Biological Unit

DefinitionDefinition

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. When a biological unit contains multiple chains that have co-evolved to bind to each other, it may also be referred to as a specific oligomer.

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, often in the PDB file format) 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.

Sources of Biological Unit ModelsSources of Biological Unit Models

REMARK 350 in the PDB File HeaderREMARK 350 in the PDB File Header

Atomic coordinates for biological units, when specified by the authors of a published structure in REMARK 350 of the PDB file format, 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). One technical problem with the files from RCSB is that when they contain more than one copy of the asymmetric unit, the chains all have identical names.

Sometimes, the specific oligomers were not known at the time the asymmetric unit was published. Also, some authors failed to specify the biological unit even when it was known. Rarely, the specified biological units might be incorrect. For all these reasons, it is advisable to consult sources other than REMARK 350.

Protein Interfaces, Surfaces and Assemblies (PISA)Protein Interfaces, Surfaces and Assemblies (PISA)

The Protein Interfaces, Surfaces and Assemblies Server (PISA) uses improved methods to predict the biological unit or probable quaternary assembly, compared to its predecessor PQS (see next section).

Probable Quaternary Structure Server (PQS)Probable Quaternary Structure Server (PQS)

The Probable Quaternary Structure Server (PQS) at the European Bioinformatics Institute examines the contacts within protein crystals, and makes an educated guess about which represent co-evolved specific oligomeric contacts, and which are artifacts of crystallization. It returns models for what it deduces to be the biological units. There are many possible relationships between the asymmetric unit and the biological units returned by PQS. Examples are given in the discussion of PQS at ProteinExplorer.Org.

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Web SitesWeb Sites

• ProtBud, a database of biological unit structures
• Biological Unit at PDBWiki

Literature CitationsLiterature Citations

• ProtBuD: a database of biological unit structures of protein families and superfamilies, Qifang Xu, Adrian Canutescu, Zoran Obradovic, and Roland L. Dunbrack Jr., 2006, Bioinformatics (doi:10.1093/bioinformatics/btl490). Abstract.