbS, this Valine sticks out and can fit into a hydrophobic pocket in the EF corner of the beta subunit of another Hb molecule
T/F the sickle mutation is found in the heme pocket region of Hb
false - the beta Glu6(A6) -> Val mutation occurs in a part of the beta subunit far from the critical regions of the heme pocket and alpha/beta contact
T/F the sickle mutation is found in the alpha/beta contact region of Hb
false - the beta Glu6(A6) -> Val mutation occurs in a part of the beta subunit far from the critical regions of the heme pocket and alpha/beta contact
what is the HbS mutation?
beta Glu6(A6) -> Val this Valine sticks out and can fit into a hydrophobic pocket in the EF corner of the beta subunit of another molecule and form aggregates
beta Glu6(A6) -> Ala mutation in helix A of Hb results in…
insignificant sickling
Glu6(A6) is the same residue involved in the HbS mutation, but Ala here probably does not fit into the hydrophobic EF pocket of the beta chain as well as Val does in Hbs
beta Glu121(GH4) -> Lys mutation in Hb results in…
enhanced sickling (but not as serious as HbS) - to + switch causes charge-charge interactions with beta Glu6(A6)
alpha His87(F8) -> Tyr mutation in Hb results in…
Fe oxidation, decreased O2 affinity alpha His87(F8) usually ligates Fe in heme. when mutant Tyr ligates Fe, Fe becomes susceptible to oxidation to the ferric state and no longer binds O2
alpha Arg141(HC3) -> His mutation in Hb results in…
R state favored, increased O2 affinity
alpha Arg 141 is a C-terminus! changing to His disrupts the alpha Arg141 interaction with Asp126 of the other alpha chain in deoxyHb
beta Gly74(E18) -> Asp mutation in Hb results in…
decreased BPG binding, increased O2 affinity
introducing negative charge in this region reduces BPG binding affinity, resulting in increased O2 affinty
beta His146(HC3) -> Asp mutation in Hb results in..
R state favored, increased O2 affinity beta His146(HC3) stabilizes deoxy Hb by binding beta Asp94 and alpha Lys40. with beta Asp94 it contributes to the Bohr effect, utilizing Asp 94 to stabilize an H+ with an abnormally high pKa. if replaced by Asp, the H+ binding and deoxyHb stabilizing through the R group is lost (but I suppose the C-terminus COO- may still interact with Lys40)
beta Gly74(E18) -> Asp mutation in Hb results in…
increased affinity for O2
beta Phe42(CD1) -> Ser mutation in Hb results in…
decreased O2 affinity (Fe oxidation, heme loss)
hydrophilic Ser opens heme pocket for water and results in heme loss
beta Glu6(A6) -> Val mutation in Hb results in…
sickling, HbS
hydrophobic Val fits into hydrophobic pocket in EF corner of a beta chain of another Hb molecule, causing polymerization and aggregation
alpha Pro95(G2) -> Arg mutation in Hb results in…
dissociation into subunits, high Mb-like O2 affinity
loss of Pro results in change of geometry and alters subunit interactions due to continuation of the helix, resulting in dissociation of tetramer, loss of cooperativity, and increased hyperbolic Mb-like O2 affinity
alpha leu136(H19) -> Pro mutation in Hb results in…
dissociation of tetramer, high Mb-like O2 affinity
Pro interrupts helix, results in dissociation of tetramer, loss of cooperativity, and increased hyperbolic Mb-like O2 affinity
the earliest Hb in a fetus is of the form…
z2e2
zeta 2 epsilon 2
(only very early fetus, soon supplanted by a2g2)
alpha leu136(H19) -> Pro mutation in Hb results in…
dissociation of tetramer, high O2 affinity
begin again p. 39
begin again p. 39
HbF subunits are…
a2g2
alpha 2 gamma 2
when is the transfer from HbF to HbA essentially complete?
~6 months of age
transcription of g-Hb ceases ~birth but transcribed HbF remains in circulation for several months
how long do RBCs remain in circulation before being turned over?
~120 days
when is the transfer from HbF to HbA essentially complete?
