11th Class Biology Biomolecules / जैव-अणु Nomenclature And Classification

Dauclax, (1883) introduced the nomenclature of enzyme. Usually enzyme names end in suffix-ase to the name of substrate e.g., Lactase acts on lactose, maltase act on maltose, amylase on amylose, sucrase on sucrose, protease on proteins, lipase on lipids and cellulase on cellulose. Sometimes arbitrary names are also popular e.g., Pepsin, Trypsin and Ptylin etc. Few names have been assigned as the basis of the source from which they are extracted e.g., Papain from papaya, bromelain from pineapple (family Bromeliaceae). Enzymes can also be named by adding suffix-ase to the nature of chemical reaction also e.g., Oxidase, dehydrogenase, catalase, DNA polymerase.

Modern names are given after chemical action. They are more systamatic, informative but slightly longer. e.g., ATP : D-glucose phosphotransferase.

Common simpler names used at the place of systematic names called trivial names.

According to older classification : The older classification of enzymes is based on the basis of reactions which they catalyse. Many earlier authors have classified enzymes into two groups :

(1) Hydrolysing enzyme : The hydrolysing enzymes of hydrolases catalyse reactions in which complex organic compounds are broken into simpler compounds with the addition of water. Depending upon the substrate hydrolysing enzymes are :

Carbohydrases : Most of the polysaccharides, disaccharides or small oligosaccharides are hydrolysed to simpler compounds, e.g., hexoses or pentoses under the influence of these enzymes.

Lactase on lactose to form glucose to galactose, sucrase/invertase on sucrose to form glucose and fructose, amylase or diastase on starch to form maltose, maltase on maltose to form glucose, cellulase on cellulose to produce glucose.

Easterases : These enzymes catalyse the hydrolysis of substances containing easter linkage, e.g., fat, pectin, etc. into an alcoholic and an acidic compound.

\[Fat\xrightarrow{Lipase}Glycerol+Fatty\text{ }acid\]

\[Phosphoric\,acid\,easters\xrightarrow{Phosphatase}\]

\[Phosphoric\text{ }acid+Other\text{ }compounds\]

Proteolytic enzymes : The hydrolysis of proteins into peptones, polypeptides and amino acids is catalysed by these enzymes

\[\Pr otein\xrightarrow{Pep\sin }Peptones\]

\[Polypeptides\xrightarrow{Peptidases}A\min o\text{ }acids\]

Amidases : They hydrolyse amides into ammonia and acids.

\[Urea\xrightarrow{Urease}Ammonia+Carbon\text{ }dioxide\]

\[Asparagine\xrightarrow{Asparaginase}Ammonia+Aspartic\text{ }acid\]

(2) Desmolysing enzymes :  Most of the desmolysing enzymes are the enzymes of respiration e.g., oxidases, dehydrogenases, (concerned with transfer of electrons), transaminases carboxylases etc.

According to IUB system to classification : In 1961 the Commission on enzymes set up by the 'International Union of Biochemistry' (IUB) framed certain rules of their nomenclature and classification.

According to IUB system of classification the major points are :

• Reactions (and enzymes catalyzing them) are divided into 6 major classes each with 4-13 subclasses.

• The enzyme name has two parts-first name is of substrate. The second ending in ase indicates type of reaction.

• The enzyme has a systematic code No. (Enzyme code/Enzyme Commission). The first digit denotes the class, the second sub-class, the third sub-sub-class and the fourth one is for the particular enzyme name. Thus, E.C. 2.7.1.1 denotes class 2 (Transferases)-subclass 7 (transfer of phosphate) sub-sub-class 1 (an alcohol functions as phosphate acceptor). The 4th digit indicates hexokinase. Major classes of enzymes are as follows :

(i) Oxidoreductases : These enzymes catalyse oxidation reduction reactions, usually  involving the transfer of hydrogen atoms or ions from one molecule to another. There are three main types of these enzymes :

Oxidases : Where the hydrogen is transferred from a molecule to oxygen, e.g., cytochrome oxidase. They play very important role in E.T.S. in photosynthesis as well as respiration,

Dehydrogenases : Where the hydrogen is transferred to a coenzyme such as NAD+, e.g., Succinic dehydrogenase. They help in oxidation of organic molecules during aerobic respiration.

Reductase :  It is cause addition of hydrogen or an electron and remove oxygen. e.g., Nitrate reductase requires NAD (coenzyme I) as coenzyme for the reaction.

(ii) Transferases : These enzyme catalyse the transfer of a specific group (e.g., amino, methyl, acyl, phosphate) from one kind of molecule to another e.g., transphosphorylases,  transaminases,  transpeptidases, transmethylases, kinases, etc.

(iii) Hydrolases : These enzyme catalyse the hydrolysis of organic foods i.e., the breakdown of large molecules by addition of water. Most of the hydrolysing (digestive) enzymes are located in lysosomes. e.g., all digestive enzymes  such as lipases (digest the stored food material of castor seeds) amylases, esterases, phosphatases, carbohydrases, proteases.

(iv) Lyases : These enzymes catalyse the breakage of specific covalent bonds and removal of groups without hydrolysis e.g., fumerases, carboxylases, aminases, histidine decarboxylase that splits \[CC-\]bond of histidine, forming \[C{{O}_{2}}\]and histamine.

(v) Isomerases : These enzymes catalyse the rearrangement molecular structure to form isomers. e.g., phosphohexose isomerase (phosphoglucomutase) act on glucose 6-phosphate to form fructose 6-phosphate (both \[{{C}_{6}}\] compounds); epimerase, racemase.

(vi) Ligases or Synthetases : These enzymes form bonds  and join two molecules together, using energy supplied from the breakdown of ATP, e.g., DNA ligase is used to repair breaks in DNA molecules. Amino-Acyl synthetase is used to activate t-RNA by attaching amino acid at 3¢ end. Tryptophan synthetase is used to convert tryptophan amino acid to IAA, etc.