Haemoglobin, zeta (IPR002340)

Short name: Haemoglobin_zeta

Overlapping homologous superfamilies

Family relationships


In vertebrates, haemoglobins 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 haem group. The alpha and beta subunits are highly similar in sequence, but differ structurally in that the beta subunit contains an alpha-helix (the D helix) that is missing in the alpha subunit [PMID: 7599114]. There is at least one haem-site ligand on each of the alpha and beta subunits. The imidazole ring of the 'proximal' His residue provides the fifth haem iron ligand; the other axial haem iron position remains essentially free for oxygen coordination. The binding of oxygen and carbon dioxide is associated with a variation of the haem 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 [PMID: 1445857, PMID: 12093902]. 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 (zeta2epsilin2, alpha2epsilon2, zeta2gamma2) and two foetal Hb (alpha2gamma2, alpha2delta2). It has been hypothesised that the embryonic alpha-haemoglobin family diverged considerably earlier than the beta-haemoglobin line, as reflected in the greater diversity found amongst alpha sequences [PMID: 6157691].

This entry represents the zeta-haemoglobin subunit, which is expressed exclusively in the primitive erythroblasts of the embryonic yolk sac and is selectively silenced during the transition from primitive to definitive erythropoesis [PMID: 9668527, PMID: 9528789].

GO terms

Biological Process

GO:0015671 oxygen transport

Molecular Function

GO:0020037 heme binding
GO:0005506 iron ion binding
GO:0019825 oxygen binding

Cellular Component

GO:0005833 hemoglobin complex

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.