~6 months of age
transcription of g-Hb ceases ~birth but transcribed HbF remains in circulation for several months
HbA subunits are…
a2b2
alpha 2 beta 2
HbA2 subunits are…
a2d2
alpha 2 delta 2
the biological purpose of HbA2 is…
unknown
HbA2 (a2d2) is expressed at low levels (1-3% of Hb) from birth to death. levels may increase in b-thalassemia patients…
expression timeframe of a-Hb
increases first few days/weeks of fetus to ~50% Hb and persists to death
expression timeframe of b-Hb
~10% Hb pre-birth, increases ~birth to ~50% Hb and persists to death
expression timeframe of g-Hb
increases first few days/weeks of fetus to ~40% Hb, transcription ceases ~birth and most g-Hb eliminated by 6 months
expression timeframe of z-Hb
first few days/weeks of fetus only, before a and g-Hb reach normal levels
expression timeframe of e-Hb
first few days/weeks of fetus only, before a and g-Hb reach normal levels
expression timeframe of z-Hb
first few days/weeks of fetus only, before a and g-Hb reach normal levels
there are 2 genes that code for this Hb subunit:
alpha Hb
how different are the 2 genes that code for a-Hb?
one amino acid different
on what chromosome are a-Hb genes located?
16
on what chromosome is the b-Hb gene located?
11
Thalassemias are…
hereditary hemolytic disease from the imbalance of Hb chain synthesis
(one or more of the genes coding for hemoglobin have been deleted or otherwise rendered non-functional)
which Hb genes are located on chromosome 11?
b, g(a), g(g), d, e
which Hb genes are located on chromosome 16?
a1, a2, z1, z2
alpha thalassemia
synthesis of a-Hb is
a+ decreased, or
a0 absent
which Hb genes are located on chromosome 16?
a1, a2, z1, z2
one defective a-Hb gene is known as…
a-thalassemia silent carrier
two defective a-Hb genes is known as…
a-thalassemia trait
- smaller RBCs and mild anemia but asymptomatic
- physicians often mistake for iron deficiency and prescribe iron supplements that have no effect on anemia
three defective a-Hb genes is known as…
hemoglobin H disease
- b-Hb are overproduced with respect to a-Hb and result in b4 tetramers (hemoglobin H)
- results in a mildy to moderately severe hemolytic anemia, e.g. may cause enlarged spleen, bone defomities, fatigue
four defective a-Hb genes is known as…
hydrops fetalis (swollen baby, or blue baby)
- usual death before or near birth, some rare cases of in-utero blood transfusions to achieve live brith and lifelong management with transfusions afterward
- fetal Hb is g4 (hemoglobin Bart) or b4, which cannot deliver oxygen due to loss of cooperativity
three defective a-Hb genes is known as…
hemoglobin H disease
a mildy to moderately severe hemolytic anemia
four defective a-Hb genes is known as…
hydrops fetalis (swollen baby) -usual death before or near birth, some rare cases of lifelong management
hemoglobin H disease
three defective a-Hb genes
a+a+a+a or a0a0a0a
-b-Hb are overproduced with respect to a-Hb and result in b4 tetramers (hemoglobin H)
-results in a mildy to moderately severe hemolytic anemia
hydrops fetalis
four defective a-Hb genes
a+a+a+a+ or a0a0a0a0
(swollen baby, or blue baby)
-usual death before or near birth, some rare cases of in-utero blood transfusions to achieve live brith and lifelong management with transfusions afterward
-fetal Hb is g4 (hemoglobin Bart) or b4, which cannot deliver oxygen due to loss of cooperativity
hemoglobin Bart
g4 hemoglobin
results from quadruple a-thalassemia
cannot deliver O2 due to loss of cooperativity
Mb-like hyperbolic binding curve
with regard to Hb, a a+ a0 stand for...
a - normal alpha chain
a+ - decreased synthesis of alpha chain
a0 - absent alpha chain
hemoglobin Bart
g4 hemoglobin
results from quadruple a-thalassemia
cannot deliver O2 due to loss of cooperativity
Mb-like hyperbolic binding curve
with regard to Hb, a a+ a0 stand for...
a - normal alpha chain
a+ - decreased synthesis of alpha chain
a0 - absent alpha chain
b-thalasemmia
synthesis of b-Hb is
b+ decreased, or
b0 absent
how many b-Hb genes are there in a normal person?
2
1 from Mom, 1 from Dad
one defective b-Hb gene is known as…
b-thalassemia minor
(b-thalassemia trait)
can live a full life and may not need to be treated
two defective b-Hb genes is known as…
b-thalassemia major
- appear healthy at birth since a2g2 compensates
- down reg of g-Hb leaves body with a-Hb only, which cannot form stable tetramers with itself and causes death of premature erythrocytes
- severe anemia in 1st or 2nd year of life
- regular transfusions are life-saving, but result in iron overload and hemosiderosis (hemosiderin is an iron storage molecule), which usually causes death in early adulthood
b-thalassemia minor
one defective b-Hb gene
(b-thalassemia trait)
can live a full life and may not need to be treated
why does a-thalassemia major usually result in death, but b-thalassemia major can be managed into adulthood?
