Blood Biochem 3
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What are the main topics covered in Blood Biochemistry 3?
Myoglobin and haemoglobin structure, allosteric proteins, 2,3 BPG, oxygen transport, haemoglobin diseases and jaundice.
What are the main functions of oxygen transport?
Oxygen is carried to tissues by haemoglobin and stored in tissues by myoglobin.
Why is oxygen transported by haemoglobin?
Oxygen has low solubility in aqueous environments, so it needs a carrier to reach tissues.
What is the role of haemoglobin?
Transports oxygen in the blood from the lungs to tissues.
What is the role of myoglobin?
Stores oxygen in muscle tissue.
Why is aerobic metabolism important?
It produces more energy than anaerobic metabolism but requires oxygen.
What are myoglobin and haemoglobin made of?
Protein and a haem group (prosthetic group).
What gives myoglobin and haemoglobin their red colour?
The haem group.
What is a prosthetic group?
A non-protein component permanently associated with a protein that is essential for its function.
What is haem?
An organic structure containing carbon, hydrogen, oxygen and nitrogen, with an iron atom in the centre.
What is the structure of the haem ring?
A tetrapyrrole ring made of four pyrrole groups.
How many nitrogen atoms bind to iron in haem?
Four nitrogen atoms.
How many additional binding positions does iron have in haem?
Two additional positions: the fifth and sixth positions.
What is the fifth binding position of iron in haem occupied by?
The proximal histidine.
What can bind to the sixth position of iron in haem?
Oxygen, water or other ligands.
What is the main structural characteristic of the haem environment?
The haem group sits within a hydrophobic protein pocket.
What is the structure of myoglobin?
One polypeptide chain containing eight alpha helices (A-H) and approximately 150 amino acids.
What percentage of myoglobin is alpha helix?
Approximately 75%.
How many haem groups does myoglobin contain?
One.
How many oxygen-binding sites does myoglobin have?
One.
Where is the haem group located in myoglobin?
In a hydrophobic pocket within the protein.
Which amino acids are particularly important in the haem pocket?
Proximal histidine and distal histidine.
What type of amino acids are found on the outside of myoglobin?
Polar amino acids.
What is the function of proximal histidine?
It binds directly to the iron atom at the fifth position and anchors haem to the protein.
Where is proximal histidine located?
At position F8 of the protein.
What is the function of distal histidine?
It helps oxygen bind correctly, stabilises oxygen binding and prevents oxidation of iron.
Does distal histidine bind directly to iron?
No. It interacts with the oxygen molecule at the sixth position.
How does distal histidine help prevent iron oxidation?
It stabilises bound oxygen and helps prevent oxidation of Fe2+ to Fe3+.
What are the three main forms of haem?
Deoxy, oxy and ferri (met).
What is the iron oxidation state in deoxyhaem?
Fe2+.
What occupies the fifth position in deoxyhaem?
Proximal histidine (His F8).
What occupies the sixth position in deoxyhaem?
Nothing; it is empty.
What is the iron oxidation state in oxyhaem?
Fe2+.
What occupies the sixth position in oxyhaem?
Oxygen (O2).
What is the iron oxidation state in ferrihaem (met)?
Fe3+.
What occupies the sixth position in ferrihaem?
Water (H2O).
What is methaemoglobin?
Haemoglobin containing oxidised Fe3+ iron, which cannot bind oxygen normally.
What is carbon monoxide (CO)?
A poisonous gas that binds to myoglobin and haemoglobin instead of oxygen.
How strongly does isolated haem bind carbon monoxide compared with oxygen?
Approximately 25,000 times more strongly.
How does distal histidine affect carbon monoxide binding?
It reduces the affinity of haem for carbon monoxide.
Why is the protein environment important to haem function?
It modifies ligand binding and controls the affinity of haem for oxygen and carbon monoxide.
How many polypeptide chains does myoglobin contain?
One.
How many polypeptide chains does haemoglobin contain?
Four.
How many haem groups does haemoglobin contain?
Four.
How many oxygen molecules can one haemoglobin molecule bind?
Four oxygen molecules.
How many oxygen molecules can one myoglobin molecule bind?
One oxygen molecule.
What is adult haemoglobin (HbA) made of?
Two alpha and two beta chains (α2β2).
What is foetal haemoglobin (HbF) made of?
Two alpha and two gamma chains (α2γ2).
What is the quaternary structure of haemoglobin?
Four interacting polypeptide chains arranged in a tetrahedral structure.
Where is each haem group located in haemoglobin?
In a crevice within each subunit.
What is an allosteric protein?
A protein that changes shape and ligand-binding affinity when a molecule binds to it.
Why is haemoglobin an allosteric protein?
It has four interacting subunits, so binding at one site affects the shape and affinity of other sites.
Is myoglobin an allosteric protein?
No. It has only one oxygen-binding site and no interacting subunits.
