Classification of Bio Enzymes

What are enzymes ?

They are biocatalysts that act on a substrate and then convert it into product. They usually have the suffix -ase.

Most of enzymes are proteins. Enzymes follow the physical and chemical reactions of proteins. They are heat labile and water soluble.

Enzymes are classified into:

  • Oxidoreductases:

Function : Transfer of hydrogen or addition of oxygen (oxidases, oxygenases and dehydrogenases).

AH2 + B ————-→ A + BH2


  • Lactate dehydrogenase (NAD).
  • Glucose-6-phosphate dehydrogenase (NADP).
  • Succinate dehydrogenase (FAD).
  • di-oxygenases.


  • Transferases:

Function: Transfer of groups other than hydrogen.

Example: Aminotransferase. (Subclass: Kinase, transfer of phosphoryl group from ATP; e.g. Hexokinase)


  • Hydrolases:

Function : Cleave bond and add water.

e.g. Acetylcholine-esterase.

Acetyl Choline + H2O —-Acetylcholine-esterase—-→ Choline + Acetate


  • Lyases:

Function : Cleave without adding water.


  • Aldolase.
  • HMG CoA lyase.
  • ATP Citrate lyase.


  • Isomerases:

Function: Intramolecular transfers.

They include racemases and epimerases.

Example: Triose phosphate isomerase.


  • Ligases:

Function : ATP dependent condensation of two molecules.


  • Acetyl CoA carboxylase.
  • Glutamine synthetase.
  • PRPP synthetase.

Note: The difference between Synthetase and Synthase :

  • Synthetase : ATP-dependent enzymes catalysing biosynthetic reactions so according to the previous classification they belong to Ligases. (notice the T mnemonics)


  • Synthase:  enzymes catalysing biosynthetic reactions, but they do not require ATP directly.
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