11th Class Biology Biomolecules / जैव-अणु Nature Of Enzymes

Mostly enzymes are proteinaceous in nature. With some exception all enzymes are proteins but all proteins are not enzymes. Enzymatic protein consist of 20 amino acids. The polypeptide chain or chains of an enzyme show tertiary structure. Their tertiary structure is very specific and important for their biological activity. Loss of tertiary structure renders the enzymic activity.

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.

Activity of enzyme is due to co-factor, which can be separated by dialysis. co-factor is small, heat stable and may be organic or inorganic in nature.

Three types of cofactors may be identified. Prosthetic group, coenzyme and metal ions.

Prosthetic group : Prosthetic groups are organic compounds distinguished from other cofactors 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.

Coenzymes : Fritz Lipmann discovered coenzymes. Coenzymes are also organic compounds but their association with the apoenzyme is transient, usually occurring only during the course of catalysis.

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 nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP) contains the vitamin niacin, coenzyme A contains pantothenic acid, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD) contains riboflavin (Vitamin\[{{B}_{2}}\]), and thiamine pyrophosphate (TPP) contains thiamine (Vitamin \[{{B}_{1}}\]).

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 \[Z{{n}^{++}},C{{u}^{++}},M{{g}^{++}}.\]

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

Enzymes activators