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Chapter: Biochemistry: Enzymes

Classification of enzymes

The most comprehensive system for the classification of enzymes was devised in 1961 by the Enzyme Commission of International Union of Biochemistry (IUB).

Classification of enzymes

The most comprehensive system for the classification of enzymes was devised in 1961 by the Enzyme Commission of International Union of Biochemistry (IUB).

The 6 major classes of enzymes are


1. oxidoreductases

Enzymes catalyzing oxidoreduction reactions between two substrates A and B are called as oxidoreductases

In this reaction cytochrome C1 is oxidised and cytochrome a is reduced simultaneously by the action of cytochrome C-oxidase.

This class includes several subclasses based on the group on which the enzymes act. The enzymes acting on

-CH -OH (1.1)

-C=O (1.2)

-C=CH (1.3)

-CH-NH2 (1.4)

-CH-NH (1.5)


2. Transferases

Enzymes catalysing the transfer of a group (x) from one substrate(AX ) to another are known as transferases.

AX + B < - - > A’ + BX


The reaction catalysed by alanine transaminase (ALT) is

In this reaction the amino group from alanine is transferred to α-ketoglutarate to form glutamate.

These enzymes are further divided into subclasses on the basis of nature of the group transferred.

Transfer of

·           one carbon compounds (2.1)

·           aldehyde or ketonic groups (2.2)

·           acyl groups (2.3)

·           glycosyl groups (2.4)

·           Phosphate groups (2.7)

·           Sulphur containing groups (2.8)


3. Hydrolases

The hydrolases are those enzymes which catalyse hydrolysis reactions i.e the direct addition of water molecule (s) across the bond, which is to be cleaved. The substrate for these enzymes are esters, ethers, peptides and glycosides.

Example : Pepsin. This enzyme is a gastro intestinal enzyme which is proteolytic in nature and involve in the hydrolysis of proteins present in the food.

The hydrolases are divided into several subclasses, depending on the nature of the group or bond being hydrolysed viz.,

(a) esterases etc. - hydrolyse ester bonds (3.1)

(b) glycosidases - hydrolyse glycosidic bonds (3.2)

(c) peptidases - hydrolyse peptide bonds (3.4)


4. Lyases

The lyases are a smaller class of enzymes that catalyse the removal of a small molecule from a larger substrate molecule. Since the reactions are reversible, lyases may also be considered to catalyse the addition of small molecules to the substrate molecule

The lyases are further classified on the basis of the linkage they attack viz., acting on

a. C-C bond (4.1)

b. C-O bond (4.2)

c. C-N bond     (4.3)

d. C-S bond (4.4)

e. C-halide bond (4.5) 

5. Isomerases

This class includes all enzymes which catalyse isomerization reactions i.e. interconversion of optical, geometrical or position isomers.


Retinene isomerase catalyses the conversion of

trans-retinene < - - > cis-retinene

Triose phosphate isomerase catalyses the conversion of

D - glyceraldehyde 3 phosphate < - - - > Dihydroxy acetone phosphate

The isomerases are further divided into the subclasses

a. racemases

b. epimerases

c. cis-trans isomerases


6. Ligases

These enzymes are otherwise known as synthetases. They catalyse synthesis reactions by joining two molecules, coupled with the breakdown of a phosphate bond of adenosine triphosphate. ATP cleavage provides energy for the new bond formation.

Example : Formation of malonyl CoA from acetyl CoA in the presence of acetyl CoA carboxylase.

ATP + acetyl CoA + CO2 < - - > ADP + Pi + malonyl CoA

The subclasses of ligases are based on the nature of bond formed in the product. Formation of

a. C-O bond

b. C-S bond

c. C-N bond

d. C-C bond


Class, subclass and enzyme numbers of some enzymes

1. Alcohol dehydrogenase (ADH)

Major class - oxidoreductase (1)

Subclass - acting on CH-OH (1)

Sub subclass - NAD or NADP as coenzymes (1)

Enzyme no - 1.1.1

2. Hexo kinase

Major class - Transferases (2)

Sub class - Transfering phosphate group (7)

Enzyme No. - 2.7.

3. Alkaline phosphatase

Major class Hydrolases (3)

Subclass - acting on esterbond (1)

Sub subclass - acting on phosphoric monoesters (3)

Enzyme No. - 3.1.3

4. Fumarase

Major class - lyases (4)

Sub class - CO lyase (2)

Sub subclass - acting on aldehyde (1)

Enzyme No. - 4.2.1

5. Ribulose - 5 - phosphate epimerase

Major class - Isomerases (5)

Subclass - Racemases and Epimerases (1)

Sub subclass - acting on carbohydrates (3)

Enzyme no - 5.1.3

6. Acetyl CoA carboxylase

Major class - ligases (6)

Sub class - forming C-C bond (4)

Sub subclass - carboxyl group (1)

Enzyme No. - 6.4.1


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