Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Enzymes
Classification and Nomenclature of Enzymes

The modern Classification and Nomenclature of Enzymes were developed by the Enzyme Commission of the International Union of Biochemistry and approved at the 5th International Biochemical Congress in Moscow in 1961*.

* The Commission included leading enzymologists from around the world, representing our country via Academician A.E. Braunshtein.

The Need for a systematic nomenclature was driven primarily by the rapid growth in the number of newly discovered enzymes, which various researchers named at their own discretion. Moreover, the same enzyme was frequently given two or more names, introducing confusion into the nomenclature. Some enzyme names did not reflect the type of catalyzed reaction at all; instead, enzymes were named after the substrate they acted upon, with the suffix -ase added. Examples include amylases (enzymes that hydrolyze CARBOHYDRATES), lipases (acting on Lipids), proteinases (hydrolyzing Proteins), and so forth.

Until 1961, there was also no unified Classification of Enzymes. The difficulties arose from the fact that different researchers based their enzyme classifications on various principles. The Commission reviewed three principles that could serve as the basis for classifying enzymes and designating their names. The first principle was the Chemical Nature of the enzyme—that is, its classification as a flavoprotein, pyridoxal-phosphate protein, hemoprotein, metalloprotein, and so on. However, this principle could not serve as a general basis for classification, as prosthetic groups are known, identifiable, and directly determinable for only a small number of enzymes. The second principle was the chemical Nature of the substrate acted upon by the enzyme. It is difficult to classify an enzyme using this principle because A wide variety of compounds within a specific Class of substances (proteins, carbohydrates, lipids, Nucleic Acids) and countless metabolic intermediates can serve as substrates. The adopted classification is based on the third principle: the type of catalyzed reaction, which is specific to the action of any enzyme. It is logical to use this principle as the foundation for the classification and nomenclature of enzymes.

Thus, the type of catalyzed chemical reaction combined with the name of the substrate(s) serves as the basis for the systematic naming of enzymes. According to the International Classification, enzymes are divided into six main classes, each containing several subclasses: 1) oxidoreductases; 2) transferases; 3) Hydrolases; 4) lyases; 5) isomerases; 6) ligases (synthetases) (Table 4.5).

Oxidoreductases. The class of oxidoreductases comprises enzymes that catalyze oxidation-reduction Reactions Involving Two substrates, which form The basis of Biological Oxidation. Their systematic names are constructed in the format "donor: acceptor oxidoreductase". For example, lactate: NAD⁺ oxidoreductase for Lactate dehydrogenase (LDH).

The MAIN TYPES OF oxidoreductases include: aerobic dehydrogenases, or oxidases, which catalyze the direct transfer of protons (electrons) to oxygen; anaerobic dehydrogenases, which accelerate The transfer of protons (electrons) to an intermediate substrate rather than oxygen; and Cytochromes, which catalyze the transfer of electrons only. This class also includes the heme-containing enzymes catalase and peroxidase, which catalyze reactions involving hydrogen peroxide.

Transferases. The class of transferases comprises enzymes that catalyze the intermolecular transfer of various atoms, groups of atoms, and radicals. Their names are constructed in the format "donor: transferred group transferase".

Transferases are categorized based on the groups they transfer, such as single-carbon residues, acyl, glycosyl, aldehyde or ketone, and nucleotide residues, nitrogenous groups, phosphoric and sulfuric acid residues, and others. Examples include methyl- and formyltransferases, acetyltransferases, aminotransferases, phosphotransferases, and others.

Table 4.5. International Classification of Enzymes

Hydrolases. The class of hydrolases includes a large group of enzymes that catalyze the Cleavage of intramolecular bonds in Organic compounds with the participation of a water molecule. Their names are constructed in the format "substrate-hydrolase". These include: esterases—enzymes catalyzing the hydrolysis and synthesis of esters; glycosidases, which accelerate the cleavage of glycosidic bonds; Phosphatases and peptidases (peptide hydrolases), catalyzing the hydrolysis of phosphoanhydride and peptide bonds; amidases, which accelerate the cleavage of non-peptide amide bonds, and others.

Lyases. The class of lyases comprises enzymes that catalyze the cleavage of C—O, C—C, C—N, and other bonds, as well as the reversible non-hydrolytic removal of various groups from substrates. These reactions are accompanied by The formation of a double bond or The addition of groups to the site of a double bond cleavage. These enzymes are designated by the term "substrate-lyase". For example, fumarate hydratase (systematic name "L-malate hydrolyase") catalyzes the reversible removal of a water molecule from malic acid to form fumaric acid. This group also includes Decarboxylases (carboxy-lyases), amidine-lyases, and others.

Isomerases. The class of isomerases comprises enzymes that catalyze the interconversion of optical and geometric isomers. Their systematic names reflect the type of reaction: "substrate cis-trans-isomerase". If the isomerization involves the intramolecular transfer of a group, the enzyme is termed a "mutase".

This class also includes racemases and epimerases acting on amino and hydroxy acids, carbohydrates and their derivatives; intramolecular oxidoreductases catalyzing the interconversion of aldoses and ketoses; and intramolecular transferases transferring acyl, phosphoryl, and other groups, etc.

Ligases (Synthetases). The class of ligases comprises enzymes that catalyze the synthesis of organic substances from two starting molecules utilizing The energy released by the cleavage of ATP (or another nucleoside triphosphate). Their systematic names are constructed in the format "X : Y ligase", where X and Y denote the starting substances. An example is L-glutamate:ammonia ligase (recommended common name "Glutamine Synthetase"), which catalyzes the Synthesis of Glutamine from glutamic acid and ammonia in the presence of ATP.