The asymmetric unit is used by the crystallographer to refine the structure against experimental data and does not necessarily represent a biologically functional molecule.

An asymmetric unit may contain:
  • one biological molecule,
  • a portion of a biological molecule,
  • multiple biological molecules.

Hemoglobin, a molecule with four protein chains (two alpha-beta dimers), provides a good example of each of these cases:

One biological molecule A portion of a biological molecule Multiple biological molecules
Entry 2hhb contains one hemoglobin molecule (4 chains) in the asymmetric unit. Entry 1hho contains half a hemoglobin molecule (2 chains) in the asymmetric unit. A crystallographic two-fold axis generates the 4 chains of the hemoglobin molecule. The two homologous portions of the molecule are structurally similar enough that only one copy appears in the asymmetric unit. Entry 1hv4 contains two hemoglobin molecules (8 chains) in the asymmetric unit.

Because of structural differences in the two hemoglobin molecules, both appear in the asymmetric unit.

You can reach the Structure Summary Page of each asymmetric unit by typing the corresponding PDB code in the search (blue) bar on the PDB home pageor you can simply click on the PDB codes in the table above.

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