70 GENERAL BIOCHEMISTRY 



Amino Sugars 



The chemistry and biochemistry of this group of compounds are 

 under intensive investigation and much is being learned. Undoubtedly 

 many amino sugars will be discovered and their biological roles 

 worked out. 1 wo are now known to be very widely distributed, espe- 

 cially in the animal kingdom. These are typical examples with a hy- 

 droxyl group replaced by an amino group. In both, the amino group 

 is on carbon two, and the configuration is the same as for the parent 

 sugar. D-Glucosamine is a reducing compound and shows mutarota- 

 tion. It is found in bacteria and in polymeric form as chitin with the 

 amino groups acetylated. Chitin is the principal component of the 

 exoskeletons of insects and crustaceans. 



D-Glucosamine, sometimes called chitosamine, also occurs in mu- 

 coitin sulfuric acid, a macromolecule made up of alternating units of 

 glucuronic acid and glucosamine linked by 1-4 bonds. Some of the 

 amino groups are acetylated; others exist as sulfamic acids, — NH — 

 SO3H. Mucoitin sulfuric acid is associated with proteins in saliva and 

 other mucous secretions, and one form, heparin, is used to prevent the 

 clotting of blood. Acetyl glucosamine is one of the constituents of 

 hyaluronic acid. This acidic substance also contains glucuronic acid 

 and is a major component of certain viscous fluids, such as the vitreous 

 humor of the eye, synovial fluid of the joints, the jelly surrounding 

 the ovum, and the umbilical cord. 



The other abundant amino sugar is D-galactosamine, also called 

 chondrosamine. It is found in certain bacterial polysaccharides but is 

 best known for its occurrence in chondroitin sulfuric acid, where it 

 appears to alternate with glucuronic acid with 1-4 linkages analogous 

 to the structure of mucoitin sulfuric acid. Small amounts of chon- 

 droitin sulfuric acid appear in the mucous secretions. The material 

 is especially plentiful in skin and cartilage, from which it is readily 

 isolated. 



RELATED SUBSTANCES 



A number of materials related to or containing carbohydrates have 

 already been discussed briefly. Among these are chondroitin and 

 mucoitin sulfuric acids, hyaluronic acid, and chitin. Other groups of 

 compounds are sufficiently widespread and biologically important to 

 warrant mention. 



