DISACCHARIDES

• When the alcohol component of a glycoside is provided by a hydroxyl function on another monosaccharide, the compound is called a Dissacharide.
• Ex. Cellulose : 4-O-β-D-Glucopyranosyl-D-Glucose(the beta-anomer is drawn)
• Maltose : 4-O- α-D-Glucopyranosyl-D-Glucose (the beta-anomer is drawn)
• Gentiobiose : 6-O-β-D-Glucopyranosyl-D-Glucose (the alpha-anomer is drawn)
• Trehalose : α-D-Glucopyranosyl-α-D-Glucopyranoside
• Although all the disaccharides shown here are made up of two glucopyranose rings, their properties differ in interesting ways.
• Maltose, sometimes called malt sugar, comes from the hydrolysis of starch. It is about one third as sweet as cane sugar(Sucrose),is easily digested by humans, and is fermented by yeast. 
•  Cellobiose is obtained by the hydrolysis of cellulose.
• It has virtually no taste, is indigestible by humans, and is not fermented by yeast.
•  Some bacteria have beta-glucosidase enzymes that hydrolyse the glycosidic bonds in cellobiose and cellulose. 
• The presence of such bacteria in the digestive tracts of cows and termites permits these animals to use cellulose as a Food. 
• Finally, it may be noted that trehalose has a distinctly sweet taste, but gentiobiose is bitter.

Figure. A four examples of disaccharides composed of two glucose units are shown

• Disaccharides made up of other sugars are known, but glucose is often one of the components. 
• Two important examples of such mixed disaccharides will be displayed above by clicking on the diagram.
• Lactose, also known as milk sugar, is a galactose-glucose compound joined as a beta-glycoside.

Figure. Lactose is a disaccharide found in milk.

• It is a reducing sugar because of the hemiacetal function remaining in the glucose moiety. Many adults, particularly those from regions where milk is not a dietary staple, have a metabolic intolerance for lactose.
• Infacts have a digestive enzyme which cleaves the beta-glycoside bond in lactose, but production of this enzyme stops with weaning.
• Cheese is less subject to the lactose intolerance problem, since most of the lactose is removed with the whey.
• Sucrose, or cane sugar, is our most commonly used sweetening agent. It is a non-reducing disaccharide composed of glucose and fructose joined at the anomeric carbon of each by glycoside bonds(one alpha and one beta).
• In the formula shown here the fructose ring has been rotated 180° from its conventional perspective.