Asymmetric Unit
The asymmetric unit (see definition in the next paragraph) is what is contained in the protein 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.
The asymmetric unit is the smallest portion of a crystal that, when duplicated and moved by crystal symmetry operations, can produce the unit cell 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 unit cell is the smallest portion of a crystal that, when duplicated and translated, can generate the entire crystal. You can see the unit cell by clicking on the word Jmol to the lower right of any molecular display in Jmol (which includes most interactive molecular displays in Proteopedia), and then on the menu that appears, Style, Unit Cell.
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- 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.