Haemoglobin Flashcards
What is the structure and function of myoglobin?
Myoglobin is an oxygen store. It is made up of 8 alpha helices with a haem group. It is a very compact globular protein - therefore it has hydrophobic residues on the inside and polar ones on the surface. It has the highest concentration in skeletal and cardiac muscle.
What is the advantage of haem being non-covalently bound to protein as opposed to no association?
- Association with the protein keeps the Fe iron in the (II) form rather than (III)
- Prevents the binding of other molecules substantially e.g. CO2
What is the role of the haem prothestic group?
It associates with oxygen at the lungs and dissociates with oxygen at the tissues. Iron binds to the protoporphyrin ring through co-ordinate bonding. The ring acts as a multi dente ligand. One coordinate bond with histidine, another with oxygen and 4 with the Nitrogen in the ring. Haem is bound non-covalently to protein.
Give the differences between myoglobin and haemoglobin structure and properties.
Structure:
- Myoglobin is made up of 8 alpha helices whereas haemoglobin is made up of 2 alpha helices and 2 beta helices.
Properties:
- Haemoglobin is an allosteric protein whereas myoglobin is not. i.e. there is cooperative binding in haemoglobin and non-competitive bonding in myoglobin.
- Oxygen affinity is affected by pH and carbon dioxide concentration in haemoglobin whereas it is not in myoglobin.
- Haemoglobin is regulated by BPG whereas myoglobin is not.
- Myoglobin has a higher affinity for oxygen than haemoglobin does.
What is cooperative bonding?
When the binding of one molecule (oxygen in this case) affects the binding of other molecules (other oxygen molecules to other subunits).
Give the details of oxygen binding to haemoglobin.
When oxygen binds to the first the Fe(II) in the first haemoglobin subunit it affects the structure of the whole molecule. It pulls iron ion into the plane of the ring. The distance between the two helices is reduced. The proximal his is pulled towards the protoporhyrin ring. This movement causes interchain salt bridges to rupture. As a result the oxygen binding sites are more available. This is known as the relaxed (R) state. Oxygen can bind easier to the binding sites as opposed to when the molecule is in the initial tense state (T state).
What is the difference between the T state and R state?
T state:
- Oxygen binding sites are relatively inaccessible
- Low affinity for oxygen
- No oxygen bound
R state:
- Fewer salt bridges
- The oxygen binding sites are more accessible
- High affinity for oxygen
What is the Bohr affect?
The oxygen saturation curve shifts to the right at a lower pH. This means at a lower pH haemoglobin dissociates more readily with oxygen and haemoglobin is less saturated. An example of this is at a high carbon dioxide concentration or a high lactic acid concentration.
Explain the biochemical mechanism of the Bohr Shift.
At a lower pH, there is a higher concentration of hydrogen ions. These can then protonate the histidine residues - these residues can form salt bridges. As a result haemoglobin is pushed back into the tense state and oxygen is released.
What is BPG? What is its function?
Bisphophoglycerate (BPG) is found in high concentrations in eukaryotes. At high altitudes and in hypoxic states, BPG concentrations increase. BPG decreases the affinity of haemoglobin for oxygen and therefore causes it to be released to the tissues. BPG is a highly negatively charged molecule. `It fits in the centre of the haemoglobin molecule. It forms salt bridges with positively charged residues. stabilising the tense state.
H+,CO2 and BPG interact at different sites and so their affects are additive.
How is carbon dioxide transported around the body?
- 85% transported as hydrogen carbonate ions
- 10% bound to the terminal amine in haemoglobin to form carbinaminohaemoglobin
- 5% dissolved directly in plasma
What is the difference between foetal haemoglobin and adult haemoglobin?
Foetal haemoglobin is made up of 2 alpha chains and 2 gamma chains. This alters the affinity of haemoglobin for oxygen. Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. It also does not bind to BPG as well. This is important as when the mother is experiencing mild hypoxia, the foetus is at an advantage. It will be able to associate with the oxygen.
What is sickle cell anaemia? What is its cause?
Sickle cell anaemia is a disease prevalent in malaria countries (this is as it results in malaria resistance). There is a mutation in which glutamate (E7) is replaced with valine. Since Valine is hydrophobic - and lies on the surface of the haemoglobin - it results in a ‘sticky patch’. These cells polymerise to form sickle cells. These are long cells that can often clog arteries leading to severe pain due to the prevention of blood flow. The effects of a sickle cell crisis only last a week as sickle cells only last a 10-20 days before being destroyed.
What is Beta-thalaessemia and what is its cause?
This is due a mutation that results in fewer beta globing molecules being produced. Therefore haemoglobin cannot be produced. Tends to be a single point mutation. Common in people of African and Mediterranean decent.
How can mutations affect haemoglobin?
- The amount of Hb synthesised
- The structure of Hb
- The stability of HB
- The affinity of Hb for oxygen
- The affinity for regulators such as BPG
