Haemoglobin Structure and Function Flashcards
Describe the structure of haemoglobin.
- Globular haemoprotein, making up about 1/3 of the RBC (haemoproteins are a group of specialised proteins that contain haem as a tightly bound prosthetic group).
- Consists of 2 α and 2 β chains, each holding its own haem molecule.
- Haem is a complex of protoporphyrin IX and ferrous iron (Fe2+). Iron is held in the centre of the haem molecule by bonds to the four nitrogens of the porphyrin ring.
- 65% of the haemoglobin in RBCs is synthesised at the erythroblast stage, and 35% at the reticulocyte stage.
How is haemoglobin synthesis stimulated?
- Stimulated by tissue hypoxia (when it isn’t getting an adequate supply of oxygen).
- Hypoxia causes the kidneys to increase the production of erythropoietin (EPO), which increases red blood cell and haemoglobin production.
Where does haem synthesis occur?
Mitochondria
Describe the events that leads to haem production.
1) IRON DELIVERY AND SUPPLY:
- Iron is delivered to the reticulocyte by transferrin.
2) SYNTHESIS OF PROTOPORPHYRINS:
- Occurs in the mitochondria of red blood cells precursors. Reactants are glycine and succinyl CoA
- Mediated by EPO and Vit B6. This is done to create Protoporphyrin IX.
3) COMBINING TO MAKE HAEM:
- Protoporphyrin IX and iron combine to make a haem molecule.
- Haem can combine with globin to form haemoglobin.
Where does globin synthesis take place?
Polyribosomes and RER
Describe haemoglobin synthesis.
- Globin mRNA is transcribed from its genes and then translated into globin by the polyribosomes.
- Proper globin synthesis depends on genes, because the precise order of amino acids in the globin chains is critical to the structure and function of haemoglobin.
- Rates of haem and globin synthesis are carefully coordinated to ensure optimal efficiency of haemoglobin assembly because haem needs to combine with globin to form haemoglobin
There are various types of globin that combine with haem to form different types of haemoglobin. Briefly outline these types
There are eight functional globin chains, arranged in two clusters:
- B-CLUSTER (β, γ, δ and ε globin genes) on the short arm of chromosome 11
- A-CLUSTER (α and ζ globin genes) on the short arm of chromosome 16
The functional haemoglobin in humans at different times of growth are:
EMBRYONIC:
- Gower I (ζ2 ε2)
- Gower II (ζ2 γ2)
- Portland (α2 ε2)
FOETAL:
- HbF (α2 γ2)
- HbA (α2 β2)
ADULT:
- HbA
- HbA2 (α2 δ2)
- HbF
- About 96-98% of the haemoglobin is HbA, with HbA2 at 1.5-3.2% and HbF at 0.5-0.8%.
- Beta globin expressed at low level in early life; main switch of adult Hb occurs about 3-6months after birth, when gamma subunits are largely replaced by beta subunits
Suggest what may happen if the globin chains were to become mutated e.g deletion or insertion of bases
Mutations or deletions may lead to:
- Altered sequence of bases and therefore an altered sequence of amino acids within the synthesised polypeptide chains
- Abnormal synthesis of globin chains (eg. Sickle Cell Disease)
- Reduced rate of synthesis of normal α or β-globin chains (eg. Thalassaemias)
List the functions of haemoglobin.
Outline how oxygen delivery occurs in haemoglobin.
- Carry oxygen from the lungs to the tissues
- Remove CO2
- Buffering action (maintains blood pH as it changes from oxyhaemoglobin to deoxyhaemoglobin)
OXYGEN DELIVERY OCCURS IN THE FOLLOWING WAY
- One Hb molecule can bind to four O2 molecules.
- When oxygenated, 2.3-DPG (disphosphateglycerate) is pushed out; the β-chains move closer.
- β-chains are pulled apart when O2 is unloaded, permitting entry of 2,3-DPG resulting in lower affinity of O2. This is why the oxygen dissociation curve is a sigmoid.
The more 2,3-DPG in the cell, the more oxygen is delivered to body tissues
The less 2,3-DPG in the cell, the less oxygen is delivered
Increasing the amount of 2,3-DPG is the body’s primary way of responding to a lack of oxygen
What does O2 binding to haemoglobin depends on?
