Editing Protein (section)

== Classification ==

{{Main|Protein family|Gene Ontology|Enzyme Commission number}}

Proteins are primarily classified by sequence and structure, although other classifications are commonly used. Especially for enzymes the EC number system provides a functional classification scheme.<ref name="McDonald Tipton 2023">{{cite journal |last1=McDonald |first1=Andrew G. |last2=Tipton |first2=Keith F. |title=Enzyme nomenclature and classification: the state of the art |journal=The FEBS Journal |volume=290 |issue=9 |date=2023 |issn=1742-464X |doi=10.1111/febs.16274 |doi-access=free |pages=2214–2231 |pmid=34773359 }}</ref> Similarly, [[Gene Ontology|gene ontology]] classifies both genes and proteins by their biological and biochemical function, and by their intracellular location.<ref name="pmid17984083">{{cite journal | vauthors = ((The Gene Ontology Consortium)) | title = The Gene Ontology project in 2008 | journal = Nucleic Acids Research | volume = 36 | issue = Database issue | pages = D440–4 | date = January 2008 | pmid = 17984083 | pmc = 2238979 | doi = 10.1093/nar/gkm883 }}</ref>

Sequence similarity is used to classify proteins both in terms of evolutionary and functional similarity. This may use either whole proteins or [[protein domain]]s, especially in [[Protein domain#Multidomain proteins|multi-domain proteins]]. Protein domains allow protein classification by a combination of sequence, structure and function, and they can be combined in many ways. In an early study of 170,000 proteins, about two-thirds were assigned at least one domain, with larger proteins containing more domains (e.g. proteins larger than 600 [[amino acid]]s having an average of more than 5 domains).<ref>{{cite journal | vauthors = Ekman D, Björklund AK, Frey-Skött J, Elofsson A | title = Multi-domain proteins in the three kingdoms of life: orphan domains and other unassigned regions | journal = Journal of Molecular Biology | volume = 348 | issue = 1 | pages = 231–243 | date = April 2005 | pmid = 15808866 | doi = 10.1016/j.jmb.2005.02.007 }}</ref>