Nomenclatures of Enzymes

Nomenclatures of Enzymes

All enzyme names end in suffix - ase, except some old names, e.g. ptyalin, pepsin, trypsin. Some old names indicate the source but not the action, e.g. papain from Papaya, bromelain from Pineapple of family Bromeliaceae.

In modern system enzyme names are given after

(i) Substrate acted upon, e.g. sucrase (after sucrose), lipase, protease, nuclease, peptidase, maltase

(ii) Chemical reaction e.g. dehydrogenase, oxidase, carboxylase, decarboxylase etc.

Group names are often qualified by the addition of the name of substrate, e.g. succinic dehydrogenase, isocitric dehydrogenase, glutamate pyruvate transaminase, DNA polymerase.

DNA polymerase catalyses synthesis of DNA segments through polymerisation of deoxyribonucleotides.

Glutamate-pyruvate transaminase transfers amino group (–NH₂) from glutamate to pyruvate.

In older times, enzymes were classified into two broad categories :

(i) Hydrolysing - Catalysing hydrolysis of larger molecules into smaller ones e.g. carbohydrases or amylases, proteases, lipases, esterases, phosphorylases, amidases. Digestive enzymes are hydrolysing in nature.

They are often grouped into three types - proteolytic, amylolytic and lipolytic.

(ii) Desmolysing - Catalysing reactions other than hydrolysis e.g. aldolases, dehydrogenases, oxidases, peroxidases, catalases, carboxylases etc.

The modern system of enzyme classification was introduced by International Union of Biochemistry (IUB) in 1961. It groups enzymes into the following six categories :

1. Oxidoreductases : They take part in oxidation and reduction reactions or transfer of electrons. Oxidoreductases are of three types - oxidases, dehydrogenases and reductases, e.g. cytochrome oxidase (oxidises cytochrome), succinate dehydrogenase, nitrate reductase.

2. Transferases : They transfer a group from one molecule to another e.g. glutamate pyruvate transaminase (transfers amino group from glutamate to pyruvate during synthesis of alanine). The chemical group transfer does not occur in the free state :

3. Hydrolases : They break up large molecules into smaller ones wiht the help of hydrogen and hydroxyl groups of water molecules. The phenomenon is called hydrolysis. Digestive enzymes belong to this group, e.g. amylase (hydrolysis of starch), sucrase, lactase.

4. Lyases : The enzymes cause cleavage, removal of groups without hydrolysis, addition of groups to double bonds or reverse, e.g. histidine decarboxylase (breaks histidine to histamine and CO2), aldolase

(fructose-1, 6-diphosphate to dihydroxy acetone phosphate and glyceraldehyde phosphate).

Fructose1, 6-diphosphate Dihydroxy acetone phosphate + Glyceraldehyde phosphate.

5. Isomerases : The enzymes cause rearrangement of molecular structure to effect isomeric changes. They are of three types

(i) Isomerases (aldose to ketose group or vice-versa)

(ii) Epimerases (change in position of one constitutent or carbon group)

(iii) Mutases (shifting the position of side group).

6. Ligases (Synthetases) : The enzymes catalyse bonding of two chemicals with the help of energy obtained from ATP, e.g. pyruvate carboxylase. It combines pyruvic acid with CO₂ to produce oxaloacetic acid.

Pyruvic acid + CO₂ + ATP + H₂O Oxaloacetic acid + ADP + Pi