S10) Protein Function — Oxygen Transport

Question 1

What is haemoglobin and what does it do?

Answer 1

• Haemoglobin is a tetrameric haem protein found in blood which carries oxygen from the respiratory organs to body tissues
• There, it releases the oxygen to permit aerobic respiration in order provide energy for metabolism

Question 2

What is myoglobin and what does it do?

Answer 2

• Myoglobin is a monomeric haem protein found mainly in muscle tissue where it serves as an intracellular storage site for oxygen
• During periods of oxygen deprivation oxymyoglobin releases its bound oxygen which is then used for metabolic purposes

Question 3

Describe the binding of oxygen to haemoglobin

Answer 3

• O₂ binds to haemoglobin via the Fe atom in the haem prosthetic group
• O₂ binding to haemoglobin shows a sigmoid shaped dependence on [O₂]

Question 4

Describe the binding of oxygen to myoglobin

Answer 4

• O₂ binds to myoglobin via the Fe atom in the haem prosthetic group
• O₂ binding to myoglobin shows a hyperbolic dependence on [O₂]

Question 5

Describe the overall structure of haemoglobin

Answer 5

• Composed of 4 polypeptide chains in α₂β₂ tetramer
• Each chain contains an essential haem prosthetic group

Question 6

Explain how haemoglobin undergoes a structural change on oxygen binding

Answer 6

• Deoxyhaemoglobin exists in either the low affinity T state or the high affinity R state
• Oxygen binding promotes stabilisation of the R state

Question 7

Why is the oxygen-dissociation curve for haemoglobin sigmoidal?

Answer 7

• Haemoglobin ‘co-operatively binds’ to oxygen
• The binding of one oxygen molecule promotes the binding of the next

Question 8

What is the benefit of the sigmoidal binding curve of haemoglobin?

Answer 8

• O₂ can be efficiently transported from the lungs to the tissues
• Haemoglobin is more sensitive to small differences in [O₂]

Question 9

2,3-BPG is a molecule which regulates oxygen binding.

What is the effect of the binding of 2,3-BPG to deoxyhaemoglobin?

Answer 9

• 1 BPG binds per haemoglobin tetramer and decreases the affinity for O₂
• [BPG] increases at high altitudes, promoting O₂ release at the tissues

Question 10

Describe he regulation of oxygen binding in terms of CO₂ and H⁺

Answer 10

Binding of H⁺ and CO₂ lowers the affinity of haemoglobin for oxygen

Question 11

What is the benefit of the Bohr effect?

Answer 11

• Metabolically active tissues produce large amounts of H⁺ and CO₂
• The Bohr effect ensures the delivery of O2 is coupled to demand

Question 12

In terms of oxygen binding, explain why is carbon monoxide poisonous?

Answer 12

Carbon monoxide (CO) is a poison because it combines with ferromyoglobin and ferrohaemoglobin to block oxygen transport

Question 13

CO binds to haemoglobin 250x more readily than O₂.

Illustrate the impact of this on the oxygen binding curve of haemoglobin

Answer 13

Question 14

HbF is the major haemoglobin in foetal blood.

Why is it important?

Answer 14

Higher binding affinity of HbF for O₂ than HbA allows transfer of O₂ to foetal blood supply from the mother

Question 15

Identify 2 properties of red blood cells with the HbS protein as seen in sickle cell anaemia

Answer 15

• More prone to lyse (anaemia)
• More rigid (black microvasculature)