In humans the embryonic and fetal forms of hemoglobin have a higher affinity for oxygen than that adults This is due to?
In humans, the embryonic and fetal forms of hemoglobin have a higher affinity for oxygen than that in adults. This is primarily due to the presence of a specific type of globin chain called the gamma globin chain. During embryonic and fetal development, the predominant hemoglobin type is hemoglobin F (HbF), which consists of two alpha globin chains and two gamma globin chains. Compared to adult hemoglobin (hemoglobin A or HbA), which has two alpha globin chains and two beta globin chains, HbF exhibits a higher affinity for oxygen.
The gamma globin chains in HbF have a higher intrinsic affinity for oxygen than the beta globin chains found in HbA. This difference is attributed to several structural variations between the gamma and beta globin chains. One key factor is the presence of a specific amino acid called serine at position 143 in the gamma globin chain. In HbA, this position is occupied by histidine. The serine residue in HbF allows for the formation of an additional hydrogen bond with oxygen, which contributes to the increased oxygen affinity.
Furthermore, the gamma globin chains have a higher degree of cooperativity compared to beta globin chains. Cooperativity refers to the phenomenon where the binding of oxygen to one subunit of a hemoglobin molecule facilitates the binding of oxygen to the other subunits. The higher cooperativity in HbF allows for a steeper oxygen-binding curve, resulting in a greater increase in oxygen saturation at lower oxygen concentrations.
The higher oxygen affinity of HbF is crucial for fetal development. It ensures that the fetus can efficiently extract oxygen from the maternal circulation, where the oxygen tension is lower compared to that in the adult lungs. As a result, the fetus can obtain sufficient oxygen for its metabolic needs despite the lower oxygen concentration in the uterine environment.
After birth, the production of HbF gradually decreases, and the production of HbA increases. The transition from HbF to HbA is typically complete within the first few months of life. This switch in hemoglobin types is regulated by various factors, including changes in gene expression, oxygen levels, and the availability of globin chains.