Hemoglobin

Hemoglobin or haemoglobin (frequently abbreviated as Hb, {{PDB|1A3N}}) is the iron-containing oxygen-transport metalloprotein in the red cells of the blood in mammals and other animals. The molecule consists of globin, the apoprotein, and four heme (or haem) groups, which are organic molecules with an iron atom in each.

Structure

In each subunit of a hemoglobin molecule, there is a heme group. A heme group consists of an iron atom held in a heterocyclic ring, known as a porphyrin. This iron atom is the site of oxygen binding. The iron atom binds equally to all four nitrogens in the center of the ring, which lie in one plane. Two additional bonds perpendicular to the plane on each side can be formed with the iron to form the fifth and sixth positions. The iron atom can either be in the Fe+2 or Fe+3 state, but ferrihemoglobin (Methemoglobin) (Fe3+) cannot bind oxygen. The name hemoglobin is the concatenation of heme and globin, a globin being a generic term for a globular protein. Since a single subunit of hemoglobin is, in fact, made of a heme imbedded in a globular protein, the name makes sense. There are a number of heme containing proteins and hemoglobins. All hemoglobins contain either two α or two ξ chains. Hemoglobin A is by far the best known.

Related Topics:
Subunit - Molecule - Heterocyclic - Porphyrin - Nitrogen - Methemoglobin - Globin - Globular protein - Heme containing proteins

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In adult humans, the most common hemoglobin is a tetramer (contains 4 subunit proteins) called hemoglobin A, consisting of two α and two β subunits noncovalently bound. The subunits are structurally similar and about the same size. Each subunit has a molecular weight of about 16,000 daltons, for a total molecular weight in the tetramer of about 64,000 daltons. Each subunit of hemoglobin contains a single heme, so that the overall binding capacity of adult human hemoglobin for oxygen is four oxygen molecules:

Related Topics:
Dalton - Molecular weight

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Stepwise Reaction:

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• Hb + O₂ <-> HbO₂
• HbO₂ + O₂ <-> Hb(O₂)₂
• Hb(O₂)₂ + O₂ <-> Hb(O₂)₃
• Hb(O₂)₃ + O₂ <-> Hb(O₂)₄

Summary Reaction:

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• Hb + 4O₂ -> Hb(O₂)₄

Quaternary structure

The four polypeptide chains are bound by hydrogen bonds, salt bridges, and hydrophobic bonds. There are two kinds of contacts between the α and β chains: α1β1 and α1β2. The contacts tend to be hydrophobic but are important in the allosteric interaction.

Related Topics:
Polypeptide chains - Hydrogen bonds - Salt bridge - Hydrophobic - Allosteric

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Types of hemoglobins

Embryonic hemoglobins:

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• Gower 1 (ξ₂ε₂)
• Gower 2 (α₂ε₂) ({{PDB|1A9W}})
• Hemoglobin Portland (ξ₂γ₂)

Adult hemoglobins (this does not mean that these hemoglobins are not created during fetal development but rather these hemoglobins still persist into adult life):

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• Hemoglobin F (α₂γ₂) ({{PDB|1FDH}}) - In adults Hemoglobin F is restricted to a limited population of red cells called F cells.
• Hemoglobin A (α₂β₂) ({{PDB|1BZ0}})
• Hemaglobin A₂ (α₂δ₂) - δ chain synthesis begins late in the third trimester and in adults, it has a normal level of 2.5%