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Physiology · Protein

Hemoglobin 3D Structure

The protein that carries oxygen in red blood cells: four chains, each holding a haem group whose iron binds one oxygen molecule.

Adult haemoglobin is built from two alpha and two beta chains, each folded around a haem group with an iron at its centre. It loads oxygen in the lungs and lets it go in the tissues, and it does both well because the four chains work together. This structure, from Max Perutz's laboratory, is human haemoglobin without oxygen bound.

Also called Haemoglobin, Hb or Haemoglobin A.

Haemoglobin as four grey ribbons packed into a rounded tetramer, with a haem group drawn as sticks tucked into each chain.
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Four chains, one molecule

Haemoglobin is a tetramer: four protein chains packed into a rounded molecule about 6 nanometres across. Each chain holds one haem, so one haemoglobin can carry up to four oxygen molecules.

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View 1 of 7: Four chains, one molecule

Download the pictureDownload the atoms (PDB)Checked against its sources on 25 October 2026.

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The Hemoglobin structure, explained

  1. 01Four chains, one molecule

    Haemoglobin is a tetramer: four protein chains packed into a rounded molecule about 6 nanometres across. Each chain holds one haem, so one haemoglobin can carry up to four oxygen molecules.

  2. 02Two alpha chains

    The two alpha chains are identical, each 141 amino acids long. Each folds into alpha helices labelled A to H, missing only a short D helix that the beta chains have: the globin fold, which myoglobin shares.

  3. 03Two beta chains

    The two beta chains are 146 amino acids long. In the molecule each pairs with an alpha chain, so haemoglobin is really two alpha-beta pairs, α1β1 and α2β2, set against each other.

  4. 04A haem in every chain

    Each chain wraps a haem group in a pocket lined with water-repelling amino acids. The pocket keeps water away from the iron, which keeps it in its ferrous form, able to bind oxygen, rather than oxidised to the ferric form of methaemoglobin, which cannot.

  5. 05Where oxygen binds

    Close up on one haem of α1. The iron is held from below by the proximal histidine, and oxygen binds on the other side, beside the distal histidine. In this structure, without oxygen, the iron sits slightly out of the plane of the ring.

  6. 06Four chains that work together

    When oxygen binds, the iron moves into the plane of the ring and pulls the proximal histidine with it. The shift spreads to the contacts between the two alpha-beta pairs, and the molecule turns from the T state, shown here, to the R state, in which the other chains bind oxygen more easily. This is why haemoglobin's oxygen binding curve is S-shaped.

  7. 07One amino acid, one disease

    In sickle cell disease the sixth amino acid of each beta chain, glutamate, is replaced by valine. The change sits on the surface, and when haemoglobin S loses its oxygen it sticks to its neighbours and forms long fibres that bend red cells out of shape.

Parts

What each part does

Alpha chain (α1)
One of the two alpha globins, 141 amino acids folded into a bundle of alpha helices that cradles one haem.
Beta chain (β1)
One of the two beta globins, 146 amino acids. It folds the same way as an alpha chain and pairs tightly with α1.
Alpha chain (α2)
The second alpha globin, identical in sequence to α1.
Beta chain (β2)
The second beta globin. The β2 chain pairs with α2, and where the α1β1 pair meets the α2β2 pair, the molecule shifts as it takes up oxygen.
Haem groups
Four flat rings of protoporphyrin IX, one in a pocket of each chain, each holding one iron. Haem is what makes blood red.
Iron
The ferrous iron (Fe²⁺) at the centre of each haem, held by four nitrogens of the ring. Oxygen binds to it, one molecule per iron.
Proximal histidine
A histidine on each chain (α87 and β92) that holds the iron from below. When oxygen binds, the iron moves into the ring and pulls this histidine, and its helix, with it.
Distal histidine
A histidine on the oxygen side of each haem (α58 and β63). It does not touch the iron but forms a hydrogen bond to oxygen once bound, which helps hold it.
Glutamate 6 of each beta chain
The amino acid changed in sickle cell disease. Valine in its place leaves a sticky patch on the surface, and deoxygenated haemoglobin S joins into long fibres.

The structure

Where these atoms come from

Structure
PDB 4HHB
What it is
Human deoxyhaemoglobin
Organism
Homo sapiens
Method
X-ray diffraction, 1.74 Å resolution
Deposited
1984
Atoms shown
4,556, without water or hydrogens
Paper
Fermi G and others, J Mol Biol 1984

Coordinates from the Protein Data Bank, free of all copyright (CC0). Water and hydrogens are left out, and the molecule is turned to face you; every atom is where the experiment placed it.

Dictionary

The terms to know

Practise it

The quizzes and cards that fit

Next to it

More molecules

Sources

Where the facts come from

Written by biofideX from these sources. Drawn with 3Dmol.js (Rego and Koes, Bioinformatics 2015).

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