Editing Enzyme (section)

== Classification and nomenclature ==
Enzymes can be classified by two main criteria: either [[Protein primary structure|amino acid sequence]] similarity (and thus evolutionary relationship) or enzymatic activity.

'''Enzyme activity'''. An enzyme's name is often derived from its substrate or the chemical reaction it catalyzes, with the word ending in ''-ase''.<ref name="Stryer_2002" />{{rp|8.1.3}} Examples are [[lactase]], [[alcohol dehydrogenase]] and [[DNA polymerase]]. Different enzymes that catalyze the same chemical reaction are called [[isozyme]]s.<ref name="Stryer_2002" />{{rp|10.3}}

The [[International Union of Biochemistry and Molecular Biology]] have developed a [[nomenclature]] for enzymes, the [[Enzyme Commission number|EC numbers (for "Enzyme Commission")]]. Each enzyme is described by "EC" followed by a sequence of four numbers which represent the hierarchy of enzymatic activity (from very general to very specific). That is, the first number broadly classifies the enzyme based on its mechanism while the other digits add more and more specificity.<ref name="moss">{{cite web | vauthors = Moss GP |title=Recommendations of the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology on the Nomenclature and Classification of Enzymes by the Reactions they Catalyse |url=https://www.qmul.ac.uk/sbcs/iubmb/enzyme/ |website=International Union of Biochemistry and Molecular Biology |access-date=28 August 2021}}</ref>

The top-level classification is:
*EC 1, [[Oxidoreductase]]s: catalyze [[oxidation]]/reduction reactions
*EC 2, [[Transferase]]s: transfer a [[functional group]] (''e.g.'' a methyl or phosphate group)
*EC 3, [[Hydrolase]]s: catalyze the [[hydrolysis]] of various bonds
*EC 4, [[Lyase]]s: cleave various bonds by means other than hydrolysis and oxidation
*EC 5, [[Isomerase]]s: catalyze [[isomer]]ization changes within a single molecule
*EC 6, [[Ligase]]s: join two molecules with [[covalent bond]]s.
*EC 7, [[Translocase]]s: catalyze the movement of ions or molecules across membranes, or their separation within membranes.

These sections are subdivided by other features such as the substrate, products, and [[chemical mechanism]]. An enzyme is fully specified by four numerical designations. For example, [[hexokinase]] (EC 2.7.1.1) is a transferase (EC 2) that adds a phosphate group (EC 2.7) to a hexose sugar, a molecule containing an alcohol group (EC 2.7.1).<ref>{{cite web | title = EC 2.7.1.1 | url = http://www.chem.qmul.ac.uk/iubmb/enzyme/EC2/7/1/1.html | author = Nomenclature Committee | work = International Union of Biochemistry and Molecular Biology (NC-IUBMB) | publisher = School of Biological and Chemical Sciences, Queen Mary, University of London | access-date = 6 March 2015 | archive-url = https://web.archive.org/web/20141201224835/http://www.chem.qmul.ac.uk/iubmb/enzyme/EC2/7/1/1.html | archive-date = 1 December 2014 | url-status = dead}}</ref>

'''Sequence similarity'''. EC categories do '''not''' reflect sequence similarity. For instance, two ligases of the same EC number that catalyze exactly the same reaction can have completely different sequences. Independent of their function, enzymes, like any other proteins, have been classified by their sequence similarity into numerous families. These families have been documented in dozens of different protein and protein family databases such as [[Pfam]].<ref>{{cite book | vauthors = Mulder NJ | chapter = Protein Family Databases|date=2007-09-28 |title =eLS|pages=a0003058.pub2|place=Chichester, UK|publisher=John Wiley & Sons, Ltd|language=en|doi=10.1002/9780470015902.a0003058.pub2|isbn=978-0-470-01617-6 }}</ref>

'''Non-homologous isofunctional enzymes'''. Unrelated enzymes that have the same enzymatic activity have been called ''non-homologous isofunctional enzymes''.<ref>{{cite journal | vauthors = Omelchenko MV, Galperin MY, Wolf YI, Koonin EV | title = Non-homologous isofunctional enzymes: a systematic analysis of alternative solutions in enzyme evolution | journal = Biology Direct | volume = 5 | issue = 1 | pages = 31 | date = April 2010 | pmid = 20433725 | pmc = 2876114 | doi = 10.1186/1745-6150-5-31 | doi-access = free }}</ref> [[Horizontal gene transfer]] may spread these genes to unrelated species, especially bacteria where they can replace endogenous genes of the same function, leading to hon-homologous gene displacement.