Fetal hemoglobin (FHB) is a specialized form of hemoglobin found in human fetuses, differing structurally from adult hemoglobin. While adult hemoglobin consists of two alpha and two beta subunits, fetal hemoglobin is composed of two alpha and two gamma subunits. This structural variation is crucial for the fetus, as it enhances the ability to extract oxygen from the mother's blood.
The presence of gamma subunits in fetal hemoglobin results in a lower affinity for 2,3-bisphosphoglycerate (BPG), a negative heterotropic allosteric inhibitor. This reduced affinity for BPG correlates with an increased affinity for oxygen, allowing fetal hemoglobin to effectively capture oxygen from maternal oxyhemoglobin (HbO2). Consequently, the oxygen saturation curve for fetal hemoglobin shifts to the left compared to that of adult hemoglobin, indicating a stronger affinity for oxygen.
In graphical representations, the oxygen saturation curve for fetal hemoglobin (often depicted in green) shows a lower dissociation constant (Kd) than that of adult hemoglobin (depicted in red). A lower Kd value signifies a stronger affinity for oxygen, confirming that fetal hemoglobin is more efficient at binding oxygen. This mechanism is vital for fetal development, ensuring that the fetus receives adequate oxygen supply from the maternal circulation.
In summary, the unique structure of fetal hemoglobin, characterized by its gamma subunits, allows for a higher oxygen affinity, facilitating the transfer of oxygen from the mother to the fetus, which is essential for proper growth and development.