Lecture 11: Hemoglobin Flashcards
Erythrocyte function
Transport oxygen from the lungs to the tissue
Hemoglobin occupies what % of volume and weight in erythrocyte
33% of volume and 90% of dry weight
2lbs of hemoglobin in a person
65% of hemoglobin is synthesized
Before the extrusion of the nucleus
What makes remaining 35% of hemoglobin
Reticulocyte
Red blood cells are made where at what stages
Yolk sac in first month of development
Liver and spleen and next few months
Bone marrow after birth
Predominant form of hemoglobin in the adult
HbA
2 alpha 2 beta subunits (a2b2)
Fetal hemoglobin
0.5% of adult hemoglobin
HbF (a2y2) (alpha2gamma2)
beta globin genes located
Chromosome 11
Alpha globin genes located
Chromosome 16
HbF predominately made where
Mostly liver, but also bone marrow
Switch from fetal to adult hemoglobin is controlled by
Time
Switching is closely related to gestational age
HbS mutation
Substitution of valine for glutamic acid in AA 6 in Beta globin gene
Non-functioning globin protein
How many alpha globin genes are there
Two
Heme structure
4 pyrrole rings form tetrapyrrole ring
4 methyl, 2 vinyl, 2 propionate side chains
Iron atom lies in the center of the protoporphyrin bonded to 4 pyrrole nitrogen atoms
Amino acid naming- F8 histidine
8th AA residue in the F helix
F helix is the 6th segment in a globin subunit
Proximal and distal histidine
F8= proximal, binds to heme group E7 = distal, O2 binds iron between the heme and distal histidine
Hemoglobin conformation change
Upon oxygenation, the iron atom moves into the plane of heme and pulls the proximal F8 histidine
This is incredibly important for hemoglobin function
Myoglobin is
An O2 storage protein
Hemoglobin is an O2 transport protein
Where is myoglobin commonly found
Muscle cells, storing oxygen for use
Myoglobin affinity Vs hemoglobin
Myoglobin has very high affinity of O2 and will not release it until pressure in tissues is very low (~0 torr)
p50 for Hb and myoglobin
P50= oxygen pressure at which molecule is half saturated with O2
P50 Hb = 26 torr
P50 myoglobin = 2.8 torr
Positive cooperativity of hemoglobin
When one molecule of O2 binds to one heme, it facilitates the binding of an O2 to another heme
Conformation change in one globin induces conformation change in another subunit in Hb
Reversability
As Hb loses oxygen in the tissue, the loss of an oxygen molecule makes it more likely the next subunit will lose its oxygen molecule
2,3-BPG
Major modulator of Hb
Intermediate by-product of glycolysis
In active tissue, there is lots of 2,3-BPG, signaling Hb to let go of O2
No 2,3-BPG - effect on hemoglobin
Hemoglobin would be an extremely inefficient oxygen transporter
2,3-BPG concentrations in lung/tissue
Lung- NO 2,3-BPG= high affinity for O2
Tissues- High [2,3-BPG]= low affinity for O2
T and R forms of Hb
T form- low affinity - in non-oxygenated hemoglobin, beta chains are farther apart
R form- High affinity - in oxygenated hemoglobin, beta chains are closer together
2,3-BPG and T/R forms
2,3-BPG stabilizes the T form of Hb, allowing it to maintain low affinity
Low 2,3-BPG in lungs leads to R form with high affinity
Smoking and 2,3-BPG
Smoking causing higher levels of 2,3-BPG, causing more T form Hb with lower affinity. Smokers have reduced O2 carrying capacity
pH and Hb
pH is lower in active tissues. Binding affinity of Hb for O2 decreases as pH decreases
Free H+ is picked up from tissue by an amino acid in Hb, changing the conformation and favoring release of O2
Fetal hemoglobin and why O2 flows from mom to fetus
Fetal Hb does not bind well to 2,3-BPG and therefore has a higher affinity for O2
HbF is mostly locked in R form
Sequential model of cooperativity
At each level of oxygen loading, it causes an adjacent globin chain to change from T to R state, leading to increased affinity for oxygen with each new one bound
Carboxyhemoglobin
Occurs when heme is combined with Carbon monoxide
The bond is 210x stronger than with oxygen
Transport of O2 to tissues is impaired, death
HbA1c
Post translational glycosylation of Hb
Usually the level is 3%
Varies with the level of blood glucose concentration that RBCs have been exposed to in 120 day lifespan
HbA1c is increased 2-3x in diabetic patients (6-9%)
Thalassemias definition
Reduced synthesis of one or more of the globin chains leads to
- imbalanced globin-chain synthesis
- defective Hb production
- Damage to red cells from effects of excess globin subunits
a-Thalassemia
Involves both alpha globin genes
Deletion of 1, 2, 3 or 4 alpha globin genes
a-globin chains are present in both fetal and adult Hbs, so deficiency of a-globin chains affects Hb production in fetal and adult life
Effect of a-Thalassemia in fetal life
Excess y(gamma)-globin chains for y^4 tetramers called Hb Barts
Effect of a-Thalassemia in adult life
Excess B-globin chains for B^4 tetramers or Hb H
a+-thalassemia
Silent carrier, one gene deleted
a-thalassemia trait
Two alpha genes deleted
-Low mean cell volume MCV
-Low mean cell hemoglobin MCH
Normal percentages of HbA2 and HbF
Hb H disease
3 alpha genes deleted
-Moderately severe anemia
-Low MCV and MCH
Excess Beta chains cause HbHB4 tetramers to precipitate and form Heinz bodies which lead to hemolytic anemia
Hydrops fetalis with Hb Barts
Both alpha genes completely inactivated (4 alpha genes deleted)
Make y^4 Hb (Barts)
-Stillborn or death within hours
-Few are saved by exchange transfusion
-Fetal onset of generalized edema and severe hypochromic anemia
What is the problem with Hb Barts
Hb barts has high O2 affinity
Binds O2 delivered by mother but releases almost none to fetal tissues
Severe hypoxia occurs and leads to profound edema
Massive hepatosplenomegaly
B-thalassemia overview
Reduced B chain synthesis from beta globin genes
Excess alpha chains precipitate and damage red cells and precursors
Severe anemia, splenomegaly, bone changes -
“Cooley’s Anemia”
B-Thalassemia major form
No B-chain expression, severe homozygous condition
0% B-globin synthesis
B-Thalassemia minor form
Partial deficiency in B-chains
Heterozygous condition with milder anemia
50% B-globin synthesis
B-thalassemia cause
Beta-globin gene mutation can affect transcription, RNA processing or translation of B-globin RNA
Excess a-chains cannot form tetramers, and aggregate and precipitate to form inclusion bodies
Cause oxidative damage to red cells, destruction of immature erythroblasts in bone marrow
Leads to ineffective erythropoiesis
Initial presentation of B-thalassemia
Disease seen when switch from HbF to HbA occurs (usually in neonates)
Clinically emerges at 6-12 months
Switch from HbF to HbA should occur at this time, but switch from y to B does not occur
Hb levels progressively drop, severe anemia occurs
Therapy for B-thalassemia
Transfusions (this is the cause of death typically) Iron chelation Stem cell transplantation Prevention by genetic counseling Upregulation of HbF expression
