TheEnzyme Commission number (EC number) is anumerical classification scheme forenzymes, based on thechemical reactions theycatalyze.[1] As a system ofenzyme nomenclature, every EC number is associated with a recommended name for the corresponding enzyme-catalyzed reaction.
EC numbers do not specify enzymes but enzyme-catalyzed reactions. If different enzymes (for instance from different organisms) catalyze the same reaction, then they receive the same EC number.[2] Furthermore, throughconvergent evolution, completely differentprotein folds can catalyze an identical reaction (these are sometimes callednon-homologous isofunctional enzymes)[3] and therefore would be assigned the same EC number. By contrast,UniProt identifiers uniquely specify a protein by its amino acid sequence.[4]
Every enzyme code consists of the letters "EC" followed by four numbers separated by periods. Those numbers represent a progressively finer classification of the enzyme. Preliminary EC numbers exist and have an 'n' as part of the fourth (serial) digit (e.g. EC 3.5.1.n3).[2]
For example, thetripeptide aminopeptidases have the code "EC 3.4.11.4", whose components indicate the following groups of enzymes:
NB:The enzyme classification number is different from the 'FORMAT NUMBER'
| Class | Reaction catalyzed | Typical reaction | Enzyme example(s) with trivial name |
|---|---|---|---|
| EC 1 Oxidoreductases | Oxidation/reduction reactions; transfer of H and O atoms orelectrons from one substance to another | AH + B → A + BH (reduced) A + O → AO (oxidized) | Dehydrogenase,oxidase |
| EC 2 Transferases | Transfer of afunctional group from one substance to another. The group may be methyl-, acyl-, amino- or phosphate group | AB + C → A + BC | Transaminase,kinase |
| EC 3 Hydrolases | Formation of two products from a substrate byhydrolysis | AB + H2O → AOH + BH | Lipase,amylase,peptidase,phosphatase |
| EC 4 Lyases | Non-hydrolytic addition or removal of groups from substrates. C-C, C-N, C-O or C-S bonds may be cleaved | RCOCOOH → RCOH + CO2 or [X-A+B-Y] → [A=B + X-Y] | Decarboxylase |
| EC 5 Isomerases | Intramolecule rearrangement, i.e.isomerization changes within a single molecule | ABC → BCA | Isomerase,mutase |
| EC 6 Ligases | Join together two molecules by synthesis of new C-O, C-S, C-N or C-Cbonds with simultaneous breakdown ofATP | X + Y + ATP → XY + ADP + Pi | Synthetase |
| EC 7 Translocases | Catalyse the movement of ions or molecules across membranes or their separation within membranes | Transporter |
Similarity between enzymatic reactions can be calculated by using bond changes, reaction centres or substructure metrics (formerly EC-BLAST], now the EMBL-EBI Enzyme Portal).[6]
Before the development of the EC number system, enzymes were named in an arbitrary fashion, and names likeold yellow enzyme andmalic enzyme that give little or no clue as to what reaction was catalyzed were in common use. Most of these names have fallen into disuse, though a few, especially proteolyic enzymes with very low specificity, such aspepsin andpapain, are still used, as rational classification on the basis of specificity has been very difficult.
By the 1950s the chaos was becoming intolerable, and after Hoffman-Ostenhof[7] and Dixon and Webb[8] had proposed somewhat similar schemes for classifying enzyme-catalyzed reactions, the International Congress of Biochemistry inBrussels set up the Commission on Enzymes under the chairmanship ofMalcolm Dixon in 1955. The first version was published in 1961, and the Enzyme Commission was dissolved at that time, though its name lives on in the termEC Number. The current sixth edition, published by theInternational Union of Biochemistry and Molecular Biology in 1992 as the last version published as a printed book, contains 3196 different enzymes. Supplements 1-4 were published 1993–1999. Subsequent supplements have been published electronically, at the website of the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology.[5] In August 2018, theIUBMB modified the system by adding the top-level EC 7 category containing translocases.[9]
