Elementary plant Biochemistry and Biotechnology (2+1)

Structure of Disaccharides & Polysaccharides

Composition, sources and properties of common disccharides

 
Disaccharides	Constituent monosaccharides	Linkage	Source	Properties
Reducing disaccharides
Maltose	a-D-glucose+ a-D-glucose	a(1à4)	Germinating cereal and malt	Forms osazone with phenylhydrazine. Fermentable by enzyme maltase present in yeast. Hydrolysed to two molecules of D-glucose. Undergoes mutarotation.
Lactose	b-D-glucose+ a-D-glucose	b(1à4)	Milk. In trace amounts it can be seen in urine during pregnancy	It shows reactions of reducing sugars including mutarotation. Decomposed by alkali. Not fermentable by yeast. Hydrolysed to one molecule of galactose and one molecule of glucose by acids and the enzyme lactase.
Non-reducing disaccharides
Sucrose	a-D-glucose+ b-D-fructose	a,b(1à2)	Sugar beet, sugarcane, sorghum and carrot roots	Fermentable. Hydrolysed by dilute acids or enzyme invertase (sucrase) to one molecule of glucose and one molecule of fructose. Relatively stable to reaction with dilute alkali.
Trehalose	a-D-glucose+ a-D-glucose	a,a(1à1)	Fungi and yeast. It is stored as a reserve food supply in insect's hemolymph	It is hydrolysable by acids to glucose with difficulty. Not hydrolysed by enzymes.


• The oligosaccharides commonly encountered in nature belong to disaccharides
• The physiologically important disaccharides are maltose, lactose, trehalose and sucrose
• Disaccharides consist of two monosaccharides joined covalently by an O-glycosidic bond.
• The hydroxyl group formed as a result of hemiacetal formation is highly reactive when compared to other hydroxyl groups.
• This hydroxyl group present in one monosaccharide reacts with any one of the hydroxyl groups attached to C-1, C-2, C-3, C-4, or C-6 of another monosaccharide to produce 1→1, 1→2, 1→3, 1→4, and 1→6 linked disaccharides
• When only one anomeric carbon is involved in glycosidic bond formation, reducing disaccharides are formed
• If both anomeric carbon atoms of monosaccharides are involved in glycosidic bond formation that results in the formation of a non-reducing disaccharides such as trehalose (aldosyl-aldosyl disaccharide) or sucrose (aldosyl-ketosyl disaccharide)
• In the case of reducing disaccharides, one end of the molecule having free anomeric carbon is called reducing end and the other end, where the anomeric carbon is involved in glycosidic bond, is called as non-reducing end