Biological Unit: Difference between revisions

The [http://www.ebi.ac.uk/msd-srv/prot_int/pistart.html Protein Interfaces, Surfaces and Assemblies Server] (PISA) at the European Bioinformatics Institute uses improved methods to predict the biological unit or probable quaternary assembly, compared to its predecessor PQS (see next section). These servers examine the contacts that occur in macromolecular crystals used in [[X-ray crystallography]]. They attempt to discriminate between [[crystal contacts]] (artifacts of crystallization) and contacts between chains that have co-evolved to maintain specific oligomeric binding.

The [http://pqs.ebi.ac.uk Probable Quaternary Structure Server] (PQS) at the European Bioinformatics Institute examines the inter-chain contacts within protein crystals, and makes an educated guess (using published methods) about which contacts represent co-evolved specific oligomeric contacts, and which are artifacts of crystallization. It was usually correct, but not always. 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 [http://proteinexplorer.org/pqs.htm PQS at ProteinExplorer.Org]. Updates to PQS stopped in August, 2009. In 2010 it is being phased out in favor of PISA (see above).