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IUB (International Union of Biochemistry) has developed a system of Enzyme Nomenclature in which each enzyme has a unique code and number that identify the type of reaction catalyzed and substrates involved. Every enzyme code consists of the letters "EC" followed by four numbers separated by periods.

Significance of 4 numbers -

1st number - class

2nd number - Sub class

3rd number - Sub sub class

4th number - Group

There are 6 classes of enzymes (mnemonic - OTHLILi) -

1. Oxidoreductase - catalyzes oxidation reduction reactions 

2. Transferase - catalyzes transfer of moieties such as methyl, phosphate, etc. groups

3. Hydrolase - catalyzes hydrolytic cleavage of C-C, C-N, C-O and other covalent bonds

4. Lyase - catalyzes bond cleavage/bond breaking reaction without the application of water molecules

5. Isomerase - catalyzes isomerisation reactions

6. Ligase - catalyzes reactions involving joining of two molecules

What you are expected to learn for NEET UG?

NCERT and from here, 6 classes of enzyme with their working

Sample Questions -

1. Ligase belong to which class?

Ans - 6

2. An enzyme cleaves bond without using water. What kind of enzyme is it?

Ans - Lyase

3. An enzyme has EC 5.2.1.3 What kind of enzyme is it?

Ans - 1st number is 5 which denotes class. Hence answer should be Isomerase

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