Various biochemical reactions continue to occur in organisms, which require corresponding enzymes to catalyze, so there are many types of enzymes. According to statistics from the enzyme website BRENDA, the number of enzymes officially included has reached 7,984.

To study and apply these enzymes with different properties and functions, a systematic and effective classification method is necessary. Although the molecular composition and cellular location of enzymes can be used as the basis for classification, classification according to the type of reaction catalyzed is the most reasonable method for functional research.

In 1961, the International Union of Biochemistry and Molecular Biology (IUBMB) classified all enzymes into six categories according to the types of reactions they catalyzed. In August 2018, the classification of translocation enzymes was added, so there are now seven major enzymes, namely: oxidoreductase (EC 1), Transferases (EC 2), hydrolase (EC 3), lyase (EC 4), isomerase (EC 5), ligase (EC 6) and translocator (EC 7). Among them, EC stands for Enzyme Commission.

Among the seven enzyme classes, each class is divided into several subclasses. The sub-categories differ according to the characteristics of various reactions. For example, oxidoreductase is based on the type of electron donor, and transfer enzyme is based on the type of transferred group, and so on.

Oxidoreductase catalyzes oxidation-reduction reactions, that is, electron transfer between molecules. It is isomerase that catalyzes intramolecular redox reactions. According to the principle of systematic classification, oxidoreductases are divided into 24 subtypes according to the type of electron donor (substrate), and the sub-subtypes are divided according to the electron acceptor.

Traditionally, oxidoreductases are often divided into 4 subtypes: dehydrogenase (receptor is a reducing coenzyme), oxidase (receptor is molecular oxygen), peroxidase (receptor is hydrogen peroxide), and oxygenase (catalyze the incorporation of oxygen atoms into organic molecules). This is also the naming convention for common names of oxidoreductases.

Oxidoreductases is the most abundant enzyme, currently there are 2375 kinds. Its oxidation, productivity, detoxification and other functions are extremely important to organisms, and its application in production is second only to hydrolytic enzymes. Such enzymes usually require cofactors, which can be determined according to the photoelectric properties of the cofactors during the reaction.

Transferase catalyzes the transfer reaction of functional groups, such as various aminotransferases and kinases catalyze the transfer of amino and phosphate groups, respectively. Coenzymes are often needed for transfer enzymes, but the reaction is not easy to determine. According to the nature of the transferred group, it is divided into 10 subclasses, the important ones are:

One-carbon transferase (EC 2.1): transfer one carbon unit, such as methyltransferase related to nucleic acid and protein methylation. Carboxyltransferases belong to this subclass, such as methylmalonyl-CoA carboxytransferase (EC However, carboxylase that consumes ATP is a ligase, such as pyruvate carboxylase (EC

Glycosyltransferase (EC 2.4): closely related to carbohydrate metabolism, such as glycogen synthase ( and glycogen phosphorylase (

Phosphotransferase (EC 2.7): often called kinase, mostly ATP as the donor. For example, hexokinase, protein tyrosine kinase, etc. It should be noted that a few proteases are also called kinases (such as enterokinase), but they are hydrolases.

Hydrolase catalyzes the hydrolysis of substrates, such as proteases and lipases. Hydrolases generally play a degrading role and are mostly located outside the cell (such as in the digestive tract) or in the lysosome. Some proteases were also called kinases (such as enterokinase, EC, which is now called enteropeptidase).

Lyases catalyze the removal of a small molecule from the substrate, leaving a double bond (or ring) or its reverse reaction, such as aldolase, hydratase, decarboxylase, etc. IUBMB adjusted the classification and transferred part of the lyase to the transfer enzyme classification. For example, citrate synthase was once included in the lyase (EC and has now been transferred to the acetyltransferase (EC

Isomerases catalyze the mutual conversion between isomers, including racemase, epizyme, cis-trans isomerase, mutase (intramolecular group transfer) and intramolecular oxidation-reduction, Intramolecular elimination-addition reaction and other 6 subclasses.

Ligases catalyze the synthesis of one substance from two substances and must be coupled with ATP decomposition, such as DNA ligase, aminoacyl-tRNA synthetase, etc. Synthetase was used as a generic name before, because many people confused it with synthase, so IUBMB changed it to ligase in 1983.

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