Introduction to chemistry of carbohydrates

1.2. Disaccharides

Disaccharides are sugars produced when two monosaccharides are covalently bonded together by glycosidic linkages. 

Glycosidic bond is formed when the hydroxyl group on one of the sugars reacts with the anomeric carbon on the second sugar.  

Examples of monosaccharides include; sucrose, maltose, and Lactose

Biologically Important Disaccharides 

Maltose (glucose + glucose)

Maltose contains two D-glucose residues joined by a glycosidic linkage between OH at the first carbon atom of the first glucose residues and OH at the fourth carbon atom of the second glucose forming a α-(1,4) glycosidic linkage. 

Maltose is the major degradative product of Starch. Maltose is hydrolysed to two molecules of D-glucose by the intestinal enzyme maltase, which is specific for the α- (1, 4) glycosidic bond.

Lactose (galactose + glucose)

Lactose is a disaccharide of β-D galactose and β-D- glucose which are linked by β-(1,4) glycosidic linkage. Lactose acts as a reducing substance since it has a free carbonyl group on the glucose. It is found exclusively in milk of mammals (Milk sugar).

Sucrose (Cane sugar) (glucose + fructose)

Sucrose is a disaccharide of α-D-glucose and β-D-fructose. It is obtained from cane sugar and present in various fruits. 

In contrast to other disaccharides, sucrose contains no free anomeric carbon atom. Since the anomeric carbons of both its component monosaccharide units are linked to each other. For this reason, sucrose is non reducing sugar.