The Types of Haemoglobin Throughout Human Life.
In this article I will be briefly running through the various forms of haemoglobin present in a human; from an embryo to an adult.
Firstly let’s recap the role and structure of haemoglobin.
The role of haemoglobin is rather similar to a delivery truck driver. This is because haemoglobin loads oxygen, transports oxygen and then finally unloads oxygen.
The process by which haemoglobin loads oxygen is called associating, and this occurs in regions of high oxygen concentrations – the lungs. Here the oxygen and haemoglobin combine forming oxyhaemoglobin.
The process in which haemoglobin unloads oxygen is called disassociating, and occurs in regions of low oxygen concentrations – in tissues. Here oxyhaemoglobin splits back into oxygen and haemoglobin.
Therefore the reversible reaction can be summarised by the equation:
Oxygen + Haemoglobin ⇌ Oxyhaemoglobin
Haemoglobin is a large protein molecule folded around four iron atoms and it has a quaternary structure. A quaternary structure is where two or more polypeptide chains join together due to chemical bonds which could be ionic, covalent or hydrogen bonds.
In the case of haemoglobin there are four polypeptide chains. Each one of these polypeptide chains contains a haem group which is able to bind to one oxygen molecule. Therefore four oxygen molecules can be transported by each haemoglobin molecule. In every red blood cell there are approximately 270 million haemoglobin molecules and so each red blood cell can carry about 1080 million oxygen molecules!
Types of Haemoglobin:
There are seven types of haemoglobin molecules throughout a human’s life. Four when you are an embryo, one once you develop into a fetus and then as an adult you have two.
The form of haemoglobin most common and in highest proportion in an embryo is Haemoglobin Gower I (ζ2ε2) The four polypeptide chains that compose this type of haemoglobin are two zeta and two epsilon chains.
The other three forms of haemoglobin are present at much lower levels and are:
- Haemoglobin Gower II (α2ε2) – Composed of two alpha and two epsilon chains.
- Haemoglobin Portland I (ζ2γ2) – Comprised of two zeta and two gamma polypeptides.
- Haemoglobin Portland II (ζ2β2) – Made of two zeta and two beta polypeptide chains.
Once an embryo develops into a fetus and the four types of embryonic haemoglobin molecules disappear they are replaced by Haemoglobin F (α2γ2)
This type of haemoglobin is used due to it having a greater affinity for oxygen than adult haemoglobin. Therefore the growing fetus is able to take its mother’s oxygen which is in her bloodstream.
Haemoglobin F remains in the child’s blood until it is around six months old and then almost all of it is replaced with adult haemoglobin.
The two types of adult Haemoglobin are:
- Haemoglobin A (α2β2) – Has two alpha chains and two beta chains
- Haemoglobin A2 (α2δ2) – Has two alpha polypeptides and two delta polypeptides.
There is also a small amount of Haemoglobin F remaining.
Haemoglobin A is the most prevalent as it makes up about 97% of adult haemoglobin
As with all biological substances mutations can occur and these mutations cause a change in the genes coding for haemoglobin and so variant forms of haemoglobin are formed. There are several hundred variant forms of haemoglobin.
Thankfully, most variant forms of haemoglobin cause little to no problems. However, there are a select few that do; most notably Haemoglobin S.
Haemoglobin S has a slight change in the coding for the beta chain in adult haemoglobin and this causes sickle cell anaemia.