Enzymes are mostly proteins that act as biological catalysts. A few are catalytic RNA molecules. Enzymes' specificity comes from their unique three-dimensional structures. As such, they will lose their activity if exposed to higher temperatures or chemical denaturants, as this disrupts their structure. They have optimum working temperatures and pH values.

The Enzyme Commission Number (EC Number) is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze. The chemical reaction catalyzed is the specific property that distinguishes one enzyme from another. EC numbers specify enzyme-catalysed reactions. The EC numbers are assigned by the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology.

Every enzyme consists of a code of the letters “EC” followed by four numbers separated by periods.
  • The first digit defines the general type of reaction catalysed by the enzyme and ranges from one to six.
  • The second figure indicates the subclass.
  • The third figure gives the sub-subclass.
  • The fourth figure is the serial number of the enzyme in its sub-subclass.

Group
Reaction Catalyzed Typical Reaction Enzyme example with trivial name

EC 1
Oxidoreductases

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
Transferases
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
Hydrolases
Formation of two products from a substrate by hydrolysis AB + H2O → AOH + BH Lipase, amylase, peptidase
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-C bonds with simultaneous breakdown of ATP. X + Y+ ATP → XY + ADP + Pi Synthetase

Class 1: Oxidoreductases

To this class belong all enzymes catalysing oxidoreduction reactions. The second figure in the code number of the oxidoreductases, unless it is 11, 13, 14 or 15, indicates the group in the hydrogen (or electron) donor that undergoes oxidation: 1 denotes a -CHOH- group, 2 a -CHO or -CO-COOH group or carbon monoxide.The third figure, except in subclasses EC 1.11, EC 1.13, EC 1.14 and EC 1.15, indicates the type of acceptor involved: 1 denotes NAD(P)+, 2 a cytochrome, 3 molecular oxygen, 4 a disulfide, 5 a quinone or similar compound, 6 a nitrogenous group, 7 an iron-sulfur protein and 8 a flavin. In subclasses EC 1.13 and EC 1.14 a different classification scheme is used and sub-subclasses are numbered from 11 onwards.

Class 2: Transferases

Transferases are enzymes transferring a group. A methyl group or a glycosyl group, from one compound (generally regarded as donor) to another compound (generally regarded as acceptor). The systematic names are formed according to the scheme donor: acceptor grouptransferase. The common names are normally formed according to acceptor grouptransferase or donor grouptransferase. The donor is a cofactor (coenzyme) charged with the group to be transferred. The second figure in the code number of transferases indicates the group transferred; a one-carbon group in EC 2.1, an aldehydic or ketonic group in EC 2.2, an acyl group in EC 2.3 and so on.The third figure gives further information on the group transferred. Subclass EC 2.1 is subdivided into methyltransferases (EC 2.1.1), hydroxymethyl- and formyltransferases (EC 2.1.2) and so on; only in subclass EC 2.7, does the third figure indicate the nature of the acceptor group.

Class 3: Hydrolases

These enzymes catalyse the hydrolytic cleavage of C-O, C-N, C-C and some other bonds, including phosphoric anhydride bonds. Although the systematic name always includes hydrolase, the common name is, in many cases, formed by the name of the substrate with the suffix -ase. The second figure in the code number of the hydrolases indicates the nature of the bond hydrolysed; EC 3.1 are the esterases; EC 3.2 the glycosylases, and so on.The third figure normally specifies the nature of the substrate. In the esterases the carboxylic ester hydrolases (EC 3.1.1), thiolester hydrolases (EC 3.1.2), phosphoric monoester hydrolases (EC 3.1.3); in the glycosylases the O-glycosidases (EC 3.2.1), N-glycosylases (EC 3.2.2). Exceptionally, in the case of the peptidyl-peptide hydrolases the third figure is based on the catalytic mechanism.

Class 4: Lyases

Lyases are enzymes cleaving C-C, C-O, C-N, and other bonds by elimination, leaving double bonds or rings, or conversely adding groups to double bonds. The systematic name is formed according to the pattern substrate group-lyase. The second figure in the code number indicates the bond broken: EC 4.1 are carbon-carbon lyases, EC 4.2 carbon-oxygen lyases and so on.The third figure gives further information on the group eliminated (e.g. CO2 in EC 4.1.1, H2O in EC 4.2.1).

Class 5: Isomerases

These enzymes catalyse geometric or structural changes within one molecule. According to the type of isomerism, they may be called racemases, epimerases, cis-trans-isomerases, isomerases, tautomerases, mutases or cycloisomerases.The subclasses are formed according to the type of isomerism, the sub-subclasses to the type of substrates.

Class 6: Ligases

Ligases are enzymes catalysing the joining together of two molecules coupled with the hydrolysis of a diphosphate bond in ATP or a similar triphosphate. The systematic names are formed on the system X:Y ligase (ADP-forming). In a few cases in Group 6, where the reaction is more complex or there is a common name for the product, a synthase name is used (e.g. EC 6.3.2.11 and EC 6.3.5.1).The second figure in the code number indicates the bond formed: EC 6.1 for C-O bonds (enzymes acylating tRNA), EC 6.2 for C-S bonds (acyl-CoA derivatives). Sub-subclasses are only in use in the C-N ligases.In a few cases it is necessary to use the word other in the description of subclasses and sub-subclasses.