22 UNITY AND DIVERSITY IN BIOCHEMISTRY 



(c) Natural Sugars 



One finds in Nature a selection of sugars, either free or combined. The 

 lower members of this group are a diose, glycollic aldehyde; two trioses, 

 glyceraldehyde and dihydroxy-acetone, and a tetrose, D-erythrose. The 

 pentoses of the biosphere which have been found up to the present are 

 D-xylose, D-ribose, D-ribulose, D-arabinose and L-arabinose. Among the 

 hexoses the commonest is D-glucose. Other aldohexoses in the biosphere 

 are D-mannose, D-galactose and L-galactose. The ketohexose D-fructose is 

 also very common. Two C7 sugars, sedoheptulose and D-mannoheptulose 

 have also been identified. It is clear that the natural sugars are most often 

 members of the D-series. 



(d) Amino Sugars 



Amongst the large number of sjmthetic compounds two are also of 

 natural origin 



H— C==0 H— C=0 



H— C— NH, H— C— NH2 



HO— C— H HO— C— H 



i I 



H— C— OH HO— C— H 



H— C— OH H— C— OH 



CH2OH CH2OH 



Glucosamine Galactosamine 



(2-desoxy-2-amino-P-D-glucose) (2-desoxy-2-amino-p-D-galactose) 



These are aldohexoses aminated in position 2. 



Glucosamine is a constituent of many polysaccharides and mucopoly- 

 saccharides, and of chitin. Galactosamine is a constituent of cartilage. 



(e) Desoxy-Sugars or "Desoses" 



These are the result of the removal of an oxygen atom from a hydroxyl 

 group. There are a certain number of natural desoses, the principal ones 

 being L-rhamnose, L-fucose and 2-desoxyribose or ribodesose. The 

 desoses give most of the reactions of sugars. They are unstable. 



