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
Proteins may also be classified based on their
celluar 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.