• on the basis of International Union of Biochemistry (IUB) system:
  1. Oxidoreductase
  2. Transferase
  3. Hydrolase
  4. Lyase
  5. Isomerase
  6. Ligase

1. Oxidoreductase enzyme:

• The enzyme that catalyze transfer of H⁺ atom, or electron pair from one substrate to another is called oxidoreductase.
• According to International union of biochemistry and molecular biology (IUBMB), this enzyme belongs to class 1 and it is designated by E.C.1.
  e.g. E.C.1.1 – CHOH group containing donor
  E.C.1.1.1.- If NAD⁺ in NADP⁺ acts as electron acceptor.
• Examples of oxidoreductase enzyme:
  • Dehydrogenase, reductase, catalase etc.

2. Transferase enzyme:

• The enzyme that catalyze transfer of any group except H-atom such as methyl group, acetyl group, phosphate group etc. among substrate is called transferase.
• E.g. Acetyltransferase, kinase, methylase, polymerase, etc.
• This enzyme belongs to E.C.2
• Nomenclature:
  • E.C.2- transferase
  • E.C.2.1- enzyme that transfer one carbon group
  • E.C.2.1.1- if the group transfer is accepted by nicotinamide.

3. Hydrolase enzyme:

• The enzyme that catalyzes the break down of bond of substrate by using water is called hydrolase.
• The bond may be ester, glycosidic, peptide etc.
• E.g. digestive enzymes- pepsin, lipase, urease, proteinase, glycosidase
• This enzyme belongs to E.C.3
• Nomenclature:
  • E.C.3- Hydrolase
  • E.C.3.1.1- carboxylic ester hydrolase
  • E.C.3.1.1.3- Lipase (Glycerol ester hydrolase)

4. Lyase enzyme:

• The enzyme that catalyze the addition of removal of group from one substrate to another by mechanism other than hydrolysis and oxidation creating double bond is called lyases.
• E.g. aldolase, fumarase, histidase, carboxylase, ketolase etc.
• Nomenclature:
  • E.C.4 – lyase
  • E.C.4.1 – C-C lyase
  • E.C.4.1.1.1- Pyruvate decarboxylase
  • E.C.4.2.1.2- Fumarase

5. Isomerase enzyme:

• The enzyme that catalyze the interconversion of optical, geometrical, positional isomers by intermolecular re-arrangement of atom or group within substrate is called isomerase.
• It belongs to enzyme class E.C.5 e.g. Isomerase, Epimerase, Racemase
• Nomenclature:
  • E.C.5- Isomerase
  • E.C.5.1 – Epimerase and Racemase
  • E.C.5.1.1.1 – Alanine racemase
  • Glyceraldehyde-3-phosphate —-triosphosphate isomerase———-> Dihydroxyacetone phosphate

6. Ligase enzyme:

• The enzyme that catalyze the formation of bond such as C-C, C-N, C-S, C-O etc. utilizing the energy due to simultaneous breaking of pyrophosphate bond of ATP molecule or similar compound. E.g. ligase, synthetase
• Nomenclature:
  • E.C 6- ligase
  • E.C.6.1- form C-O bond
  • E.C.6.1.1 – aminoacyl tRNA synthase

Enzyme classification with examples

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