Structural Classification of Proteins

The Structural Classification of Proteins (SCOP) database is a largely manual classification of protein structural domains based on similarities of their amino acid sequences and three-dimensional structures.

Originally published in 1995 it is usually updated at least once yearly by Alexei G. Murzin and his colleagues, upon whose expertise the classification rests.

Hierarchical structure
SCOP utilizes four levels of hierarchic structural classification:
 * 1) class - general "structural architecture" of the domain
 * 2) fold - similar arrangement of regular secondary structures but without evidence of evolutionary relatedness
 * 3) superfamily - sufficient structural and functional similarity to infer a divergent evolutionary relationship but not necessarily detectable sequence homology
 * 4) family - some sequence similarity can be detected.

Comparison to other classification systems
This classification is more significantly based on the human expertise than semi-automatic CATH, its chief rival. It is usually accepted that SCOP provides a better justified classification. Human expertise is needed to decide whether certain proteins are evolutionary related and therefore should be assigned to the same superfamily, or their similarity is a result of structural constraints and therefore they belong to the same fold. Another database, FSSP, is purely automatically generated (including regular automatic updates) but offers no classification, allowing the user to draw their own conclusion as to the significance of structural relationships based on the pairwise comparisons of individual protein structures.

Structural classes
SCOP includes the following structural classes:


 * 1) &alpha;-helical domains
 * 2) &beta;-sheet domains
 * 3) α/β domains which consist of from "beta-alpha-beta" structural units or "motifs" that form mainly parallel &beta;-sheets
 * 4) α+β domains formed by independent &alpha;-helices and mainly antiparallel &beta;-sheets
 * 5) multi-domain proteins
 * 6) membrane and cell surface proteins and peptides (not including those involved in the immune system)
 * 7) "small" proteins
 * 8) coiled-coil proteins
 * 9) low-resolution protein structures
 * 10) peptides and fragments
 * 11) designed proteins of non-natural sequence

Wikilinks to SCOP
To cite a particular SCOP page in Wikipedia, use the template of the form, where xxxxxx is a SCOP accession number, for instance.