Classification of Enzymes with Examples : The 7 types of enzymes

Classification of Enzymes in Details

  • To study enzymes with different properties and functions, a systematic and effective classification method is necessary.
  • Although the molecular composition and cellular localization of enzymes can be used as a basis for classification.
  • For functional research, it is more important to classify enzymes according to the type of reaction they catalyze.
  • In 1961, the International Union of Biochemistry and Molecular Biology (IUBMB) grouped all enzymes into six categories according to the type of reaction they catalyze. 
  • In August 2018, the classification of translocases was added, so there are now seven major enzymes, namely: oxidase-reductase (EC 1), transferase (EC 2), hydrolase (EC 3), lyase (EC 4), isomerase (EC 5), ligase (EC 6) and translocase (EC 7). Wherein EC stands for Enzyme Commission.
  • In these seven major enzyme classes (class), each is divided into several subclasses (subclass), and subclasses are divided into sub-subclasses (sub-subclass). 
  • The classification criteria of subclasses are different according to the characteristics of each type of reaction.

1. Oxidoreductases (EC 1)

  • These enzymes catalyze oxidation-reduction reactions.
  • According to the principle of systematic classification enzymes oxidoreductases are divided into 24 subclasses according to the type of electron donor (substrate) and the sub-subclasses are divided according to the electron acceptor.
  • The coenzymes of this class are NAD, NADP, FAD, FMN, ubiquinone, glutathione, and lipoic acid.
  • Oxidoreductase enzymes help in energy production, detoxification, and other functions that are extremely important to living organisms.

Subclasses of Oxido-reductases Enzymes:

If we consider all subclasses, then groups of enzymes acting on are :

  • 1.1. CH-OH donor group;
  • 1.2. aldehyde or ketone group of donors;
  • 1.3. CH-CH group of donors;
  • 1.4. CH-NH 2 donor group;
  • 1.5. CH-NH donor group;
  • 1.6. NADH or NADPH as donors;
  • 1.8. containing sulfur groups of donors;
  • 1.9. heme-containing donors;
  • 1.10. diphenols as donors;
  • 1.11. hydrogen peroxide as an acceptor;
  • 1.11. hydrogen as a donor;
  • 1.13. one donor with the inclusion of molecular oxygen;
  • 1.14. two donors with the inclusion of molecular oxygen;
  • 1.15. superoxide radicals as an acceptor;
  • 1.17. CH 2 group of donors;
  • 1.18. ferredoxin as a donor;
  • 1.19. flavodoxin as a donor;
  • 1.20. phosphorus or arsenic as a donor;
  • 1.21. on substances X-H and Y-H with the formation of an XY-bond;
  • 1.22. halogen as a donor;
  • 1.97. other oxidoreductases

Examples of Oxido-reductases Enzymes

The most common names for oxidoreductase enzymes are:

  • Oxidases: are oxidoreductases that catalyze the oxidation of substrates with molecular oxygen as an electron acceptor without incorporating oxygen into the substrate molecule.

  • Dehydrogenases: are oxidoreductases that catalyze the dehydrogenation of a substrate using any molecules other than oxygen as a hydrogen acceptor. For example, alcohol dehydrogenase has an EC number of – it is an oxidoreductase that acts on the OH group of the donor with NAD as an acceptor with the first serial number in its subclass; lactate dehydrogenase – EC, acts on the OH group of the donor with NAD as an acceptor with serial number 27 in its subclass.

  • Reductases: If the transfer of hydrogen from a donor molecule is difficult to prove, then such oxidoreductases are called reductases.

  • Peroxidases are oxidoreductases that catalyze reactions with hydrogen peroxide as an electron acceptor.

  • Monooxygenases: are oxidoreductases that catalyze the introduction of one oxygen atom into a substrate molecule with molecular oxygen as an oxygen donor.

  • Dioxygenases: are oxidoreductases that catalyze the introduction of 2 oxygen atoms into a substrate molecule with molecular oxygen as an oxygen donor.

2. Transferase (EC 2)

  • Transferases catalyze the reactions of the transfer of various functional groups from one substrate (donor) to another (acceptor).
  • These enzymes take place in the interconversion of various substances, neutralizing natural and foreign compounds. 
  • The coenzymes of transferases are pyridoxal phosphate, coenzyme A, tetrahydrofolic acid, and methylcobalamin. 
  • The class is subdivided into 9 subclasses depending on the structure of the transferred groups.

