Protein Structure Classification


To understand and map the universe of protein structures, it is necessary to collate, annotate and classify these structures in a rational scheme. While the individual protein structures reveal much about the specific molecular mechanisms that underlie a particular biological function, taken together this body of data also allows biologists to explore the evolution of structure and function. As structure is much better conserved than sequence, these data also facilitate the recognition of evolutionary relationships that are hidden at the sequence level. The different approaches that have been taken to tackle this problem include the identification of protein domains, phylogenetic and phenetic classification and hierarchical and nearest‐neighbour clustering. Powerful sequence searching methods then enable structural assignments to be allocated to genomic data.

Key Concepts

  • Proteins comprise recognisable smaller sequence domains.
  • Domains usually consist of secondary and supersecondary structures and have an average size of 150 ± 50 residues.
  • These domains may be thought of as units of evolution, which recur in many proteins in various combinations.
  • Structural classifications have been developed that group domains into fold and sequence families.
  • The phenetic approach groups the proteins according to their structural characteristics.
  • The phylogenetic approach groups proteins into families according to their evolutionary history.

Keywords: protein structure classification; common folds; protein architecture; structural comparison; genomes

Figure 1. Schematic representation of the Class (C), Architecture (A) and Topology/fold (T) levels in the CATH database.
Figure 2. There are over 2700 recognised homologous superfamilies in the CATH database, however more than half of all known protein domains are found within just 100 of the most highly populated superfamilies.
Figure 3. This figure shows the structural superposition of 120 nonredundant CATH domains from the Trypsin‐like Serine Proteases CATH Superfamily (id: These superpositions are calculated for all superfamilies in CATH and can be used to identify the highly conserved structural core that acts like a fingerprint for each superfamily.
Figure 4. Domain motions.


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Orengo CA, Thornton JM and Jones DT (eds) (2003) Bioinformatics: Genes, Proteins & Computers. Oxford: BIOS Scientific.

Williamson M (2012) How Proteins Work. New York: Garland Science, Taylor & Francis Group, LLC.

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Pearl, Frances MG, Sillitoe, Ian, and Orengo, Christine A(Oct 2015) Protein Structure Classification. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0003033.pub3]