Disaccharide

A disaccharide (also called a double sugar or biose) is a sugar formed when two monosaccharides are joined by glycosidic linkage. It is therefore a dimer, with a polymerization degree of 2, even if the two component sugars are not the same type. Like monosaccharides, disaccharides are simple sugars and generally soluble in water. Common examples include sucrose (glucose–fructose), lactose (galactose–glucose), and maltose (glucose–glucose). In living organisms, disaccharides are widely used for structural and energy storage, and may be further polymerized by glycosyltransferases to create polysaccharides of repeating disaccharide units, such as glycosaminoglycans or amylose. Disaccharides are one of the four chemical groupings of carbohydrates (monosaccharides, disaccharides, oligosaccharides, and polysaccharides). The most common types of disaccharides—sucrose, lactose, and maltose—have 12 carbon atoms, with the general formula C12H22O11. The differences in these disaccharides are due to atomic arrangements within the molecule and the type of glycosidic linkage. The joining of monosaccharides into a double sugar happens by a condensation reaction, which involves the elimination of a water molecule from the functional groups only. Breaking apart a double sugar into its two monosaccharides is accomplished by hydrolysis with the help of a type of enzyme called a disaccharidase. As building the larger sugar ejects a water molecule, breaking it down consumes a water molecule. These reactions are vital in metabolism. Each disaccharide is broken down with the help of a corresponding disaccharidase (sucrase, lactase, and maltase).