Mb and Hb Flashcards
Where is myoglobin?
-Cardiac and Skeletal muscle
What are Mb and Hb?
- Oxygen-carrier proteins
- O2 not soluble
- O2 does not diffuse efficiently
What is the function of myoglobin?
- Binds O2 released from Hb to act as a reserve store of O2 for use during muscle contraction
- O2 used in muscle contraction for generation of ATP
Where is Hb and what is its function?
- In RBCs
- Carries O2 to tissues and transports CO2 back to the lungs for excretion
What is the structure of Mb?
- One polypeptide
- Predominantly a-helices stabilised by H bonds
- Hydrophobic interior
- Hydrophillic exterior
- Contains a haem group
How does the globin structure of Mb permit O2 binding to haem?
- Haem sits in crevice of globin
- Globin has 2 polar histidine a’a
- One histidine binds to Fe2+ of haem
- The other binds to Fe2+:O2 complex to stabilise it
What is the structure of Hb?
- 4 polypeptides
- 2a and 2b subunits
- each subunit has hydrophobic haem binding pocket
How does O2 binding cause a change in conformation in Mb?
- Fe2+ too large to sit in plane of ring
- O2 binds
- Fe2+ becomes smaller
- Pulled up into the ring, pulling the attached histidine with it causing a conformational change
-What shape is the O2 binding curve for Mb and why?
- Hyperbolic due to O2 binding being in hyperbolic dependance of pO2
- This is due to Mbs high affinity for O2
How does Mbs high affinity for O2 relate to its function?
- High affinity of O2 means that Mb is highly saturated, even at low pO2
- Therefore Mb is saturated at normal pO2 of the tissues and will only release its O2 at v.low pO2, i.e on demand during exercise
What shape is the Hb dissociation curve and why?
-Sigmoidal due to cooperative binding
What is cooperative binding?
Binding of one molecule of O2 to one of the subunits increases the O2 affinity of the other subunits in the same tetramer
Why is there a change in conformation when O2 binds to Hb?
-Weak interactions between the two polypeptide dimers allows conformational change when O2 binds by pulling histidine up into the plane of the ring
What are the two states of Hb?
Relaxed and Taut
Why does the T state have a lower affinity for O2?
The dimers interact through ionic bonds which constraints movement and thus does not allow O2 to bind
Why does the R state have a higher affinity for O2?
The binding of one molecule of O2 induces a conformational change which ruptures the polar bonds between the dimers, this permits movement and exposes the O2 binding sites in the other subunits
Why is it important to have cooperative binding and intermediate R and T states?
- Cooperative bindiing permits effecient O2 binding/release in the lungs and the tissues
- T state is inefficient at binding O2 at high pO2 in the lungs due to low affinity
- R state is inefficient at releasing O2 at low pO2 in the tissues due to high affinity
- Having both states allows Hbs affinity to change over a range of pO2 which permits O2 delivery from high pO2 to low pO2
What is the Bohr effect?
- CO2 causes Hb to have a decreased affinity for O2 and shifts the oxygen dissociation curve to the right
- This means that in high concentrations of CO2, O2 is released into the tissues
- High concentrations of CO2 is correlational to high H+
- High H+ lowers the pH and this favours the unloading of O2 as it promotes the T state of Hb through increasing ionic bonds
- H+ is naturally in a higher conc at the tissues due to higher levels of CO2, this creates a pH gradient between the lungs and tissues, thus Hb can respond to small changes in pH and allows O2 delivery to be coupled to demand
How does 2,3-BPG effect Hb O2 affinity and why is it important?
- Binds to Hb site other than active site and decreases affinity for O2
- Without 2,3-BPG Hb affinity for O2 would be too great and O2 would not be released at the pO2 of the tissues
- In absence of 2,3-BPG the O2dissociation curve shifts to the left
How does CO effect Hb affinity for O2 and why is it a problem?
- Binds tightly to Hb at a haem and increases the affinity for O2 of the other subunits
- Shifts the O2 dissociation curve to the left
- Effects the release of O2 at the level of the tissues as affinity is so high at pO2 of the tissues it is not released
How is CO poisoning treated?
