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
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)
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.
· 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)
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)
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)
This class includes all enzymes which catalyse
isomerization reactions i.e. interconversion of optical, geometrical or
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
c. cis-trans isomerases
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
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
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