Schematic representation of structural classes of protein according to theCATH classification scheme.[1]
Proteins are a class ofmacromolecular organic compounds that are essential to life. They consist of a longpolypeptide chain that usually adopts a single stablethree-dimensional structure. They fulfill a wide variety offunctions including providingstructural stability to cells, catalyze chemical reactions that produce or store energy or synthesize otherbiomolecules includingnucleic acids and proteins, transport essential nutrients, or serve other roles such assignal transduction. They are selectivelytransported 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.
Both classification schemes are based on a hierarchy of fold types. At the top level are all alpha proteins (domains consisting ofalpha helices), all beta proteins (domains consisting ofbeta 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 asmetamorphic proteins.[5] Finally other proteins appear not to adopt any stable conformation and are referred to asintrinsically disordered.[6]
Proteins frequently contain two or moredomains, each have a different fold separated by intrinsically disordered regions. These are referred to asmulti-domain proteins.
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 theircellular function. A widely used classification isPANTHER (protein analysis through evolutionary relationships) classification system.[7]
Enzymes classified according to theirEnzyme Commission number (EC). Note that strictly speaking, anEC 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.
