Enzymes are classified by the International Union of Biochemistry and Molecular Biology into six main classes based on the type of reaction they catalyze. Each enzyme receives a unique Enzyme Commission number consisting of four digits, providing a systematic way to identify and categorize enzymatic activities regardless of species or common name.
EC Numbering System
The EC number consists of four digits separated by periods. The first digit denotes one of six main classes based on the reaction type. The second and third digits specify subclasses and sub-subclasses that further define the reaction. The fourth digit is a serial number unique to each enzyme within that sub-subclass. For example, hexokinase is EC 2.7.1.1, indicating it is a transferase (class 2) that transfers a phosphate group (subclass 7), uses an alcohol as the acceptor (sub-subclass 1), and is the first enzyme described with this activity.
Class 1: Oxidoreductases
Oxidoreductases catalyze oxidation-reduction reactions involving the transfer of electrons or hydrogen atoms. They include dehydrogenases, oxidases, reductases, and peroxidases. Dehydrogenases typically transfer hydride ions from a substrate to a coenzyme such as NAD+ or FAD. Lactate dehydrogenase (EC 1.1.1.27) catalyzes the interconversion of lactate and pyruvate. Cytochrome c oxidase (EC 1.9.3.1) is the terminal enzyme of the electron transport chain.
Class 2: Transferases
Transferases catalyze the transfer of functional groups from a donor to an acceptor molecule. They include kinases, which transfer phosphate groups from ATP to substrates, transaminases, which transfer amino groups, and glycosyltransferases, which transfer sugar residues. Hexokinase (EC 2.7.1.1) phosphorylates glucose in the first step of glycolysis. Alanine transaminase (EC 2.6.1.2) catalyzes the transfer of an amino group between alanine and alpha-ketoglutarate.
Class 3: Hydrolases
Hydrolases catalyze hydrolytic cleavage of bonds using water. They include proteases, which hydrolyze peptide bonds, lipases, which hydrolyze ester bonds in lipids, and glycosidases, which hydrolyze glycosidic bonds. Trypsin (EC 3.4.21.4) is a serine protease that cleaves peptide bonds on the carboxyl side of basic amino acids. Acetylcholinesterase (EC 3.1.1.7) hydrolyzes the neurotransmitter acetylcholine.
Class 4: Lyases
Lyases catalyze the addition or removal of groups without hydrolysis or oxidation, often forming or breaking carbon-carbon, carbon-oxygen, or carbon-nitrogen bonds. They catalyze reactions in both directions in vivo but are named for the direction of bond cleavage leading to product formation. Pyruvate decarboxylase (EC 4.1.1.1) removes carbon dioxide from pyruvate. Fumarase (EC 4.2.1.2) catalyzes the reversible hydration of fumarate to malate in the citric acid cycle.
Class 5: Isomerases
Isomerases catalyze geometric or structural rearrangements within a molecule. They include racemases, epimerases, cis-trans isomerases, and mutases. Triose phosphate isomerase (EC 5.3.1.1) interconverts dihydroxyacetone phosphate and glyceraldehyde-3-phosphate in glycolysis. Phosphoglucomutase (EC 5.4.2.2) transfers a phosphate group within glucose, converting glucose-1-phosphate to glucose-6-phosphate.
Class 6: Ligases
Ligases catalyze the joining of two molecules coupled with the hydrolysis of ATP or another nucleoside triphosphate. They are also called synthetases. DNA ligase (EC 6.5.1.1) joins DNA fragments by forming phosphodiester bonds. Aminoacyl-tRNA synthetases (EC 6.1.1.x) attach amino acids to their corresponding tRNA molecules. Acetyl-CoA carboxylase (EC 6.4.1.2) carboxylates acetyl-CoA to form malonyl-CoA in fatty acid synthesis.
Common Names and Trivial Names
While the systematic EC classification is unambiguous, many enzymes are better known by their common names, which typically end in -ase and describe the substrate and reaction type. Common names often emphasize the reverse reaction in a pathway or the physiological direction. Isoenzymes are different enzyme forms that catalyze the same reaction but differ in amino acid sequence, kinetic properties, and tissue distribution, such as the five lactate dehydrogenase isoenzymes formed by different combinations of H and M subunits.
