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.