The haemoglobin molecule and thalassaemia Flashcards
What is the function of RBCs?
- Carry oxygen from lungs to tissues
- Transfer CO2 from tissues to lungs
State some characteristics of RBCs.
RBCs:
- Have a count of 3.5-5 x 1012/L (in an average sized person)
- Contain haemoglobin (Hb)
- Contains approximately 640 million molecules of Hb per cell
- Do not have nucleus or mitochondria
State some characterisistics of heamoglobin.
- Found exclusively in RBCs
- Molecular weight 64-64.5 kDa
- i.e. Hb has a molecular mass of approximately 64000-65000
- Normal concentration in adults: 120-165g/L
- Approximately 90 mg/kg of Hb produced and destroyed in the body every day - /kg?
- Each gram of Hb contains 3.4mg Fe
NOTE: Hb outside RBCs is toxic - come back to this if not mentioned elsewhere are write down what he says on panopto
During which stage of erythropoeisis does Hb synthesis occur?
- Synthesis begins in pro-erythroblast stage
- 65% of the Hb molecules are synthesised in the erythroblast stage
- 35% aresynthesised in the reticulocyte stage
Where is haem synthesised?
In mitochondria
Where is globin synthesised?
In ribosomes
Describe the process of haemoglobin synthesis.
- Fe binds to transferrin
- Fe-transferrin complex endocytosed into RBC
- Fe transported to mitochondria and is used in the synthesis of haem
- In the mitochondria a protoporphyrin ring is combined with the Fe to form haem
- δ-ALAS (δ-aminolevulinic acid synthase) - regulatory enzyme in haem synthesis
- Excess haem production exerts negative feedback on this enzyme to reduce haem production
- Globin chains produced in cytosol - by normal process of protein synthesis
State some key facts about haem.
- Haem is not specific to Hb but is also contained in other proteins e.g:
- Myoglobin
- Cytochromes
- Peroxidases
- Catalases
- Tryptophan
- The haem part is the same in all types of Hb
- Formed by the combination of protoporphyrin ring with central iron atom → ferroprotoporphyrin
- Iron usually in ferrous form (Fe2+)
- Able to combine reversibly with oxygen
- Synthesised mainly in mitochondria which contain the enzyme ALAS which regulates heam production
Describe the genes for the functional globin chains.
Various types of globin chains which combine with haem to form different haemoglobin molecules
Eight functional globin genes, arranged in two gene clusters:
β-cluster:
- β, γ, δ and ε globin genes
- 2 gamma therefore 5 genes in this cluster
- On the short arm of chromosome 11
α-cluster:
- α and ζ globin genes
- 2 alpha therefore 3 genes in this cluster
- On the short arm of chromosome 16
Describe the process of globin gene expression and switching.
Alpha globin cluster:
- Zeta globin chain - first globin to be produced from alpha cluster
- Zeta production begins very early in embryogenesis (soon after conception) but stops after about 7 weeks
- Then production of alpha globin chains take over
- Alpha globin is the predominant globin chain produced from the alpha globin cluster throughout the rest of foetal and adult life
- Complete inability to synthesis alpha globin chains → embryo cannot produce Hb → death in utero
Beta globin cluster:
- Epsilon globin chain - first globin to be produced from beta cluster but production also stops quite early on
- Then production of gamma chains take over
- Gamma chains most important and predominant in foetal life
- Form part of foetal haemoglobin
- Foetal Hb = 2 alpha + 2 gamma
- Even after birth the baby will be producing significant quantities of gamma globin for the first few months
- Normally between 3-6 months
- Then gamma globin production declines and beta gloin production takes over
- So adult Hb becomes the predominant Hb
- HbA = 2 alpha + 2 beta
- Defect in beta globin production doesn’t affect baby in utero but symptoms present after birth when beta globin production takes over from gamma globin production
- Beta thalassaemia presents after about 3 months
What are the 3 variants of Hb present in adults?
- Hb A - 2 alpha chains + 2 beta chains
- 96-98 %
- Hb A2 - 2 alpha chains + 2 delta chains
- 1.5-3.2%
- Normal Hb variant
- Hb F - 2 alpha chains + 2 gamma chains
- 0.5-0.8%
- Foetal Hb - present in trace amounts in adults
Describe the primary, secondary and tertiary globin structure.
Primary structure:
- α globin chain has 141 AAs
- Non-α globin chains have 146 AAs
Secondary structure:
- 75% of the α and β chains are in a helical arrangement
Tertiary structure:
- Approximate sphere
- Hydrophilic surface (charged polar side chains), hydrophobic core
- Haem pocket - i.e. when each globin chain assembles into its tertiary structure there is an area within it to house the haem molecule
Describe the oxygen dissociation curve.
- The oxygen carrying capacity of Hb at different pO2
- The oxygen dissociation curve has sigmoid shape
- Binding of one molecule facilitates the second molecule binding (cooperativity)
- P50 (partial pressure of O2 at which Hb is half saturated with O2) = 26.6mmHg
What is the normal position of the oxygen dissociation curve dependent on?
- Concentration of 2,3-DPG
- H+ ion concentration (pH)
- CO2 in red blood cells
- Structure of Hb
What factors cause a right or left shift of the oxygen dissociation curve?
Right shift - gives up oxygen more readily (to deliver oxygen more easily to tissues):
- High 2,3-DPG
- High H+
- High CO2
- HbS (sickle Hb)
Left shift - gives up oxygen less readily
- Low 2,3-DPG
- HbF
NOTE:
- As oxygen binds to Hb, there is a conformational change which makes the middle a binding site for 2,3-DPG
- 2,3-DPG is a glycolytic product
- So when ATP is being produced in large amounts more 2,3-DPG is being produced - reflective of tissue metabolism
- When metabolism is higher, you want more oxygen being offloaded to tissues
- So 2,3-DPG binds to the Hb molecule it causes the oxygen to be squeezed out so there is more available for the respiring tissue
