1. Classification on the basis of site of action
Enzymes are of two types based on site of action.
(i) Endoenzymes: Enzymes which act inside the cell are called endoenzymes. Such as Calvin cycle enzymes, Hatch-Slack cycle enzymes, Krebs cycle enzymes and glycolysis process enzymes.
(ii) Exoenzymes: Enzymes that act outside the cell are called exoenzymes. For example, lysozyme, digestive enzymes etc.
2. Structural classification of enzymes
Enzymes are divided into two groups based on structure. These are-
(i) Simple Enzymes: Enzymes which are made up of proteins only are called simple enzymes. For example, protease, amylase etc.
(ii) Composite Enzymes: Enzymes which are composed of proteins and other substances are called conjugated enzymes. A conjugated enzyme consists of two parts. Protein part and non-protein part. The protein part is called apoenzyme and the non-protein part is called prosthetic group.
3. Classification of enzymes based on chemical reactions
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.