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.
This list aims to organize information on how proteins are most often classified: by structure, by function, or by location.
[6] Proteins frequently contain two or more domains, each have a different fold separated by intrinsically disordered regions.
Note that strictly speaking, an EC number corresponds to the reaction the enzyme catalyzes, not the protein per se.