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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


The Enzyme Commission number (EC number) is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze. As a system of enzyme nomenclature, every EC number is associated with a recommended name for the respective enzyme.

Strictly speaking, 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. By contrast, UniProt identifiers uniquely specify a protein by its amino acid sequence.[1]

Format of number

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.

For example, the tripeptide aminopeptidases have the code "EC", whose components indicate the following groups of enzymes:

  • EC 3 enzymes are hydrolases (enzymes that use water to break up some other molecule)
  • EC 3.4 are hydrolases that act on peptide bonds
  • EC 3.4.11 are those hydrolases that cleave off the amino-terminal amino acid from a polypeptide
  • EC are those that cleave off the amino-terminal end from a tripeptide

Top level codes

Top-level EC numbers[2]
Group Reaction catalyzed Typical reaction Enzyme example(s) with trivial name
EC 1
To catalyze oxidation/reduction reactions; transfer of H and O atoms or electrons from one substance to another AH + B → A + BH (reduced)
A + O → AO (oxidized)
Dehydrogenase, oxidase
EC 2
Transfer of a functional group from one substance to another. The group may be methyl-, acyl-, amino- or phosphate group AB + C → A + BC Transaminase, kinase
EC 3
Formation of two products from a substrate by hydrolysis AB + H2O → AOH + BH Lipase, amylase, peptidase
EC 4
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
EC 5
Intramolecule rearrangement, i.e. isomerization changes within a single molecule AB → BA Isomerase, mutase
EC 6
Join together two molecules by synthesis of new C-O, C-S, C-N or C-C bonds with simultaneous breakdown of ATP X + Y+ ATP → XY + ADP + Pi Synthetase


The enzyme nomenclature scheme was developed starting in 1955, when the International Congress of Biochemistry in Brussels set up an Enzyme Commission.

The first version was published in 1961.

The current sixth edition, published by the International Union of Biochemistry and Molecular Biology in 1992, contains 3196 different enzymes.


  1. "ENZYME (Enzyme nomenclature database)". ExPASy. Retrieved 2006-03-14.
  2. Moss, G.P. "Recommendations of the Nomenclature Committee". International Union of Biochemistry and Molecular Biology on the Nomenclature and Classification of Enzymes by the Reactions they Catalyse. Retrieved 2006-03-14.

See also

de:EC-Nummer id:Nomor EC it:Classificazione EC


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