Enzymes are biocatalysts, primarily proteins, that accelerate biochemical reactions and are classified by the International Union of Biochemistry into six categories based on the types of reactions they facilitate. The classification system includes oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases, each performing specific functions in cellular reactions. The document provides examples and systematic naming, including the enzyme commission (EC) number for identification.

2. Enzymes - Introduction
 Enzymes are biocatalysts present in cells that speed up
biochemical reactions without getting itself destroyed in
the reaction.
 All types of biochemical reactions in the cell require
enzymes.
 Enzymes are typically proteins.
 Certain types of RNA can also serve as catalysts. These
RNA molecules are called ribozymes and DNAzymes.

5. Nomenclature and Classification of Enzymes
 Enzymes are classified according the report of a Nomenclature
Committee appointed by the International Union of Biochemistry.
 This enzyme commission assigned each enzyme a systematic name and
a code number, called enzyme commission (EC) number.
Common name : Alcohol dehydrogenase
Systematic name : Alcohol: NAD+ oxidoreductase
EC number : EC 1.1.1.1

6. Nomenclature and Classification of
Enzymes
The code numbers, prefixed by EC, which are now widely in use,
contain four elements separated by points, with the following
meaning:
(i) the first number shows to which of the six main classes the enzyme
belongs,
(ii) the second figure indicates the subclass,
(iii) the third figure gives the sub-subclass,
(iv) the fourth figure is the serial number of the enzyme in its sub-
subclass.

7. Nomenclature and Classification of Enzymes
 Example: EC 1.1.1.1 refers to the enzyme alcohol dehydrogenase (common
name).
 Its systematic name is alcohol : NAD+ oxido reductase.
 It catalyses the oxidation of alcohol to an aldehyde or ketone.
 Reaction: an alcohol + NAD+ an aldehyde or ketone + NADH + H+
 First digit 1 indicates the major class oxidoreductase
 Second digit 1 indicates the subclass acting on CH-OH group of donors
 Third digit 1 indicates the sub-subclass NAD+ as the electron acceptor
 Fourth digit 1 indicates the serial number of the enzyme alcohol
dehydrogenase

8. Enzyme - Classification
 Enzymes are classified into six categories (EC number classification)
by the International Union Of Biochemists (IUB) on the basis of the
types of reactions that they catalyze:
 EC 1 Oxidoreductases,
 EC 2 transferases,
 EC 3 hydrolases,
 EC 4 lyases,
 EC 5 ligases,
 EC 6 isomerases.

9. Classification of Enzymes
1. Oxidoreductases
Enzymes involved in oxidation-reduction reactions. Eg. Alcohol dehydrogenase
2. Transferases
Enzymes involved in transfer of groups from one molecule to another. Eg.
Hexokinase
3. Hydrolases
Enzymes that bring about hydrolysis by addition of water. Eg. Lipase
4. Lyases
Enzymes that catalyze the breaking of a chemical bond through means not
involving hydrolysis, and forms a double bond or adds a group to a double bond.
Eg. Fumarase
5. Isomerases
Enzymes involved in all kinds of isomerization reactions. Eg. Phosphohexose
isomerase
6. Ligases
Enzymes catalyzing the joining of two molecules with hydrolysis of ATP. Eg.
Glutamine synthetase

10. 1. Oxidoreductases
 Oxidation-reduction (redox) reactions - hydrogen or oxygen atoms
or electrons are transferred between molecules.
 Example: dehydrogenases, oxidases, oxygenases and peroxidases
EC 1.1.1.1
Common name: alcohol dehydrogenase
Systematic name: alcohol:NAD+ oxidoreductase

11. 2. Transferases
 These enzymes catalyse the transfer of a group between
substrates
 E.g: transfer of acyl, alkyl, amino, phosphate and
glycosyl.
 Involves transfer of amino group

12. 3. Hydrolases
 These enzymes catalyze reactions that involve the process of
hydrolysis.
 They break single bonds by adding water.
 Some hydrolases function as digestive enzymes.
CH-O-CO-R2
CH2-O-CO-R1
CH2-O-CO-R3
+
CH2-OH
CH-O-CO-R2
R3-COOH
lipase
OH
Triacylglycerol
Diacylglycerol
Fatty acid
CH2-O-CO-R1
H
Water

13.  Enzymes that catalyze the breaking of a chemical bond
through means not involving hydrolysis, and forms a double
bond or adds a group to a double bond.
 Eg: Fumarase
4. Lyases

14.  Enzymes involved in all kinds of isomerization reactions.
 Example: triosephosphate isomerase and phosphoglucose
isomerase
5. Isomerases

15.  Enzymes catalyzing the joining of two molecules with
hydrolysis of ATP. Eg. Pyruvate carboxylase
6. Ligases