Fetal hemoglobin ($\mathrm{HbF}={\alpha }_2{\gamma }_2$)

Fetal hemoglobin, commonly HbF, is a hemoglobin form built from two alpha-globin and two gamma-globin subunits, written ${\alpha }_2{\gamma }_2$. The major adult form, hemoglobin A, instead has the ${\alpha }_2{\beta }_2$ combination. Fetal identity concerns subunit composition; oxyhemoglobin and deoxyhemoglobin concern oxygen occupancy. The same fetal molecule can change its oxygenation state without becoming adult hemoglobin.

Primary structural evidence is available in the human fetal deoxyhemoglobin crystal structure 1FDH. Its experimental record identifies an alpha/gamma tetramer determined by X-ray diffraction at $2.50\mathring{\mathrm{A}}$ resolution.^[ref] The related paper connects structural differences with weaker interaction with 2,3-Bisphosphoglycerate, an effector relevant to oxygen-binding behavior.^[ref] A structure constrains a mechanism, while an affinity curve requires a functional measurement under specified conditions.

An original Hill equation model illustrates what higher oxygen affinity means. Let fractional occupancy at oxygen partial pressure $p$ beWith the same stipulated $n=2$, compare midpoints $P_{50}=20$ and 30 in arbitrary pressure units. At $p=25$, their occupancies areThe lower midpoint gives greater occupancy at the same pressure. It does not imply that the protein carries more than one oxygen molecule per available haem site. These midpoints and the exponent are teaching parameters, not measured fetal/adult reference values.

Subunit identity also matters analytically. A primary 1991 experiment measured fetal and adult hemoglobin derivative spectra over 450–1000 nm and found differences relevant to multicomponent analysis.^[ref] An optical model that distinguishes oxygenation states should therefore also justify which protein-form spectra it uses. Oxygen occupancy, subunit composition and total protein concentration are different quantities.