| Description | In vertebrates, hemoglobins (Hb) function to transport oxygen in blood plasma. Hb binds oxygen in the reduced [Fe(II)] state. Hb is composed of four globins in a tetrahedral arrangement, typically two alpha and two beta globins, where each monomer binds a heme group. The alpha and beta subunits are highly similar in sequence, but differ structurally in that the beta subunit contains an α-helix (the D helix) that is missing in the alpha subunit []. There is at least one heme-site ligand on each of the alpha and beta subunits. The imidazole ring of the 'proximal' His residue provides the fifth heme iron ligand; the other axial heme iron position remains essentially free for oxygen coordination. The binding of oxygen and carbon dioxide is associated with a variation of the heme iron coordination. Oxygen binding results in a transition from high-spin to low-spin iron, with accompanying changes in the Fe-N bond lengths and coordination geometry. In Hb, these subtle changes lead to the well-known cooperative effect of oxygen binding, which involves a relaxed (R) state when oxygen is bound and a tense (T) state upon oxygen release [, ]. The alpha or beta subunits are substituted in embryo and foetal Hb with subunits that have higher oxygen affinity (gamma, delta, epsilon, pi or zeta subunits). There are at least three types of human embryonic Hb (zeta2epsilon2, alpha2epsilon2, zeta2gamma2) and two foetal Hb (alpha2gamma2, alpha2delta2) [, ]. It has been hypothesised that the embryonic alpha-hemoglobin family diverged considerably earlier than the beta-hemoglobin line, as reflected in the greater diversity found amongst alpha sequences []. Alpha-like globins derived from a common ancestor and have a more stable history than beta-globins [].This entry represents the adult and embryonic alpha-like hemoglobin subunits. | Name | Hemoglobin, alpha-type |
| Short Name | Hemoglobin_a-typ | Type | Family |
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