EC numbers (Enzyme Commission numbers) are 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.

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 enzyme tripeptide aminopeptidase has the code EC 3.4.11.4 which is constructed as follows: 3 stands for hydrolases (enzymes that use water to break up some other molecule), 3.4 for hydrolases that act on peptide bonds, 3.4.11 for those that cleave off the amino-terminal amino acid from a polypeptide, and 3.4.11.4 for those that cleave off the amino-terminal end from a tripeptide.

The toplevel classification is

• EC 1 Oxidoreductases: catalyze oxidation/reduction reactions
• EC 2 Transferases: transfer a functional group (e.g. a methyl or phosphate group)
• EC 3 Hydrolases: catalyze the hydrolysis of various bonds
• EC 4 Lyases: cleave various bonds by means other than hydrolysis and oxidation
• EC 5 Isomerases: catalyze isomerization changes within a single molecule
• EC 6 Ligases: join two molecules with covalent bonds

The complete nomenclature can be browsed at http://www.chem.qmul.ac.uk/iubmb/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. To uniquely specify a protein by its amino acid sequence, Swiss-Prot identifiers can be used. An EC tree that is cross-linked with the corresponding Swiss-Prot entries, as well as with other databases and a Medline literature search, is available at http://www.expasy.org/enzyme/

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.

See also: TC number (classification of membrane transport proteins)