- With 100% O2 in a hyperbaric chamber
- Facilitates CO diffusion by competing with CO to bind to haem
What chains are in HbA
- 2a
- 2b
- Predominant adult Hb
What chains are in HbF?
- 2a
- 2g
- Foetal Hb
Why does HbF have a higher affinity than HbA and why is it important?
- HbF only binds weakly to 2,3-BPG as the g chains lack the pos a’a to which it binds
- Important because the high affinity HbF facilitates the diffusion of O2 across the placenta
What chains are in HbA2?
- 2a
- 2d
- minor component of adult Hb
What is HbA1c?
-Non-enzymatic glycosylated HbA
What type of inheritance is sickle cell anaemia?
-Autosomal homozygous recessive
Which population does it mainly affect?
- Africans
- African-Americans
What nucleotide mutation causes sickle cell anaemia?
-Point mutation of A->T
What a’a mutation occurs in sickle cell anaemia?
-Glutamate->Valine
What causes sickled cells to aggregate?
-Non-polar valine causes protrusion out of b-chain into b-chain of another Hb
Why do RBCs sickle in sickle cell anaemia?
- When RBCs are in the T state (deoxygenated), the conformation allows the mutated valine to form hydrophobic interactions with other a’a side chains.
- This forms a network of fibrous polymers of Hb which stiffen and distort the cell into the characteristic sickle shape
Why do RBCs undergo cycles of sickling?
-When in the R state (oxygenated) the Hb is in a conformation where hydrophobic interactions cannot occur and thus polymers of Hb do not form and cells won’t be sickled
Why do people with sickle cell anaemia suffer with anaemia?
- The RBCs are continually undergoing sickling until a point where the stress causes the cell to remain in the distorted sickle shape
- The sickled cells undergo early destruction and the bone marrow cannot match the rate of destruction and anaemia develops
Why is anaemia not a major problem in sickle cell?
-The HbS has a lower affinity for O2 and so it is released more easily which counteracts the lack of Hb
Why can sickled cells cause pain?
- Sickled cells do not readily distort
- Capillaries which supply blood to tissues have a smaller diameter than the sickled cell
- Because the sickled cell has lost its flexibility it can lead to occlusion of the microvasculature
- Deprives tissues of oxygen causing local damage and sever pain
What is a sickle crisis?
When multiple cells sickle and cause occlusion of blood cells leading to local anoxia
What can precipitate a sickle cell crisis?
- Infection
- Cold
- Acid blood
- dehydration
In which areas of the body is pain normally associated with sickle cell?
- Bone
- Chest
- Brain
- Kidney
Why can jaundice and splenomegaly occur in sickle cell?
- Increased destruction of RBCs causes hypertrophy of the spleen due to increased work load
- Increased destruction of RBCs increases haem to be broken down to bilirubin
- Can not excrete high amount of bilirubin and jaundice occurs
What are thalassaemias?
-A group of genetic disorders caused by an imbalance in a and b chains in Hb
What is b-thalassaemia, what mutation causes it and what happens to RBCs?
- Reduction or absence in b chains
- Typically caused by a point mutation
- RBC precursors undergo early destruction as Hb precipitates out.
Can a-chains form a stable tetramer?
-No
Are thalassaemias present from birth?
-No occurs in the first few months as HbF changes to HbA
What is the genetic difference between B thalassaemia minor and B thalassaemia major?
- B-thalassaemia minor caused by a mutation in one of the two genes (located on separate chromosomes)
- B thalassaemia major caused by a mutation in both genes at both chromosomes
What is a-thalassaemia and what kind of mutation typically causes it?
- Reduction or absence of a-chains
- Deletion mutations
Why are there varying levels of a-thalassaemia?
- Because there are 4 a-globin genes (2 on each chromosome)
- Different amounts and levels of mutations causes different severities of a-thalassaemia
Why is a-thlassaemia not as critical as b-thalassaemia?
-B-globin chains can form a stable Hb tetramer but just with an increased oxygen affinity (don’t give up oxygen so readily at the tissues)