Subclasses of Transferases Enzymes:

If we consider all  subclasses, then groups of enzymes are distinguished in them depending on the composition of the transferred group

  • 2.1. transferring one-carbon fragments;
  • 2.2. carrying aldehyde and keto groups;
  • 2.3. carrying acyl groups;
  • 2.4. carrying glycosyl groups;
  • 2.5. carrying non-methyl alkyl and aryl groups;
  • 2.6. transferring nitrogen-containing groups;
  • 2.7. carrying phosphorus-containing groups.
  • 2.8. transferring sulfonic groups;
  • 2.9. carrying selenium-containing groups.

Examples of Transferases Enzymes

  • Carbonyltransferases (EC 2.1): Transfer of one-carbon units, such as methyltransferases involved in the methylation of nucleic acids and proteins. Carboxyltransferases belong to this subclass, such as methylmalonyl-CoA carboxyltransferases (EC

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

  • Phosphotransferases (EC 2.7): Often called kinases, mostly use ATP as a donor. For example, hexokinase, protein tyrosine kinase, etc. It is important to note that a few proteases are also called kinases (eg enterokinase), but they are hydrolases.

3. Hydrolases (EC 3)
  • Hydrolases are the class of enzymes that break intramolecular bonds in the substrate by adding H2O elements.
  • Hydrolases are divided into 13 subclasses. 
  • Due to the complexity of many substrates, several enzymes have retained trivial names for example, pepsin, and trypsin. 
  • There are no coenzymes for these types of enzymes.
  • Hydrolases are widely present in the gastrointestinal tract (pepsin, trypsin, lipase, amylase, and others) and lysosome. 
  • These enzymes carry out various functions such as the decomposition of macromolecules to their monomers.
Subclasses of Hydrolases Enzymes
If we consider all subclasses, then they distinguish groups of enzymes that catalyze hydrolysis:
  • 3.1. esters;
  • 3.2. O-glycosides;
  • 3.3. ethers;
  • 3.4. peptides;
  • 3.5. non-peptide nitrogen-carbon bonds;
  • 3.6. acid anhydrides;
  • 3.7. carbon-carbon bonds;
  • 3.8. bonds involving halogen;
  • 3.9. phosphorus-nitrogen bonds;
  • 3.10. sulfur-nitrogen bonds;
  • 3.11. carbon-phosphorus bonds;
  • 3.12. sulfur-sulfur bonds;
  • 3.13. carbon-sulfur bonds.
Examples of Hydrolases Enzymes
Historically, the names of hydrolases were formed from the name of the substrate with the ending “-ase” – collagenase, amylase, lipase, DNA-ase. The most common working names for hydrolases are:
  • Proteases and peptidases – hydrolysis of proteins and peptides.
  • Nucleases – hydrolysis of nucleic acids.
  • Esterases – hydrolysis of ester bonds.
  • Lipases – hydrolysis of neutral fats (triacylglycerols).
  • Phosphatases – cleavage of phosphoric acid from substances.
  • Glycosidases – hydrolyze O- and S-glycosidic bonds.
4. Lyases (EC 4)
  • Lyases are enzymes that catalyze the breaking of C-O, C-C, CN, and other bonds, as well as reversible reactions of cleavage of various groups in a non-hydrolytic way. 
  • There are 7 subclasses. 
  • These reactions are accompanied by the formation of a double bond or the addition of groups to the site of the double bond. 
  • Lyases are complex enzymes. 
  • Coenzymes are pyridoxal phosphate, thiamine diphosphate, magnesium, and cobalt are involved.
  • Enzymes are divided into subclasses depending on the nature of the bond being attacked. 
  • Examples are enzymes that act on carbon-carbon bonds, carbon-oxygen bonds, and carbon-nitrogen bonds.
Subclasses of Lyases Enzymes
If we consider all subclasses, then they stand out:
  • 4.1. carbon-carbon lyases;
  • 4.2. carbon-oxygen lyases;
  • 4.3. carbon-nitrogen lyases;
  • 4.4. carbon-sulfur lyases;
  • 4.5. carbon-halogen lyase;
  • 4.6. phosphorus-oxygen lyase;
  • 4.7. carbon-phosphorus lyase.
Example of Lyases Enzymes
  • Carbon-Carbon Lyases: Pyruvate decarboxylase, Phosphoenolpyruvate carboxylase, Oxaloacetate decarboxylase, Glutamate decarboxylase, Lysine decarboxylase, Histidine decarboxylase, RUBISCO, etc.
  • Carbon-Oxygen Lyases: Fumarase, Enolase (Alpha), aconitase, Tryptophan synthase, nitrile hydratase, etc.
  • Carbon-Nitrogen Lyases: Ammonia-Lyases, Serine dehydratase, Amidine-Lyases, Adenylosuccinate lyase, etc.
  • Carbon-Sulphur Lyases: Cystathionine gamma-lyase, Cystathionine-beta-lyase, Leukotriene C4 synthase, etc.
  • Carbon-Halide Lyases: Dichloromethane dehalogenase, Halohydrin dehalogenase etc.
  • Phosphorus-Oxygen Lyases: guanylate cyclase, adenylate cyclase, etc.
5. Isomerases (EC 5):
  • Isomerases are enzymes that catalyze isomeric transformations within a single molecule. 
  • Isomerases are complex enzymes. 
  • Their coenzymes include pyridoxal phosphate, deoxyadenosylcobalamin, glutathione, monosaccharide phosphates (glucose-1,6-diphosphate), etc.
  • Subclasses of isomerases are distinguished depending on the type of reaction. 
  • For example, Racemases are responsible for interconversions of L- and D-isomers, and S- and R-isomers. 
  • Epimerases change their configuration at one of the chiral carbon atoms, for example, interconversion of α- and β-isomers, transformations of ribulose into xylulose, galactose into glucose, and mannose into galactose.
  • Mutases (transfer of chemical groups inside molecules, for example, phosphoglucomutase converts glucose-1-phosphate to glucose-6-phosphate).

