F
IPR002339

Hemoglobin, pi

InterPro entry
Short nameHemoglobin_pi
Overlapping
homologous
superfamilies
 
Globin/Protoglobin (IPR012292)
family relationships

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
[5]
. 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
[3, 4]
. 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)
[6, 7]
. 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
[2]
. Alpha-like globins derived from a common ancestor and have a more stable history than beta-globins
[7]
.

This entry represents the pi-haemoglobin subunit, which is the counterpart of the alpha chain in the major early embryonic hemoglobin P. Pi is expressed during development only in the primitive erythrocyte lineage and not in the definitive lineage in chicken
[1]
.

References

1.Mechanism of developmental regulation of alpha pi, the chicken embryonic alpha-globin gene. Knezetic JA, Felsenfeld G. Mol. Cell. Biol. 13, 4632-9, (1993). View articlePMID: 8336706

2.Complete amino acid sequences of the major early embryonic alpha-like globins of the chicken. Chapman BS, Tobin AJ, Hood LE. J. Biol. Chem. 255, 9051-9, (1980). View articlePMID: 6157691

3.Cooperative oxygen binding, subunit assembly, and sulfhydryl reaction kinetics of the eight cyanomet intermediate ligation states of human hemoglobin. Doyle ML, Ackers GK. Biochemistry 31, 11182-95, (1992). View articlePMID: 1445857

4.The crystal structure of a tetrameric hemoglobin in a partial hemichrome state. Riccio A, Vitagliano L, di Prisco G, Zagari A, Mazzarella L. Proc. Natl. Acad. Sci. U.S.A. 99, 9801-6, (2002). View articlePMID: 12093902

5.The D-helix in myoglobin and in the beta subunit of hemoglobin is required for the retention of heme. Whitaker TL, Berry MB, Ho EL, Hargrove MS, Phillips GN Jr, Komiyama NH, Nagai K, Olson JS. Biochemistry 34, 8221-6, (1995). View articlePMID: 7599114

6.Human embryonic, fetal, and adult hemoglobins have different subunit interface strengths. Correlation with lifespan in the red cell. Manning LR, Russell JE, Padovan JC, Chait BT, Popowicz A, Manning RS, Manning JM. Protein Sci 16, 1641-58, (2007). PMID: 17656582

7.Evolution of hemoglobin and its genes. Hardison RC. Cold Spring Harb Perspect Med 2, a011627, (2012). PMID: 23209182

GO terms

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