Accessing And Understanding Biological Units

In PDB entry 2hhb, the biological unit and the asymmetric unit are the same.

PDB ID: 2hhb

Fermi, G., Perutz, M. F., Shaanan, B., Fourme, R.: The crystal structure of human deoxyhaemoglobin at 1.74Å resolution. J Mol Biol (1984) 175: 159.

When crystallographic structures are deposited in the PDB, the primary coordinate file generally contains one asymmetric unit - a concept that has applicability only to crystallography, but is important to understanding the process in obtaining the functional biological molecule. An introduction to biological units in the PDB archive is now accessible at This useful guide:

* Defines "asymmetric unit" and "biological molecule."

* Indicates where information about the biological unit can be found in PDB and mmCIF coordinate files.

* Describes how the biological unit files in the PDB have been derived.

Coordinate files for the biological units for applicable structures are accessible from the View Structure and Download/Display File sections of the Structure Explorer pages on the primary PDB website and its mirrors. The biological unit coordinate files can also be downloaded from the PDB FTP site at Images for the asymmetric and biological units are available from each entry's View Structure section of the Structure Explorer page.