EC numbers (Enzyme Commission numbers) are a numerical classification scheme for enzymes, based on the chemical reactions they catalize. 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