Schematic representation of structural classes of protein according to the CATH classification scheme.[1]
Proteins are a class of macromolecular organic compounds that are essential to life. They consist of a long polypeptide chain that usually adopts a single stable three-dimensional structure. They fulfill a wide variety of functions including providing structural stability to cells, catalyze chemical reactions that produce or store energy or synthesize other biomolecules including nucleic acids and proteins, transport essential nutrients, or serve other roles such as signal transduction. They are selectively transported to various compartments of the cell or in some cases, secreted from the cell.
This list aims to organize information on how proteins are most often classified: by structure, by function, or by location.
Structure
Proteins may be classified as to their three-dimensional structure (also known a protein fold). The two most widely used classification schemes are:[2]
Both classification schemes are based on a hierarchy of fold types. At the top level are all alpha proteins (domains consisting of alpha helices), all beta proteins (domains consisting of beta sheets), and mixed alpha helix/beta sheet proteins.
While most proteins adopt a single stable fold, a few proteins can rapidly interconvert between one or more folds. These are referred to as metamorphic proteins.[5] Finally other proteins appear not to adopt any stable conformation and are referred to as intrinsically disordered.[6]
Proteins frequently contain two or more domains, each have a different fold separated by intrinsically disordered regions. These are referred to as multi-domain proteins.
Function
The human genome, categorized by function of each gene product, given both as number of genes and as percentage of all genes.[7]
Proteins may also be classified based on their cellular function. A widely used classification is PANTHER (protein analysis through evolutionary relationships) classification system.[7]
Enzymes classified according to their Enzyme Commission number (EC). Note that strictly speaking, an EC number corresponds to the reaction the enzyme catalyzes, not the protein per se. However each EC number has been mapped to one or more specific proteins.
^Thul PJ, Åkesson L, Wiking M, Mahdessian D, Geladaki A, Ait Blal H, et al. (May 2017). "A subcellular map of the human proteome". Science. 356 (6340). doi:10.1126/science.aal3321. PMID28495876. S2CID10744558.