Subclasses of Isomerases Enzymes
If we consider all subclasses, then isomerases are divided according to the type of isomerization into:
5.1. racemases and epimerases;
5.2. cis-trans isomerase;
5.3. intramolecular oxidoreductases;
5.4. intramolecular transferases – mutases ;
5.5. intramolecular lyases.
Examples of Isomerases Enzymes
  • Some examples of isomerases include glucoisomarase, triose phosphate isomerase, bisphosphoglycerate mutase, photoisomerase, etc
6. Ligases / Synthetases (EC 6):
  • Ligases are enzymes that catalyze the attachment of two molecules to each other using the energy of high-energy ATP bonds (or other macros). 
  • Ligases are complex enzymes. 
  • They contain nucleotide (UTP), biotin (vitamin H), and folic coenzymes. There are 6 subclasses.
  • An example of subclasses are groups of enzymes according to the type of bond formed: carbon-oxygen (C-O), carbon-sulfur (C-S), carbon-nitrogen (C-N), and carbon-carbon (C-C).
Subclasses of Ligases Enzymes
If we consider the class as a whole, then there are 6 subclasses of enzymes that form bonds:
  • 6.1. carbon-oxygen;
  • 6.2. carbon-sulfur;
  • 6.3. carbon-nitrogen;
  • 6.4. carbon-carbon;
  • 6.5. phosphorus-oxygen;
  • 6.6. nitrogen metal.
Examples of ligases enzymes include Acetyl-CoA carboxylase (C-C bond),
Glutamate–cysteine ligase (C-N bond),
Aminoacyl tRNA synthetase (C-O bond), Succinyl coenzyme A synthetase (C-S bond),  Acetyl—CoA synthetase (C-S bond), Pyruvate carboxylase (C-C bond), etc.
Frequently Asked Questions on Enzyme Classification:

1. What are enzymes and their properties?
Answer: Enzymes are biological catalysts. Their properties are enzymes are biological catalysts, and almost all enzymes are proteins (although it has recently been found that some reactions are catalyzed by RNA, not proteins). Substances involved in the reaction catalyzed by an enzyme are called substrates.
2. How many groups of enzymes?
Answer: According to the International Classification, enzymes are divided into six main classes, each of which has several subclasses: 1) oxidoreductases; 2) transferases; 3) hydrolases; 4) lyases; 5) isomerases; 6) ligases (synthetases).
3. What are the functions of enzymes?
Answer: Enzymes act as catalysts in almost all biochemical reactions occurring in living organisms – they catalyze about 4000 bioreactions. Enzymes play an essential role in all life processes, directing and regulating the body’s metabolism.
4. How many enzymes are known?
Answer: The Enzyme Committee (EC) of the International Union of Biochemistry and Molecular Biology classifies enzymes into 6 classes, according to the type of reaction they catalyze. More than 5000 enzymes are currently known, some work alone and others require a cofactor.
5. How are ligase enzymes classified?
Answer: Ligases are classified as EC6 in the EC enzyme classification scheme. Ligases are further divided into six subclasses.
6. How are hydrolase enzymes classified?
Answer: Hydrolases are enzymes that catalyze the breaking of different types of chemical bonds by hydrolysis. They are classified by EC number 3. 
7. How are oxidoreductase enzymes classified?
Answer: Oxidoreductases are classified as number 1 according to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology,

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