Classification of Enzymes

  Enzymes are classified according to the type of reaction they catalyse. All enzymes have formal 'EC' (Enzyme Commission) number and names, and most have trivial names.

International Union of Biochemistry had formulated a committee to look into uniform system of nomenclature and classification of enzymes. It was known as Enzyme Commission. Classification of enzyme is essential for ready reference about the enzyme as well as for the identification of new enzyme. The system of classification accepted by IUB is applied.

According to the International Union of Biochemistry (IUB) enzymes are classified into seven major classes.

IUB - EC numbers

Each enzyme is described by a sequence of four numbers preceded by "EC".

  • First digit represents the Class - Seven classes on the basis of the nature of reaction.
  • Second digit stands for the Subclass - Where, the enzyme classes are further divided on the basis of groups or chemical bond attacked by the enzyme.
  • Third digit is for the Group - indicates Coenzyme/ Cofactor required for enzyme action.
  • Fourth digit gives the number of the particular enzyme in the list indicates - serial number of individual enzyme.

e.g. EC 1:1:1:1 is alcohol NAD Oxidoreductase.

The seven classes as per IUB are as follows:

1]. EC-1 : Oxidoreductase
2]. EC-2 : Transferase
3]. EC-3 : Hydrolase
4]. EC-4 : Lyase
5]. EC-5 : Isomerase
6]. EC-6 : Ligase
7]. EC-7 : Translocases

IUB system of enzyme classification; classes have been formed on the basis of nature of reaction catalyzed.

EC-1 Oxidoreductases

Catalyse removal or addition of Hydrogen atoms, Oxygen atoms or Electrons from one substrate to another.
Oxidoreductases catalyzes oxidation-reduction reactions.

A(red) + B(ox) A(ox) + B(red).

Enzymes in this category include :  

• Dehydrogenases
• Reductases
• Oxidases
• Peroxidases.

Examples of Oxidoreductases

• Lactate dehydrogenase (LDH)
• Glucose 6-phosphate dehydrogenase (G-6-PD)
• Cytochrome oxidase
• Alcohol Dehydrogenase

EC-2 Transferases

Transferase Catalyses the transfer of a group such as, Amino, Carboxyl, Methyl or Phosphate, etc (except hydrogen) from one molecule to another.

A-X + B A + B-X

Enzymes in this category include : 

• Amino transferase or Transaminase

• Kinase: catalyzes the transfer of phosphate groups

Examples of Transferase

• Aspartate amino transaminase(AST)
• Alanine amino transaminase (ALT)
• Hexokinase
• Transmethylase

EC-3 Hydrolases

Hydrolases catalyze the cleavage of Peptide, Glycosidic, Ester and Ether and some other bonds with the addition of water(H₂O).

A-B + H₂O ➞ A-OH + B-H

Enzymes in this category are:

• All digestive enzymes like -
   - α-amylase,
   - Pepsin,
   - Trypsin,
   - Chymotrypsin,
   - Lipase
• Acid phosphatase and alkaline phosphatase.

EC-4 Lyases

Lyases catalyze the cleavage of Carbon-Oxygen(C-O), Carbon-Carbon(C-C) and Carbon-Nitrogen (C-N) bonds by means other than hydrolysis or oxidation, giving rise to compound with double bonds or catalyze the reverse reaction, by the addition of group to a double bond.

X-A-B-Y A=B + X-Y

In cases where reverse reaction is important, then synthase, (not synthetase of group EC-6) is used in the name.

Examples of  Lyases

• Aldolase
• Fumarase
• Argininosuccinase
• Carbonic anhydrase
• HMG CoA Lyases

EC-5 Isomerases

Isomerases catalyze intramolecular structural rearrangement in a molecule. They are called Epimerases, Isomerases or Mutases, depending on the type of isomerism involved.

      ABC CAB

Examples of Isomerases 

 All racemases and epimerases
  - Phosphoglucomutase
  - Triphosphate isomerase
  - Phosphohexose isomerase

EC-6 Ligases (Synthetases)

Ligases catalyze the joining of two molecules coupled with the hydrolysis of ATP.

A + B + ATP ➞ A B+ ADP + Pi

Examples of Ligases

• Glutamine synthetase
• Pyruvate carboxylase
• DNA ligases
• PRPP synthetase

EC-7: Translocases (A new EC Class)

Translocases catalyze the movement of ions or molecules across membranes or their separation within membranes.

Examples of Translocases

  • Enzymes catalyzing the translocation of : Hydrogen ions (H+), Inorganic cations, Inorganic anions, amino acids and Peptides, and carbohydrates and their derivatives.
  • Enzymes of the reaction that provided the driving force for the translocation linked to Oxidoreductase reactions, Hydrolysis of a nucleoside Triphosphate, Hydrolysis of a diphosphate, and Decarboxylation reaction.