The modern system of enzyme classification was introduced by International Union of Biochemistry. 

It grouped enzymes into the following six categories : 

(i) Oxidoreductases : They take part in oxidation and reduction reactions or transfer of electrons. 

(ii) Transferases : They transfer group from one molecule to another e.g., glutamate pyruvate transaminase. 

(iii) Hydrolases : They break up large molecules into smaller ones with the help of hydrogen and hydroxyl groups of water molecules. The phenomenon is called hydrolysis. 

(iv) Lyases : The enzyme cause cleavage, removal of groups without hydrolysis, addition of groups to double bonds or reverse. e.g. aldolase. 

(v) Isomerase : The enzyme causes rearrangment of molecular structure to effect isomeric changes. These are of three types-isomerases, epimerases, mutase. 

(vi) Ligases : The enzyme catalyse bonding of two chemicals with the help of energy obtained from ATP. Е.g. pyruvate carboxylase.