Polysaccharides heteropolysaccharides

Xanthan gum [11138-66-2] is an anionic heteropolysaccharide produced by several species of bacteria in the genus Aanthomonas A. campestris NRRL B-1459 produces the biopolymer with the most desirable physical properties and is used for commercial production of xanthan gum (see Gums). This strain was identified in the 1950s as part of a program to develop microbial polysaccharides derived from fermentations utilizing com sugar (333,334). The primary... 

Polysaccharides composed of only one kind of monosaccharide are described as homopolysaccharides (homoglycans). Similarly, if two or more different kinds of monomeric unit are present, the class name heteropolysaccharide (heteroglycan) may be used. (See 2-Carb-39.)... 

Heteropolysaccharides contain two or more different monosaccharides. Glycosaminoglycans are polysaccharides that contain aminosugar units. Most of them are of animal origin. 

Most of the biochemical studies on polysaccharide synthesis to date have been concerned with the formation of homopolymers even when it is known that the synthesis of the homopolymer chain occurs in vivo as part of a heteropolysaccharide (4-6). Cytochemical investigations have made no such distinctions and the polymers located by these studies have nearly always been sites at which heteropolymers were present and where deposition in the wall occurred. The bulk of the polysaccharides that occur in the wall, with the exception of cellulose and callose, are heteropolymers. Generally the polysaccharides of the hemicelluloses and pectins are composed of poly-... 

Synthesis. The synthases are present at the endomembrane system of the cell and have been isolated on membrane fractions prepared from the cells (5,6). The nucleoside diphosphate sugars which are used by the synthases are formed in the cytoplasm, and usually the epimerases and the other enzymes (e.g., dehydrogenases and decarboxylases) which interconvert them are also soluble and probably occur in the cytoplasm (14). Nevertheless some epimerases are membrane bound and this may be important for the regulation of the synthases which use the different epimers in a heteropolysaccharide. This is especially significant because the availability of the donor compounds at the site of the transglycosylases (the synthases) is of obvious importance for control of the synthesis. The synthases are located at the lumen side of the membrane and the nucleoside diphosphate sugars must therefore cross the membrane in order to take part in the reaction. Modulation of this transport mechanism is an obvious point for the control not only for the rate of synthesis but for the type of synthesis which occurs in the particular lumen of the membrane system. Obviously the synthase cannot function unless the donor molecule is transported to its active site and the transporters may only be present at certain regions within the endomembrane system. It has been observed that when intact cells are fed radioactive monosaccharides which will form and label polysaccharides, these cannot always be found at all the membrane sites within the cell where the synthase activities are known to occur (15). A possible reason for this difference may be the selection of precursors by the transport mechanism. 

Proteoglycans are widely distributed throughout the body, being most abundant in connective tissue, where they may contribute up to 30% of that tissue s dry weight. They consist of a polypeptide backbone to which heteropolysaccharide chains are attached. However, unlike glycoproteins, proteolycans consist of up to, or in excess of, 95% carbohydrate and their properties resemble those of polysaccharides more than those of proteins. 

FIGURE 7-13 Homo- and heteropolysaccharides. Polysaccharides may be composed of one, two, or several different monosaccharides, in straight or branched chains of varying length. 

The involvement of glycolipid and glycoprotein intermediates in the synthesis of polysaccharides from glycosyl-nucleotides in plants is considered to be a likely possibility. Such intermediates could act as specific primers, or acceptor substrates, for the formation of polysaccharides. Furthermore, subunits of complex heteropolysaccharides could be assembled on such intermediates, and later incorporated into polysaccharides, or directly cross-linked into the cell wall. Evidence of the involvement of such intermediates in the synthesis of polysaccharides in a number of organisms is presented in Sections XII,3,b and XII,3,c. 

Comparison of the activity of sulfated homopolysaccharides such as dextran and cellulose esters with that of neutral homopolysaccharides and sulfated heteropolysaccharides such as heparin and heparan sulfuric acid half esters shows potent virucidal activity against human T-cell lymphotropic virus type III (HTLV-III) for the sulfated homopolysaccharides. In contrast, neutral homopolysaccharides have no effect and sulfated heteropolysaccharides exhibit only a little effect on HTLV-III activities. This suggests that the sulfate moiety and the type of polysaccharide are most important in inhibiting growth of HTLV-III [126]. 

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