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MOLECULES GENES AND DISEASE > Mb and Hb > Flashcards

Mb and Hb Flashcards

0
Q

Where is myoglobin?

A

-Cardiac and Skeletal muscle

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1
Q

What are Mb and Hb?

A
  • Oxygen-carrier proteins
  • O2 not soluble
  • O2 does not diffuse efficiently
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2
Q

What is the function of myoglobin?

A
  • 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
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3
Q

Where is Hb and what is its function?

A
  • In RBCs

- Carries O2 to tissues and transports CO2 back to the lungs for excretion

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4
Q

What is the structure of Mb?

A
  • One polypeptide
  • Predominantly a-helices stabilised by H bonds
  • Hydrophobic interior
  • Hydrophillic exterior
  • Contains a haem group
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5
Q

How does the globin structure of Mb permit O2 binding to haem?

A
  • 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
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6
Q

What is the structure of Hb?

A
  • 4 polypeptides
  • 2a and 2b subunits
  • each subunit has hydrophobic haem binding pocket
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7
Q

How does O2 binding cause a change in conformation in Mb?

A
  • 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
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8
Q

-What shape is the O2 binding curve for Mb and why?

A
  • Hyperbolic due to O2 binding being in hyperbolic dependance of pO2
  • This is due to Mbs high affinity for O2
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9
Q

How does Mbs high affinity for O2 relate to its function?

A
  • 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
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10
Q

What shape is the Hb dissociation curve and why?

A

-Sigmoidal due to cooperative binding

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11
Q

What is cooperative binding?

A

Binding of one molecule of O2 to one of the subunits increases the O2 affinity of the other subunits in the same tetramer

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12
Q

Why is there a change in conformation when O2 binds to Hb?

A

-Weak interactions between the two polypeptide dimers allows conformational change when O2 binds by pulling histidine up into the plane of the ring

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13
Q

What are the two states of Hb?

A

Relaxed and Taut

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14
Q

Why does the T state have a lower affinity for O2?

A

The dimers interact through ionic bonds which constraints movement and thus does not allow O2 to bind

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15
Q

Why does the R state have a higher affinity for O2?

A

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

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16
Q

Why is it important to have cooperative binding and intermediate R and T states?

A
  • 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
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17
Q

What is the Bohr effect?

A
  • 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
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18
Q

How does 2,3-BPG effect Hb O2 affinity and why is it important?

A
  • 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
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19
Q

How does CO effect Hb affinity for O2 and why is it a problem?

A
  • 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
20
Q

How is CO poisoning treated?

A
  • With 100% O2 in a hyperbaric chamber

- Facilitates CO diffusion by competing with CO to bind to haem

21
Q

What chains are in HbA

A
  • 2a
  • 2b
  • Predominant adult Hb
22
Q

What chains are in HbF?

A
  • 2a
  • 2g
  • Foetal Hb
23
Q

Why does HbF have a higher affinity than HbA and why is it important?

A
  • 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
24
Q

What chains are in HbA2?

A
  • 2a
  • 2d
  • minor component of adult Hb
25
Q

What is HbA1c?

A

-Non-enzymatic glycosylated HbA

26
Q

What type of inheritance is sickle cell anaemia?

A

-Autosomal homozygous recessive

27
Q

Which population does it mainly affect?

A
  • Africans

- African-Americans

28
Q

What nucleotide mutation causes sickle cell anaemia?

A

-Point mutation of A->T

29
Q

What a’a mutation occurs in sickle cell anaemia?

A

-Glutamate->Valine

30
Q

What causes sickled cells to aggregate?

A

-Non-polar valine causes protrusion out of b-chain into b-chain of another Hb

31
Q

Why do RBCs sickle in sickle cell anaemia?

A
  • 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
32
Q

Why do RBCs undergo cycles of sickling?

A

-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

33
Q

Why do people with sickle cell anaemia suffer with anaemia?

A
  • 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
34
Q

Why is anaemia not a major problem in sickle cell?

A

-The HbS has a lower affinity for O2 and so it is released more easily which counteracts the lack of Hb

35
Q

Why can sickled cells cause pain?

A
  • 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
36
Q

What is a sickle crisis?

A

When multiple cells sickle and cause occlusion of blood cells leading to local anoxia

37
Q

What can precipitate a sickle cell crisis?

A
  • Infection
  • Cold
  • Acid blood
  • dehydration
38
Q

In which areas of the body is pain normally associated with sickle cell?

A
  • Bone
  • Chest
  • Brain
  • Kidney
39
Q

Why can jaundice and splenomegaly occur in sickle cell?

A
  • 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
40
Q

What are thalassaemias?

A

-A group of genetic disorders caused by an imbalance in a and b chains in Hb

41
Q

What is b-thalassaemia, what mutation causes it and what happens to RBCs?

A
  • Reduction or absence in b chains
  • Typically caused by a point mutation
  • RBC precursors undergo early destruction as Hb precipitates out.
42
Q

Can a-chains form a stable tetramer?

A

-No

43
Q

Are thalassaemias present from birth?

A

-No occurs in the first few months as HbF changes to HbA

44
Q

What is the genetic difference between B thalassaemia minor and B thalassaemia major?

A
  • 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
45
Q

What is a-thalassaemia and what kind of mutation typically causes it?

A
  • Reduction or absence of a-chains

- Deletion mutations

46
Q

Why are there varying levels of a-thalassaemia?

A
  • Because there are 4 a-globin genes (2 on each chromosome)

- Different amounts and levels of mutations causes different severities of a-thalassaemia

47
Q

Why is a-thlassaemia not as critical as b-thalassaemia?

A

-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)

MOLECULES GENES AND DISEASE (12 decks)

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