Haemoglobin as a protein

Question 1

Explain the structure of Haemoglobin

Answer 1

Haemoglobin sub-units contain:
• Globular protein
• Haem: an iron containing molecule where oxygen binds

Each haemoglobin can bind with 4 molecules

In adults, the quaternary structure of haemoglobin is made up of 4 sub-units joint together. 2 of these sub-units are alpha chains, 2 of these are beta chains.
Sometimes, instead of 2 beta chains there are 2 delta chains.

In foetuses and infants:
There are 2 alpha chains, but 2 gamma chains. As infants grow, the 2 gamma chains are replaced by 2 beta chains

Question 2

Explain oxygen binding and structural changes of haemoglobin

Answer 2

There are two conformational states:
• Relaxed ( R ) state: The quaternary structure is of oxyhaemoglobin, favouring oxygen binding
• Taut ( T ) state: The quaternary structure is of deoxyhaemoglobin, favouring lower oxygen binding

When there is no oxygen, T is more stable (because no oxygen is on it, and so it doesn’t need to work to get rid of it).

When there is oxygen, R is more stable (because it doesn’t need to work hard to get oxygen to bind to it).

Question 3

List the factors affecting oxygen-haemoglobin binding (6)

Answer 3

• Temperature = high temp means oxygen release
• Acidity/CO2: high acidity = high oxygen release
• 2,3-DPG: hormone which lowers affinity for O2
• pO2: low PO2 = more binding
• Fetal haemoglobin: gamma units have higher affinity than beta units
• Sickle cell anaemia: Instead of coding for glutamate at the beta chains, it codes for valine which gives the defective haemoglobin a sticky patch on its beta chains
  As a result, the shape of the red blood cell is affected, becoming sickle instead of a bi-concave disc. The cell breaks down prematurely, and the sticky cells end up causing blood clots.

Question 4

Describe co-transport with an example.

Answer 4

• Co transport is when two substances are simultaneously transported across a membrane by one protein.
• An example is Na+ and glucose.
• Na+ follows down the concentration gradient, and glucose follows Na+.
  Glucose is against the concentration gradient.
• Moving a molecule against its concentration gradient requires energy.
• This energy in co-transport is supplied by the molecule moving along its concentration gradient