Carbohydrates- Disaccharides By Dr. Bhushan R. Kavimandan
Disaccharides ofbiological importance: Disaccharides consist of two monosaccharides joined by glycosidic linkages. They are crystalline, water-soluble and sweet to taste. The disaccharides are of two types: 1.Reducing disaccharides: they are with free aldehyde or keto group e.g. maltose, lactose 1.Non-reducing disaccharides: they are with no free aldehyde or keto group e.g. sucrose, trehalose.
Maltose: Maltose or malt sugar (C 12 H 22 O 11 ), as such, is not found in free form in the body. Itis produced during the course of digestion of starch by the enzyme amylase. Maltose is composed of two α-d-glucose units held together by α (1 4) glycosidic bond. The free aldehyde group present on C 1 of second glucose answers the reducing reactions, besides the osazone formations (sunflower-shaped). Maltose can be hydrolysed by dilute acid or enzyme maltase to liberate two molecules of α-d-glucose. Inisomaltose, the glucose units are held together by α (1 6) glycosidic bond. Cellobiose is another disaccharide identical in structure with maltose, except that the cellobiose has β (1 4) glycosidic bond. Cellobiose is formed during the hydrolysis of cellulose.
Sucrose: Sucrose produced by sugar cane and sugar beets. Sucrose is made up of α-d-glucose and β-d-fructose, are held together by glycosidic bond between C 1 α-d-glucose and C 2 β-d-fructose. The reducing groups of glucose and fructose are involved in the glycosidic bond, hence sucrose is non-reducing sugar and can not form osazones. Sucrose is the most abundant among the naturally occurring sugars. It has distinct advantages over other sugars as storage and transport form. This is due to the fact that in sucrose, both the functional groups (aldehydes and keto) are held together and protected from oxidative attacks. It is sweeter than most other common sugars (except fructose) namely glucose, lactose and maltose. α-d-glucosyl (1 2) β-d-fructose
Inversion of sucrose: Sucrose, as such is dextrorotatory (+66.5 o ). But, when hydrolysed, sucrose becomes levorotatory. The process of change in optical rotation from dextro to levo is referred to as inversion. The hydrolysed mixture of sucrose, containing glucose and fructose, is known as invert sugar. The process of inversion is explained as follows: Hydrolysis of sucrose by the enzyme sucrase (invertase) or dilute acid liberates one molecule each of glucose and fructose. It is postulated that sucrose is first split into α-d-glucopyranose (+52.5 o ) and β-d-fructofuranose both being dextrorotatory. However, β-d-fructofuranose is less stable and immediately converted to β-dfructopyranose which is strongly levorotatory (-92 o ). The overall effect is that dextro sucrose on inversion is converted to levo form (-28.2 o ).
Lactose: Lactose is commonly known as milk sugar since it is disaccharide found in milk. It is composed of β-d-galactose and β-d-glucose held together by β (1 4) glycosidic bond. The anomeric carbon of C 1 glucose is free, hence lactose exhibit reducing properties andforms osazones (powder-puff or hedgehog shape). Lactose of milk is the most important carbohydrate in the nutrition of young mammals. It is hydrolysed by intestinal enzyme- lactase to glucose andgalactose. This enzyme is abundant in the early years of life and gradually disappears with age in certain individuals causing lactose intolerance.
Trehalose: It is major constituent of the circulating fluid (hemolymph) of insects, in which it serves asanenergy storage compound. It is also found in yeast and other fungi. Itis composed of two α-d-glucose moieties. Anomeric carbon atoms of both α-d-glucose are held together by glycosidic linkage. Consequently, it resembles sucrose in being non-reducing sugar as it has no free aldehyde group. It does not form osazone. On hydrolysis it yields 2 moles of glucose.