BIOSYNTHESIS OF CARBOHYDRATES

Carbohydrates Carbohydrates constitute one of the most important groups of natural products. Earlierly, carbohydrates were defined as compounds containing carbon, hydrogen and oxygen in which the hydrogen and oxygen are generally in the same ratio as in water and thus corresponded to the formula C x (H 2 O) y, e.g., glucose C 6 H 12 O 6, cane-sugar C 12 H 22 O 11 etc.

Cont… But later on it was found that certain carbohydrates do not correspond to this formula e.g., rhamnose, C 6 H 12 O 5, rhamnohexose C 7 H 14 O 6, digitoxose C 6 H 12 O 4 ; while several compounds althouth not carbohydrates, correspond to this formula, e.g. formaldehyde CH 2 O acetic acid C 2 H 4 O 2, etc. So, now-a-days carbohydrates are defined as the optically active polyhydroxy aldehydes or ketones or substances which yield these on hydrolysis.

Cont… The ultimate source of all carbohydrates is the plants which built up them from carbon -dioxide and water by photosynthesis. Light xCO 2 + x H 2 O (CH 2 O)x + O 2 Chlorophyll Carbohydrate

Classification of Carbohydrates Carbohydrates may be classified into two broad groups: Sugars and Non-sugars (polysaccharides)

Sugars Sugars are monosaccharides (i.e., compounds that cannot be hydrolyzed to simpler sugars); disaccharides (which yield 2 monosaccharide molecules on hydrolysis); trisaccharides (which yield 3); tetrasaccharides (which yield 4) and so forth. Monosaccharides are classified by the number of crabon atoms in the molecule. Those with three carbon atoms are triose; 4 carbon atoms pentoses; 6 carbon atoms hexoses and others. Sugars are crystalline, soluble in water and sweet tasting.

Polysaccharides The more complex, high-molecular weight polysaccharides are represented by starch, inulin and celluloses. These polysaccharides can usually be hydrolyzed to a component hexose and are therefore called hexosans; starch, which yields glucose, is known as glucosan and inulin which yields fructose, is known as fructosan.

Significance of Carbohydrates Carbohydrates are the first products formed in photosynthesis. Moreover, carbohydrates are the products from which by subsequent organic reactions, the plant synthesizes a greater number of other constituents. Sugars and starch are important products in the economy of mankind. They are extensively used as foods and pharmaceuticals.

Cont… The plant also builds its structural skeleton from carbohydrate material. Cellulose, a polysaccharide composed of glucose units joined by β-1,4 linkages, forms the primary cell walls in plants. Gums and mucilages constitute an important group of drugs both from the pharmaceutic and therapeutic (e.g., protective after injury) viewpoint. Another purified carbohydrate pectin have some pharmaceutical application (e.g., used as suspending agent).

SUCROSE (C 12 H 22 O 11 )

Sucrose(1-α - D-glucopyranosyl- β-D- Fructofuranside) Sucrose is the commonest sugar known. The most important sources are sugar cane and sugar beets; other sources are maple spas, honey and fruit juices. The primary use of sugar is as food in various forms. Sucrose is a pharmaceutic necessity for syrups. It is also a demulcent and a nutrient. In sufficient concentration in aqueous solution, sucrose is bacteriostatic and preservative. Sucrose masks disagreeable tastes in troches and tablets and retards oxidation in certain preparations.

Biosynthesis of Sucrose Fructose 6-phosphate, derived from the photosynthetic cycle, is converted to glucose 1-phosphate which, in turn, reacts with UTP to form UDP-glucose (uridinediphosphate- glucose). This UDP-glucose reacts with fructose-6-phosphate to form sucrose phosphate and ultimately sucrose.