Biological Unit
The "Biological Unit" is the quaternary structure of a protein that is believed to be the functional form of the molecule. It can be a single chain, or a quaternary assembly of multiple identical or non-identical chains. For example, the biological unit of hemoglobin includes two alpha chains and two beta chains, making it a tetrameric α2β2 structure.
Of course, what is the biological unit under one set of conditions may change under a different set of conditions, so there may be more than one biological unit that includes a given protein chain.
Published macromolecular structure data files (Atomic Coordinate Files or PDB Files) contain the Asymmetric Unit, which may be identical with the biological unit, or only a portion of it, or may contain multiple biological units. When publishing a macromolecular structure, the authors may elect to specify the biological unit. In the PDB File Format, this is done in REMARK 350.
Atomic coordinates for biological units, when specified by the authors of a published structure, are available from the Protein Data Bank. When this was written (April, 2008), biological units were available at the bottom of the list under the Download Files section, under the Structure tab (upper left of the page).
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