- a-Hb is necessary in HbF. without a functional a-subunit, g-subunits form g4 Hb (hemoglobin Bart), which has no cooperativity and Mb-like hyperbolic binding curve, and results in hypoxia and tissue death (hydrops fetalis), unless in-utero blood transfusions are successful
- b-Hb is necessary for HbA only, so fetus survives birth and can be treated with regular blood transfusions throughout life
Hb jamaica plain
BBs+
one normal beta
one double mutant beta with sickle mutation (Glu6Val) and mutation stabilizing oxyHb form
(thus baby presents with symptomatic sickling and mild hypoxemia despite having only sickle cell trait
why does quadruple a-thalasemmia result in hyrdrops fetalis but b-thalassemmia major can be managed with regular transfusions?
b-Hb is not
Hb jamaica plain
BBs+
one normal beta
one double mutant beta with sickle mutation (Glu6Val) and mutation stabilizing oxyHb form
(thus baby presents with symptomatic sickling and mild hypoxemia despite having only sickle cell trait
methemoglobin
Fe++ is oxidized to Fe+++, which does not bind O2
hemoglobin variant caused by nitrates and nitrites from fertilizer runoff in drinking water is likely…
methemoglobin
Fe++ is oxidized to Fe+++, which does not bind O2
Hb H
b4
arises in cases of triple a-thalassemia
a+a+a+a or a0a0a0a
-b-Hb are overproduced with respect to a-Hb and result in b4 tetramers
-results in a mildy to moderately severe hemolytic anemia
cyanosis
appearance of a blue or purple coloration of the skin or mucous membranes due to the tissues near the skin surface being low on oxygen
Hb H
b4
arises in cases of triple a-thalassemia
a+a+a+a or a0a0a0a
-b-Hb are overproduced with respect to a-Hb and result in b4 tetramers
-results in a mildy to moderately severe hemolytic anemia
this Hb variant may cause serious health problems, such as enlarged spleen, bone defomities, fatigue
hemoglobin H disease
(triple a-thalassemia)
-b-Hb are overproduced with respect to a-Hb and result in b4 tetramers (hemoglobin H)
-results in a mildy to moderately severe hemolytic anemia and possibly serious health problems
physicians often mistake this Hb variant for iron deficiency and prescribe iron supplements that have no effect on anemia
a-thalassemia trait
(double a-thalassemia)
-smaller RBCs and mild anemia but asymptomatic
-physicians often mistake for iron deficiency and prescribe iron supplements that have no effect on anemia
this Hb variant results in smaller RBCs and mild anemia but no serious symptoms
a-thalassemia trait
double a-thalassemia
a-thalassemia major
quadruple a-thalassemia
hydrops fetalis (swollen baby, or blue baby)
-usual death before or near birth, some rare cases of in-utero blood transfusions and lifelong management with transfusions
-fetal Hb is g4 (hemoglobin Bart) or b4, which cannot deliver oxygen due to loss of cooperativity
(quadruple a-thalassemia)
what are 2 therapies currently under consideration for treatment of b-thalassemia major?
- gene therapy (e.g. switching on g-Hb for life)
- bone marrow replacements
this Hb variant is commonly found in people of African, Mediterranean (Italian, Greek, Turkish), and Caribbean ancestry
HbS
what are 2 therapies currently under consideration for treatment of b-thalassemia major?
- gene therapy (e.g. switching on g-Hb for life)
- bone marrow replacements
what are the symptoms with <10% met-Hb
no symptoms
what are the symptoms with 10%<30%
cyanotic skin discoloration (blue/purple due to low O2)
what are symptoms with 30-50% met-Hb?
severe
In a patient suspected of having sickle cell anemia, which of the following tests would most
definitively establish the diagnosis?
A. Analysis of packed cell volume.
B. Peripheral blood smears.
C. Hemoglobin electrophoresis (isoelectric focusing).
D. Genetic analyses of the patient’s parents.
E. Bone marrow biopsy
C. Hemoglobin electrophoresis (isoelectric focusing).
which has more negative charge, HbA or HbS?
HbA
will migrate further toward + end on isoelectric focusing (anode)
what happens if his F8 is changed to a hyrdophobic residue?
heme loss
does not coordinate with heme, opens pocket to water and Fe oxidation
