5.4 Fetal Hemoglobin

Instead of having a \(\beta\) subunit, fetal hemoglobin has a \(\gamma\) subunit. This generates a hemoglobin tetramer that is composed of \(\alpha_2\gamma_2\) subunits.

Binding Affinities of Fetal and Adult Hemoglobin

Figure 5.16: Binding Affinities of Fetal and Adult Hemoglobin

Consequently, the fetal form of hemoglobin has a higher O2 affinity in vivo compared to the adult form of hemoglobin.

Furthermore, the \(\alpha_2\gamma_2\) hemoglobin tetramer of fetal hemoglobin binds to BPG more weakly than adult hemoglobin. Hence, this also increases fetal hemoglobin’s affinity for O2 (see figure 5.17 below).

BPG Binding in Adult and Fetal Hemoglobin

Figure 5.17: BPG Binding in Adult and Fetal Hemoglobin

To be more specific, it is the His143 in the adult \(\beta\) chain that is replaced by Ser143 in the \(\beta\) chain of fetal hemoglobin. This removal of two positive charges and associated interactions is responsible for reducing hemoglobin’s affinity for hemoglobin.