BIOCHEMISTRY - Textbook - L. I. Ostapchenko - 2012

Chapter 7. ENZYMOLOGY

7.2. Classification and Nomenclature of Enzymes

Each enzyme has two names. The first is a short, working name, which is convenient for everyday use. The second (more complete) name is systematic, used for the unambiguous identification of the enzyme. The names of most Enzymes contain the suffix "ase" added to the name of the reaction substrate (urease, sucrase, lipase, nuclease) or to the type of chemical Transformation of a particular substrate (Lactate dehydrogenase, adenylate cyclase, phosphoglucomutase, Pyruvate carboxylase). According to the Ukrainian Classification of Enzymes (EC), enzyme names are written as a single word. However, usage has preserved A number of trivial, historically established enzyme names that give no indication of either the substrate or the type of chemical transformation—these include Trypsin, Pepsin, renin, and Thrombin.

In 1961, the International Union of Biochemistry and Molecular Biology developed a systematic nomenclature, according to which all enzymes are divided into six main classes depending on the type of chemical reaction they catalyze. Each Class consists of numerous subclasses and sub-subclasses, taking into account the chemically transformed substrate group, the donor and acceptor of the transferred groups, the presence of additional molecules, and so forth. Each of the six classes has a specific, fixed serial number.

The first class of enzymes, "Oxidoreductases," catalyzes various oxidation-reduction Reactions Involving Two substrates (The transfer of electrons or hydrogen atoms from one substrate to another). The systematic name of these enzymes is formed according to the formula "donor: acceptor oxidoreductase," while the working name follows the format "substrate - oxidoreductase subclass."

The subclass of dehydrogenases includes enzymes that catalyze dehydrogenation reactions (the removal of hydrogen). The Coenzymes NAD⁺, NADP⁺, FAD, and FMN are used as hydrogen acceptors. All enzymes in this group exhibit high substrate Specificity. An example of a reaction:

Enzymes belonging to the oxidase subclass catalyze reactions in which molecular oxygen serves as the electron acceptor. An example of a reaction catalyzed by cytochrome c oxidase:

Oxygenases (hydroxylases) catalyze Reactions Involving the transfer of an oxygen atom from an oxygen molecule to a substrate. An example of a reaction:

The second class of enzymes, "Transferases," catalyzes the transfer of functional groups from one compound to another. They are classified according to the group being transferred. The names of these enzymes are constructed using the formula "donor: acceptor - transferred group - transferase." The class of transferases includes aminotransferases, Acyltransferases, methyltransferases, Glycosyltransferases, and Kinases (phosphotransferases). Examples of reactions:

The third class of enzymes, "Hydrolases," catalyzes Hydrolysis reactions (the Cleavage of a covalent bond with The addition of a Water molecule at the site of the break). They are divided into subclasses depending on the bond being cleaved. Enzyme names are formed according to the formula "substrate - hydrolase" or by directly adding the suffix "ase" to the substrate name: protease, lipase, phospholipase, Ribonuclease. An example of a reaction:

For certain classes of hydrolases, special terms are used that characterize the hydrolysis of a specific chemical bond: esterases, Phosphatases, etc.

The fourth class of enzymes, "Lyases," removes a specific group from substrates via a non-hydrolytic pathway (releasing groups such as CO2, Н2О, NH2, H2S, etc.) or adds a water molecule (most commonly) across a double bond. Enzyme names are formed according to the formula "substrate - group added or removed." Examples of reactions:

Isomerases can catalyze intramolecular oxidation-reduction reactions, bringing about the interconversion of aldoses and ketoses, ketone and enol groups, or the shifting of double bonds within a molecule:

The fifth class of enzymes, "Isomerases," catalyzes various intramolecular transformations. They are classified according to the type of isomerization reaction. The general term "isomerase" is used as the general designation for enzymes of this class, for example:

When isomerization consists of the intramolecular transfer of a group, the enzyme is called a mutase:

The sixth class of enzymes, "Ligases" (synthetases), catalyze the joining of two molecules together with The formation of a covalent bond. This process is coupled with the cleavage of a phosphoester bond in an ATP molecule (or other nucleoside triphosphates) or with the cleavage of high-energy bonds in Other Compounds. In the first case (utilizing the energy of ATP hydrolysis), such enzymes are referred to as ligases or synthetases:

If the energy source is any high-energy compound other than ATP, the enzymes are called synthases:

According to the classification, each enzyme has a systematic name that uniquely defines the chemical reaction it catalyzes. For example, the substrate D-glyceraldehyde-3-phosphate: enzyme NAD-oxidoreductase (common name: glyceraldehyde phosphate dehydrogenase). The enzyme's name indicates that its substrate is D-glyceraldehyde-3-phosphate, and the type of catalyzed reaction is a redox reaction in the presence of the coenzyme NAD⁺.

In 1972, the Commission on Biochemical Nomenclature of the International Union of Pure and Applied Chemistry proposed the "Rules of Enzyme Nomenclature," which assign a four-digit code number where the first digit denotes the enzyme class, the second (subclass) specifies the group undergoing transformation, the third (sub-subclass) specifies additional reaction participants (such as the donor and acceptor), and the fourth digit is the serial number of the enzyme in that subgroup. For instance, the enzyme malate dehydrogenase has the systematic name L-malate:NAD-oxidoreductase and the code number 1.1.1.38. This code indicates that the enzyme belongs to the first class of enzymes—oxidoreductases, the oxidized group is a hydroxyl group (1) in the presence of the coenzyme NAD⁺ (1), and its serial number within this subgroup is 38. This numerical enzyme nomenclature is primarily used in scientific literature.