Explain what oxygen affinity is and its importance in oxygen delivery.
The amount of O2 bound to haemoglobin and released to tissues depends on:
- PO2
- PCO2
- Affinity of haemoglobin for O2 (i.e oxygen affinity)
Oxygen affinity is the ease with which haemoglobin binds and releases oxygen.
- Oxygen affinity determines the proportion of O2 released to the tissues or loaded onto the cell at a given oxygen partial pressure.
- Increases in oxygen affinity mean that haemoglobin has an increased affinity for O2, so greater and stronger binding occurs.
- Decreases in oxygen affinity causes O2 to be released.
What is the oxygen dissociation curve?
What is the Bohr effect and how is it shown on the curve?
Explain what happens during both the right and left shift of the curve?
- Oxyhaemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2), and is determined by oxygen affinity.
- The P50 is the oxygen tension/partial pressure at which haemoglobin is 50% saturated with oxygen.
- The Bohr effect states that haemoglobin’s oxygen binding affinity is inversely related both to acidity. This is because of carbon dioxide produced at tissues and then released into blood. This results in generation of H+ ions i.e lower pH. Based on concept that alterations in blood pH CAN alter the position of the curve.
- This means that in acidic pH i.e greater concentrations of CO2, the curve shifts to the right; this results in an enhanced capacity to release O2 where it is needed.
RIGHT SHIFT
- Increased P50
- Decreased affinity to O2 when CO2 concentration increases or pH decreases (Bohr effect) or when 2,3-DPG level rises
- Hb more willing to release O2 allowing it to diffuse to tissue
Examples of situations where this happens would be acidosis and anaemia (lowered number of RBCs compensated for by increased efficiency of oxygen delivery).
LEFT SHIFT
- Decreased P50
- Increased affinity to O2
- Hb less willing to release O2 to tissue
Examples of situations where this happens would be alkalosis and the presence of abnormal haemoglobins.
Normal position of curve depends on:
- Concentration of 2,3-DPG
- H+ ion concentration (pH)
- CO2 in RBCs
- Cellular environment
- Structure of haemoglobin i.e if there are any abnormalities
The standard conditions are:
- TEMP: 37ºC
- pH: 7.4
- BASE EXCESS (BE): 0
What are the three mechanisms for carbon dioxide transport?
- Dissolution of CO2 in plasma
- Formation of carbonic acid
- Binding to form carbaminohaemoglobin
What are the following made up of?
Oxyhaemoglobin
Deoxyhaemoglobin
Carbaminohaemoglobin
Methemoglobin
OXYHAEMOGLOBIN - Oxygen bound to haemoglobin
DEOXYHAEMOGLOBIN - Haemoglobin not bound to oxygen
CARBAMINOHAEMOGLOBIN - Non-covalent binding of CO2 to haemoglobin. Allows transport of CO2 in blood
METHEMEGLOBIN - Haemoglobin that contains Fe3+
CARBOXYHAEMOGLOBIN - Binding of CO to Fe2+ - responsible for ‘oxygen starvation’ during carbon monoxide poisoning
What is sickle cell disease and suggest a possible treatment?
- Inheritance of a sickle beta-globin gene - i.e Mendelian recessive inheritance
- Example is sickle cell anaemia (HbSS) - homozygous - substitution of glutamate to valine at 6th position
- In low partial pressures of oxygen, HbS polymerises due to binding of valine to hydrophobic patch which is exposed upon deoxygenation - and forms precipitates with the unique ‘sickle’ shape - causing haemolysis
- Some heterozygous conditions - HbSD and HbSC
- Treatment - induce HbF as an alternative which inhibits HbS polymerisation
- Tested for using electrophoresis
Describe α and β-thalassemia
β-thalassemia
- Loss of 1 β-chain causes mild microcytic anaemia (thalassaemia trait)
- Loss of both (β0) causes thalassaemia major
- Excess α-chains precipitate in erythroblasts causing haemolysis and
ineffective erythropoiesis.
α-thalassemia
- Can be a loss of 1 up to 4 alpha chains
