mucopolysaccharide (myōˌkəpŏlēsăkˈərĪd) [key], class of polysaccharide molecules, also known as glycosaminoglycans, composed of amino-sugars chemically linked into repeating units that give a linear unbranched polymeric compound. The monomeric amino-sugar constituents are ordinary monosaccharides that contain a nitrogen atom covalently bound to one of the ring carbons of the sugar portion. The nitrogen is, in turn, either bonded to two atoms of hydrogen (termed a primary amino-group) or to another carbon atom (hence, a substituted amino-group). The mucopolysaccharides are quite similar structurally to the more well-known animal and plant polysaccharides such as glycogen and starch. Chitin is a particularly plentiful mucopolysaccharide and serves, like cellulose does in plants, as a structural polysaccharide for many phyla of lower plants and animals. The shells of lobsters, crayfish, crabs, insects, and many other invertebrate organisms contain mostly chitin complexed with inorganic salts. The copepods, a group of microscopic marine organisms of the subphylum Crustacea, alone are considered to synthesize about 109 tons of chitin per year. Chitin is probably the second most abundant large organic compound on earth (the first being cellulose). Heparin, an anticoagulant used widely in the treatment of blood clotting disorders, such as pulmonary embolus, is a mucopolysaccharide. Another important compound of this class is hyaluronic acid, a molecule found universally in the connective tissues of animals and in the fluids of their eyes and joints. Hyaluronic acid in association with protein has been isolated from various organisms, and such complexes are thought to bind water in the cellular spaces, thus holding cells together in a jellylike matrix. In addition, such substances may provide the fluids of joints with lubricating and shock-absorbing qualities. Many other mucopolysaccharides are, like hyaluronic acid, associated with proteins; the separation between such proteoglycans and glycoproteins is somewhat arbitrary, but the latter compounds are distinguished by their relative paucity of sugars.