Enzymes are divided into 9 classes according to the nature of the chemical reaction. These are-
(i) Oxidoreductase: An enzyme that adds hydrogen, oxygen or electrons to a substance or releases hydrogen, oxygen or electrons from a substance is called an oxidoreductase enzyme. It is also called oxidative enzyme as it causes oxidation. For example, phosphoglyceraldehyde dehydrogenase, catalase, cytochrome oxidase, glucooxidase, reductase etc.
(ii) Hydrolytic/Hydrolase Enzyme: An enzyme that breaks down a substance by binding water molecules to specific bonds is called a hydrolytic enzyme. For example, protease, sucrase, phosphatase, esterase, lipase, carbohydrase, nuclease, pepsin, invertase, urease, amylase, maltase, trypsin etc.
(iii) Transferase: An enzyme that releases a group from a substance and attaches it to another substance is called a transferase enzyme. Such as hexokinase, kinase, trans amylase, trans acetylase, decarboxylase, methylase, protein kinase, polymerase etc.
(iv) Isomerase: An enzyme which converts a substance into its isomer is called an isomerase enzyme. Eg – phosphoglucoisomerase, phosphoglucomutase, racemase, mutase etc.
(v) Ligase: Enzyme which joins two or more substrates to form a new compound by taking energy from ATP is called ligase. Such as glutamic synthetase, acetyl Co-A synthetase, aspartic synthetase, succinic thiokinase, pyruvic carboxylase etc.
(vi) Lyase: Enzymes which act on carbon-carbon, carbon-oxygen and carbon-nitrogen bonds of substances are called lyases. Such as isocitrate, lyase, citric synthetase, aldolase, fumarase, decarboxylase, dehydratase, hydrolyase etc.
(vii) Carboxylase: An enzyme which combines CO2 with a substance or releases CO2 from a substance is called a carboxylase enzyme. Eg – carboxylase.
(viii) Phosphorylase Enzyme: An enzyme which adds phosphate group to a substance or removes phosphate group from a substance is called phosphorylase enzyme. For example – phosphofructokinase, phosphorylase, pyruvic acid kinase etc.
(ix) Epimerase: An enzyme which converts a substance into its epimer is called an epimerase enzyme. Epimer molecules differ only by the configuration of one carbon atom. Eg – Epimerase.