The modem system of enzyme classification was introduced by International Union of Biochemistry. It groups enzymes into the following six categories :
(1) Oxidoreductases : They take part in oxidation and reduction reactions or transfer of electrons. They are of three types: oxidases, dehydrogenases and reductases, e.g., cytochrome oxidases, succinate dehydrogenase, nitrate reductases.
(2) Transferases : They transfer group from one molecule to another e.g., glutamate pyruvate transaminase. The chemical group transfer does not occur in free state. Glutamic acid + Oxaloacetic acid Transfers Ketoglutaric acid + Aspartic acid.
(3) 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.
+ H20 > C6H1206
(4) Lyases : The enzyme cause cleavage, removal of groups without hydrolysis, addition of groups to double bonds or reverse, e.g., aldolase.
(5) Isomerase : The enzyme causes rearrangement of molecular structure to effect isomeric changes. These are of three types - isomerases, epimerases, mutase.
Glucose 6 phosphate —isomerases > Fructose
(6) Ligases : The enzyme catalyse bonding of two chemicals with the help of energy obtained from ATP e.g., pyruvate carboxylase. It combines with pyruvic acid with C02 to form oxaloacetic acid