= Hemoglobin A
{title2=$\mathrm{HbA}=\alpha_2\beta_2$}
\b[<Hemoglobin A>], commonly HbA, is the major adult human hemoglobin assembly made of two <alpha-globin> and two <beta-globin> chains, $\alpha_2\beta_2$.<Human globin gene records in NCBI Gene>[HBA1 and HBB summaries, HbA chain composition]{ref} It is one hemoglobin form rather than a name for every hemoglobin molecule present in an adult. <Fetal hemoglobin> has a different non-alpha-chain composition.
Human deoxyhemoglobin structure 4HHB is a primary structural example. The associated study refined the model at $1.74\,\mathrm{\mathring A}$ using room-temperature diffraction data recorded on film with a synchrotron X-ray source.<The crystal structure of human deoxyhaemoglobin at 1.74 A resolution by G. Fermi and others>[Primary abstract and RCSB Experimental Data]{ref} This connects the abstract $\alpha_2\beta_2$ notation with experimentally determined chain and haem organization.
Composition and oxygenation are independent descriptors. Hemoglobin A can occur in oxygenated or deoxygenated states while retaining its alpha/beta identity. Each intact tetramer contains four haem sites, so a concentration expressed per tetramer differs by a factor of four from a haem-equivalent concentration.
For an original accounting example, take a pure preparation at 1.00 g/L and use a stipulated nominal tetramer <molar mass> of $64{,}500\,\mathrm{g\,mol^{-1}}$. Then
$$
c_{\mathrm{tetramer}}=\frac{1.00}{64{,}500}\,\mathrm{mol\,L^{-1}}
\approx15.50\,\mathrm{\mu mol\,L^{-1}},
$$
and haem-site concentration is about $62.02\,\mathrm{\mu mol\,L^{-1}}$. If all sites are functional and mean oxygen occupancy is 0.80, bound oxygen concentration is about $49.61\,\mathrm{\mu mol\,L^{-1}}$. Oxygen dissolved outside the protein is an additional pool. The assumptions are explicit teaching conditions, not a physiological reference range.
A primary spectral study measured both fetal and adult hemoglobin derivatives and found differences relevant to multicomponent analysis.<Absorption spectra of human fetal and adult oxyhemoglobin, de-oxyhemoglobin, carboxyhemoglobin, and methemoglobin by W. G. Zijlstra and others>[Abstract]{ref} A measurement model should therefore state which protein-form and oxygenation-state spectra it uses, alongside the basis of the reported concentration.
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