Hemoglobins are tetramers composed of pairs of two different polypeptide subunits. The subunit composition of the principal hemoglobins are α2β2 (HbA; normal adult hemoglobin), α2γ2 (HbF; fetal hemoglobin) and α2δ2 (HbA2; a minor adult hemoglobin)
The total number of amino acids in beta chain and gamma chain is the same-146. The higher affinity of HbF to oxygen is due to presence of serine(in gamma chain) in place of histidine(in beta chain), which inhibits its binding with 2,3-bisphosphoglyceric acid.
Is this the only difference, or are there more such differences in the amino acid sequence?