NEET Biology Biomolecules / जैव-अणु Notes - Introduction Properties Action and Inhibition of Enzyme

Important Terms And Classification Of Animals

Introduction and history of cellular enzymes.

Enzymes (Gk. en = in; zyme = yeast) are proteinaceous substances which are capable of catalysing chemical reactions of biological origins without themselves undergoing any change. Enzymes are biocatalysts. An enzyme may be defined as "a protein that enhances the rate of biochemical reactions but does not affect the nature of final product." Like the catalyst the enzymes regulate the speed and specificity of a reaction, but unlike the catalyst they are produced by living cells only. All components of cell including cell wall and cell membrane have enzymes. Every cell produces its own enzymes because they can not move from cell to cell due to having high molecular weight. Maximum enzymes (70%) in the cell are found in mitochondrion. The study of the composition and function of the enzyme is known as enzymology.

The term enzyme (meaning in yeast) was used by Willy Kuhne (1878) while working on fermentation. At that time living cells of yeast were thought to be essential for fermentation of sugar. Edward Buchner (1897), a German chemist proved that extract zymase, obtained from yeast cells, has the power of fermenting sugar (alcoholic fermentation). Zymase is complex of enzymes (Buchner isolated enzyme for the first time).

Later J.B. Sumner (1926) prepared a pure crystalline form of urease enzyme from Jack Bean (Canavalia ensiformis)  and suggested that enzymes are proteins. Northrop and Kunitz prepared crystals of pepsin, trypsin and chymotrpsin  Arber and Nathans got noble prize in 1978 for the discovery of restriction endonucleases which break both strands of DNA at specific sites and produce sticky ends. These enzymes are used as microscissors in genetic engineering.

 Nature of enzymes. 

Mostly enzymes are proteinaceous in nature. With some exception all enzymes are proteins but all proteins are not enzymes. Enzymic protein consist of 20 amino acids, which constitute other proteins. More than 100 amino acids linked to form an active enzyme. The polypeptide chain or chains of an enzyme show tertiary structure. Sequence of the amino acid in specific enzymic proteins. Their tertiary structure is very specific and important for their biological activity. Loss of tertiary structure renders the enzyme activity.

DNA is the master molecule, which contains genetic information for the synthesis of proteins. It has been found that DNA makes RNA and RNA finally makes proteins. The process of RNA formation from DNA template is known as transcription and synthesis of proteins as per information coded in mRNA is called translation. The above relation can be given by the formula given below.

Some enzymes like pepsin, amylase, urease, etc., are exclusively made up of protein i.e. simple proteins. But most of the other enzymes have a protein and a non-protein component, both of which are essential for enzyme activity. The protein component of such enzymes is known as apoenzyme whereas the non-protein component is called cofactor or prosthetic group. The apoenzyme and prosthetic group together form a complete enzyme called holoenzyme.

 Apoenzyme + Prosthetic group = Holoenzyme

Activity of enzyme is due to cofactor which can be separated by dialysis. After separation of cofactor the activity of holoenzyme or conjugated enzyme is lost.

Co-factor is small, heat stable and may be organic or inorganic in nature.

Three types of co-factors may be identified. Prosthetic group, coenzyme, and metal ions.

(1) Prosthetic group : Prosthetic groups are organic compounds distinguished from other co-factors in that they are permanently bound to the apoenzyme, e.g., in peroxisomal enzymes peroxidase and catalase which catalyzes breakdown of hydrogen peroxide to water and oxygen, heme is the prosthetic group and is a permanent part of the enzymes active site.

(2) Coenzymes : Coenzymes are also organic compounds but their association with the apoenzyme is transient, usually occurring only during the course of catalysis. Furthermore, the same coenzyme molecule may serve as the co-factor in a number of different enzymes catalyzed reactions. In general coenzymes not only assist enzymes in the cleavage of the substrate but also serve as temporary acceptor for one of the product of the reaction. The essential chemical component of many coenzymes are vitamins, e.g., coenzyme Nicotineamide adenine dinucleotide (NAD), Nicotineamide adenine dinucleotide phosphate (NADP) contains the vitamin niacin, coenzyme A contains pantothenic acid, Flavin mononucleotide (FMN), Flavin adenine dinucleotide (FAD) contains riboflavin (Vitamin B2), and thiamine pyrophosphate (TPP) contains thiamine (Vitamin B­1).

(3) Metal ions : A number of enzymes require metal ions for their activity. The metal ions form coordination bonds with specific side chains at the active site and at the same time form one or more coordination bonds with the substrate. The latter assist in the polarization of substrate bonds to be cleaved by the enzyme. The common metal ions are Zn++, Cu++, Mg++.

Inorganic part of enzyme acts as prosthetic group in few enzyme they are called activator. These activators are generally metals. Hence these enzymes are called "Metallo enzyme" such as

Differences between apoenzyme and coenzyme.