Asymmetric Unit
From Proteopedia
The asymmetric unit (see definition below) is what is contained in the model that crystallographers publish in the Protein Data Bank. It is not to be confused with the biologically functional unit (Biological Unit), which may be the same, smaller, or larger than the asymmetric unit.
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- (obtained from the Probable Quaternary Structure server at the European Bioinformatics Institute). In the case of 2qcb, it is a homotetramer of the asymmetric unit.
- . In the case of 2qcb, the unit cell contains portions of 16 copies of the asymmetric unit.
The asymmetric unit is the smallest portion of a crystal that provides a complete solution. When duplicated and moved by crystal symmetry operations, the unit cell of the crystal can be generated, and from the unit cell, large portions of the crystal. The symmetry operations that are common for biological molecules are rotation, translation, and screw operations (which combine rotation and translation). Mirror symmetry planes occur rarely in crystals of biological molecules due to the presence of chiral centers, e.g. L or D amino acids.
The biological unit is defined elsewhere.
The unit cell is the smallest portion of a crystal that, when duplicated and translated, can generate the entire crystal.
If the green link "Unit Cell" above is not working, you can see the unit cell by right-clicking on the molecule in JSmol, and then on the menu that appears, Symmetry, Reload {1 1 1}.
Websites
- Diagrams illustrating these concepts are available at the RCSB Protein Data Bank.
- Asymmetric unit at the Online Dictionary of Crystallography of the International Union of Crystallographers.
