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Enzymes are biocatalyst synthesized by the living cell. They are proteins in nature, colloidal and heat labile in character and specific in their action. Enzymes are grouped under six major classes according to the type of reactions they catalyze. They are listed in the table below:

 

Enzyme Class

Description

Reaction catalyzed

Example

1.

Oxidoreductases

Enzymes involved in oxidation-reduction reaction

Oxidation \longrightarrow Reduction

AH2 + B ―→ A + BH2

Alcohol dehydrogenase

2.

Transferases

Enzymes that catalyze transfer of functional groups

Group Transfer

C – X + D \longrightarrow C + D – X

Transaminase, hexokinase

3.

Hydrolases

Enzymes that bring about hydrolysis of various complex molecules

Hydrolysis

A – B + HOH \longrightarrow AH + BOH

Urease, Lipase, Pepsin

4.

Lyases

Enzyme involved in addition or loss of groups to double bond

Addition \longrightarrow Elimination

C – D + X – Y \longrightarrow CX + DY

Fumarase, Aldolase

5.

Isomerase

Enzymes involved in isomerization reactions

Isomerization

\longrightarrow B′

Ketoisomerase, retinol isomerase

6.

Ligases

Enzymes involved in synthesis by condensation of two groups requiring ATP

Condensation

X + Y ―\longrightarrow X – Y
            ATP

Acetyl CoA carboxylase

 

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