Classification of Enzymes

Enzymes are biological catalysts that speed up chemical reactions without being consumed. In 1961, the International Union of Biochemistry (IUB) developed a systematic classification based on the type of reaction they catalyze.

Each enzyme is assigned an Enzyme Commission (EC) number consisting of four digits, where the first digit represents one of these six major classes:

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1. Oxidoreductases

These enzymes catalyze oxidation-reduction (redox) reactions, involving the transfer of electrons, hydrogen, or oxygen atoms from one substrate (the electron donor) to another (the electron acceptor).

• Common Subclasses: Dehydrogenases, oxidases, reductases.
• Example: Lactate dehydrogenase, which converts lactate to pyruvate.

2. Transferases

Transferases facilitate the transfer of a functional group (such as a methyl, ethyl, phosphate, or amino group) from one molecule (donor) to another (acceptor).

• Common Subclasses: Kinases (transfer phosphate), transaminases.
• Example: Hexokinase, which transfers a phosphate group from ATP to glucose.

3. Hydrolases

These enzymes catalyze the cleavage of bonds by the addition of water (hydrolysis). They break down large molecules into smaller units.

• Common Subclasses: Esterases, peptidases, amylases.
• Example: Lipase, which breaks down lipids into fatty acids and glycerol.

4. Lyases

Lyases catalyze the breaking of various chemical bonds (C-C, C-O, C-N) by means other than hydrolysis or oxidation, often resulting in the formation of a double bond or the addition of groups to double bonds.

• Common Subclasses: Decarboxylases, dehydratases, synthases.
• Example: Fumarase, which converts fumarate to malate.

5. Isomerases

These enzymes catalyze structural or geometric changes within a single molecule. They rearrange the atoms to convert a molecule into its isomer.

• Common Subclasses: Epimerases, mutases, racemases.
• Example: Phosphoglucoisomerase, which converts glucose-6-phosphate into fructose-6-phosphate.

6. Ligases

Ligases (also known as synthetases) catalyze the joining of two large molecules by forming a new chemical bond. This reaction typically requires the input of energy from ATP hydrolysis.

• Common Subclasses: Carboxylases, DNA ligase.
• Example: DNA ligase, which joins DNA fragments together.

Class	Type of Reaction	Key Feature
Oxidoreductases	Redox	Transfer of H or O atoms
Transferases	Group Transfer	Moving functional groups
Hydrolases	Hydrolysis	Breaking bonds using a water molecule
Lyases	Non-hydrolytic Bond Breaking	Formation of double bonds
Isomerases	Isomerization	Intramolecular rearrangement
Ligases	Bond Formation	Joining molecules using ATP