What are enzymes? Discuss briefly the classification of enzymes according to the IUB system giving classical examples of each class.

Hint: Thousands of chemical reactions are taking place in the body of living organisms that are mediated by the enzymes while operating at optimal biological temperatures.

Complete answer:
Enzymes are commonly proteinaceous substances that are involved in catalyzing chemical reactions occurring in the living organisms without themselves undergoing any change and thus they are called biocatalysts. Whenever needed, enzymes are synthesized by the living cells and are used in the required reaction.
In 1961, the International Union of Biochemistry (IUB) introduced the modern system of enzyme classification that grouped enzymes into the following six classes –

1) Oxidoreductases – This class includes the enzymes that take part in oxidation and reduction reactions. Based on the action of enzymes they can be oxidases, dehydrogenases, or reductases. Examples of this class of enzymes are – cytochrome oxidase, succinate dehydrogenase, nitrate reductase, etc.
2) Transferases – they transfer a group from one molecule to another. An example of such an enzyme is – glutamate-pyruvate transaminase.
3) Hydrolases – they break up large molecules into smaller ones with the help of hydrogen and hydroxyl groups of water molecules. Examples are – amylase, sucrase, lactase, etc.
4) Lyases – these enzymes cause cleavage or removal of groups without hydrolysis. Histidine decarboxylase and aldolase are examples of such enzymes.
5) Isomerases – these enzymes cause rearrangement of molecular structure to effect isomeric changes. Mutases and epimerases are examples of such enzymes.
6) Ligases – these catalyze bonding of two chemicals with the help of energy obtained from ATP. Pyruvate carboxylase is an example of such an enzyme.

Note: In the older classification of enzymes, there were only two classes of enzymes – hydrolyzing and desmolysing. Due to a variety of reactions occurring in the body of living organisms, the function of the enzyme was not clear from those two groups.