Q. What is the difference between adult and fetal haemoglobin?
Haemoglobin (Hb), the oxygen carrying protein present in our red blood cells, is made up of an iron containing Haeme molecule surrounded by a protein called Globin. Each Hb molecule contains two pairs of globin chains. Different Hbs are found in humans during the embryonic, fetal and adult life and they differ from each other in the type of globin chain they possess.
Except in the first few weeks of embryo formation, one globin chain pair is always alpha while the second pair is termed non-alpha - (beta , gamma , delta). It is the combination of 2 alpha-chains with different non-alpha chains that gives rise to different types of Hbs. The production of these different globin chains is governed by distinct genes called a, b, g, d, etc.
An adult normally has the following types of haemoglobins (Hb) present in blood:
Hb A – made up of 2 alpha & 2 beta globin chains: 95% of total
Hb A2 – made up of 2 alpha & 2 delta globin chains: 3% of total
Hb F – made up of 2 alpha & 2 gamma globin chains: 1-2% of total
In order to deliver oxygen to the tissues, haemoglobin must be able both to bind oxygen and to release oxygen. Hb A and Hb F differ (besides their structure), in their oxygen affinity i.e. the tightness with which they bind to oxygen. This is primarily due to the different function they perform in adult and fetal life. The developing fetus does not breathe the way we do by drawing air into its lungs (where there is plenty of oxygen). Instead, it draws its oxygen from the mother's blood (where the oxygen content is low). Hb F is equipped to do this as it binds to oxygen with greater affinity than does Hb A thus extracting more oxygen for the developing fetus, thus facilitating oxygen transfer from the mother to the developing baby. The levels of fetal Hb rapidly fall after birth as now the lungs are functional and Hb A is better suited for oxygen delivery and this occurs by the switching off of the g-gene.