Introduction to The Cellular Enzymes

Enzymes are the proteinaceous substances which are capable of catalyzing 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 reaction but does not affect the nature of final product’. Like the catalyst, the enzyme regulates it is speed and specificity of a reaction, but unlike the catalyst, they are produced by living cells only.

All components of self including cell wall and cell membrane have enzymes. Every cell produces its own enzymes because they cannot move from cell to cell due to having high molecular weight. Maximum enzymes (70%) in the cell are found in mitochondria. 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 in 1878 while working on fermentation. At that time the living cell of yeast was thought to be essential for fermentation of sugar. Edward Buckner in 1897, German chemist proved that extract zymase, obtained from yeast cells, has the power of fermenting sugar (alcoholic fermentation). Zymase is a complex of enzymes (Buckner isolated enzyme for the first time).

Later JB Sumner (1926) prepared the 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 chymotrypsin.

Arber and Nathans got Nobel Prize in 1978 for the discovery of restriction endonucleases which break both strands of DNA at a specific site and produces sticky ends. These enzymes are used as micro-scissors in genetic engineering.

Nature of enzymes

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

DNA is the master molecules, which contain genetic information for the synthesis of proteins. It has been found that DNA makes RNA and RNA finally make protein. 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.

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

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

A cofactor is a small, heat stable and may be organic or inorganic in nature.

Three Types of cofactors may be identified. The prosthetic group, coenzymes, and metal ions.

Prosthetic group

Prosthetic group or organic compounds distinguished from other cofactors in that they are permanently bound to the apoenzymes, e.g, in peroxisomal enzymes peroxidase and catalase which catalyzes the breakdown of hydrogen peroxide to water and oxygen, heme is a prosthetic group and is a permanent part of the enzyme’s active site.

Coenzymes are also organic compounds but their association with apoenzyme is transient, usually occurring only during the course of catalysis. Furthermore, the same coenzyme molecules may serve as the cofactor 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 a temporary acceptor for one of the products of the reaction. The essential chemical component of many coenzymes are vitamins, e.g. coenzyme nicotinamide adenine dinucleotide (NAD) nicotinamide adenine dinucleotide phosphate (NADP) contains the vitamin niacin, coenzyme contains pantothenic acid, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD) contains riboflavin (vitamin B2), and thiamine pyrophosphate (TPP) contains thiamine (vitamin B1).

Metal Ions

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