Compounds Formed from Sugar and Hydroxyl Groups | Carbohydrates

The following points highlight the two main compounds formed from sugar and hydroxyl groups. The compounds are: 1. Glycosides 2. Amino Sugars (Hexosamines).

Compound # 1. Glycosides:

Glycosides are compounds formed by the conden­sation reaction between a sugar and the hydroxyl group of a second compound or aglycone which may or may not be another sugar. If the carbohy­drate portion is glucose, the resulting compound is a glucoside: if galactose, a galactoside, etc.

All reducing sugars will condense with dry methyl alcohol under catalytic action of dry HCl to form glycosides. Thus, glucose forms α- and β- methyl glucosides. The aglycone may be -OH containing sub­stances such as methyl alcohol, glycerol, a sterol, a phenol or another sugar (as in the disaccharides).

Importance:

(i) Glycosides are found in many drugs, spices and in the constituents of animal tissues.

(ii) The glycosides are important in medicine because of their action on the heart (car­diac glycosides). All the glycosides con­tain steroids as the aglycone component.

(iii) The glycosides include derivatives of digi­talis and strophanthus such as Ouabain, which is an inhibitor of the Na⁺– K⁺ ATPase of cell membranes.

(iv) Other glycosides include antibiotics such as streptomycin.

Compound # 2. Amino Sugars (Hexosamines):

Sugars containing an amino group are called amino sugars. Examples are D-Glucosamine, D-Galactosamine, and D-Mannosamine, all have been identi­fied in nature. These amino sugars are formed by the replacement of hydroxyl group attached to car­bon atom 2 of the sugar by an amino group. The structure of an amino sugar is given in Fig. 3.13.

Importance:

(i) Glucosamine is a constituent of hy­aluronic acid. It is the organic constituent of lobster shell and fairly distributed in nature.

(ii) Galactosamine is a constituent of glycoproteins and of chondroitin as the N-acetyl derivative.

(iii) Mannosamine is an important constituent of mucoprotein.

(iv) Several antibiotics (erythromycin, carbomycin) contain amino sugars.

Erythromy­cin contains a dimethyl-amino sugar. Carbomycin contains the first known 3-amino sugar, 3-amino-D-ribose. The amino sug­ars are believed to be related to the antibi­otic activity of these drugs.