Disaccharides

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 16:21

The sugarA small carbohydrate that either contains five or six carbon atoms or is a dimer of two units, each containing five or six carbon atoms. molecules listed in Fig. 1 are usually referred to as monosaccharides. This distinguishes them from the disaccharides which are made up by condensing two sugar units.

Figure 1 The linear form of some important monosaccharides: (a) ribose; (b) fructose; (c) mannose; (d) glucose.
A familiar example of a disaccharide is ordinary cane sugar, sucrose, which may be obtained by condensing a molecule of α-glucose with one of the cyclic forms of fructose called β-fructose. The structure of sucrose is shown in Fig. 2.
Figure 2 The formation of sucrose from glucose and fructose.'

Other, less familiar, examples of disaccharides are lactose, which occurs in milk, and maltose, which are shown in Fig. 3. In order to digest a disaccharide like sucrose or lactose, the human body must have an enzymeA highly effective, highly specific biochemical catalyst; usually a protein, but RNA enzymes also exist. which can catalyze hydrolysisAny reaction in which water (hydro) is split into two parts (lysis). Examples include the reaction of an anion with water to form the conjugate acid and hydroxide ion and hydrolysis of an ester or amide, in which the H from water bonds to form an alcohol or amine and the OH bonds to a carbonyl carbon to form a carboxylic acid. of the linkage between the two monosaccharide units. Many Asians, Africans, and American Indians are incapable of synthesizing lactase, the enzyme that speeds hydrolysis of lactose. If such persons drink milk, the undigested lactose makes them sick.

Figure 3 Two other common disaccharides, maltose and lactose.