What is cooperative oxygen binding?
Binding of oxygen to one haemoglobin subunit increases the oxygen affinity of the remaining subunits.
What are the two main conformational forms of haemoglobin?
Tense (T) and relaxed (R).
What is the T form of haemoglobin?
Deoxygenated haemoglobin with lower oxygen affinity.
What is the R form of haemoglobin?
Oxygenated haemoglobin with higher oxygen affinity.
Which form of haemoglobin has lower oxygen affinity?
Tense (T) form.
Which form of haemoglobin has higher oxygen affinity?
Relaxed (R) form.
What stabilises the T form of haemoglobin?
Extra ionic bonds and binding of 2,3 BPG.
What happens to haemoglobin when oxygen binds?
It changes from the tense (T) form towards the relaxed (R) form.
How do deoxyhaemoglobin and oxyhaemoglobin differ structurally?
They have different quaternary structures and different hydrogen and ionic bond interactions.
What happens at the contact points between haemoglobin subunits during oxygen binding?
Subunits rotate, changing the interactions and stabilising the R form.
What is the oxygen dissociation curve?
A graph showing the relationship between oxygen partial pressure (pO2) and haemoglobin or myoglobin oxygen saturation.
What does P50 mean?
The oxygen partial pressure at which a protein is 50% saturated with oxygen.
What does a low P50 indicate?
High oxygen affinity.
What does a high P50 indicate?
Low oxygen affinity.
What is the P50 of myoglobin?
Approximately 1 mmHg.
What is the P50 of haemoglobin?
Approximately 26 mmHg.
Which has a higher oxygen affinity: myoglobin or haemoglobin?
Myoglobin.
Which has a lower P50: myoglobin or haemoglobin?
Myoglobin.
What shape is the myoglobin oxygen dissociation curve?
Hyperbolic.
Why is the myoglobin oxygen dissociation curve hyperbolic?
It has only one oxygen-binding site, so there is no cooperativity.
What shape is the haemoglobin oxygen dissociation curve?
Sigmoidal (S-shaped).
Why is the haemoglobin oxygen dissociation curve sigmoidal?
Cooperative oxygen binding occurs between its four interacting subunits.
Why does myoglobin have a higher oxygen affinity than haemoglobin?
It needs to store oxygen in muscle and release it only when oxygen levels are very low.
How does haemoglobin transport oxygen?
It binds oxygen in the lungs at high pO2 and releases it in tissues at low pO2.
What are allosteric effectors of haemoglobin?
CO2, H+ and 2,3-bisphosphoglycerate (2,3 BPG).
What are negative allosteric effectors?
Molecules that bind to haemoglobin at sites other than the oxygen-binding site and decrease its oxygen affinity.
How do CO2, H+ and 2,3 BPG affect haemoglobin?
They decrease oxygen affinity, stabilise the T form and promote oxygen release.
What happens to the oxygen dissociation curve when negative allosteric effectors bind?
It shifts to the right.
What does a rightward shift of the oxygen dissociation curve indicate?
Reduced oxygen affinity and increased oxygen release to tissues.
What is 2,3 BPG?
2,3-bisphosphoglycerate, a molecule found at high concentrations in red blood cells that regulates haemoglobin oxygen affinity.
Where does 2,3 BPG bind to haemoglobin?
In the central cavity between the beta chains of deoxyhaemoglobin.
Does 2,3 BPG bind at the oxygen-binding site?
No. It binds at a separate allosteric site.
Which form of haemoglobin does 2,3 BPG bind to preferentially?
Deoxyhaemoglobin (T form).
How does 2,3 BPG affect haemoglobin?
It stabilises deoxyhaemoglobin, decreases oxygen affinity and promotes oxygen release.
How does 2,3 BPG affect the oxygen dissociation curve?
It shifts the curve to the right.
By approximately how much can 2,3 BPG lower haemoglobin oxygen affinity according to the lecture?
Approximately 26-fold.
When does 2,3 BPG concentration increase?
During oxygen deprivation or hypoxia.
What conditions can increase 2,3 BPG?
Anaemia, cardiac failure and high altitude.
Why is increased 2,3 BPG beneficial during hypoxia?
It promotes oxygen release from haemoglobin to tissues.
What is hypoxia?
Inadequate oxygen availability to body tissues.
What is the role of erythropoietin (EPO)?
It stimulates red blood cell production.
Where is erythropoietin produced?
Mainly in the kidneys.
What happens to EPO production during high altitude or hypoxia?
It increases, stimulating production of more red blood cells.
Why does high altitude increase red blood cell production?
Lower oxygen availability stimulates EPO release from the kidneys.
How can EPO affect athletic performance?
It increases red blood cell production and oxygen-carrying capacity.
Why can EPO use be relevant in horses?
EPO can enhance oxygen transport and athletic performance.
What is bovine oxyglobin used for?
Treatment of anaemia or hypoxia.