Synthesis of Heteropolysaccharides

D-Gluco-D-mannans are plant cell-wall constituents which are considered to belong to the hemicellulose groups that are closely associated with cellulose. 

The insoluble polysaccharide was characterized by examination of the soluble, G-labeled oligosaccharides isolated by a variety of treatments, including enzymic hydrolysis, partial hydrolysis with acid, or acetolysis. From the polymer containing D-mannose- C, several oligosaccharides were isolated having various ratios of D-mannose to D-glucose, from 1 1 to 3 or 4 1. One of these oligosaccharides was a disaccharide tentatively identified as an 0-/3-D-man- 

Based on the ratios of D-mannose to D-glucose in the larger oligosaccharides, the polymer formed from GDP-D-mannose- C or GDP-d-glucose- C (in the presence of D-mannose) appears to be a /8-D-(1 — 4)-linked D-gluco-D-mannan containing 3 or 4 D-mannose residues per D-glucose residue. 

An exploration of the sugar nucleotides in this alga disclosed the presence of UDP-D-galactose, GDP-L-galactose, UDP-D-glucose, GDP-D-mannose, and UDP-D-glucuronic acid. Moreover, AMP, UMP, GMP, IMP, ADP, UDP, IDP, NAD , NADP , and a new nucleotide, adenosine 3, 5 -pyrophosphate, were isolated. 

Inasmuch as GDP-L-galactose is found together with GDP-D-mannose in this red alga, it is suggested that the formation of l-galactose nucleotide probably occurs by a mechanism similar to 

---

Finally, details of the synthesis of heteropolysaccharides in plants are as yet completely unknown. The structural similarities among some plant gums and such bacterial exopolysaccharides as xanthan gum suggest that similar mechanisms may be operative in bacteria and in plants. Lipid intermediates could be suggested as potential glycosyl donors in the formation of plant gums and mucilages. 

The synthesis of heteropolysaccharides is somewhat more complex than that of homopolysaccharides, because at each step a different primer is required and a different activated monosaccharide is added. 

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-... 

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. 

Peptidoglycans are components of bacterial cell walls and consist of heteropolysaccharide chains cross-linked by short peptide chains. These cell walls bear the antigenic determinants when exposed to them, humans (and other mammalian species) develop specific antibodies to defend against bacteria. Bacterial virulence is also related to substances associated with the cell wall. Cell wall synthesis is the target for the action of the penicillins and cephalosporins. 

Wheat farming produces two abundant by-products, straw and bran, the dry matter of which contains more than 25% of hemicellulose, a heteropolysaccharide made up mainly of two aldopentoses o-xylose, and L-arabinose (Fig. 1). These two 4-epi-meric sugars are obtained by hydrolysis of hemicellulose. They constitute polar and enantiopure synthons used in the synthesis of surfactants and fine organic chemicals. A third important synthon, furfural, results from dehydration of pentoses. Hemi-celluloses are the second most abundant natural organic chemicals next to cellulose. 

The assembly line principle is used in the synthesis of fatty acids, heteropolysaccharides and the carbohydrate side chains of glycoproteins. Here one unit is added after another according to the dictates of an enzyme array. In the carbohydrate-containing compounds the length of the chain and the numbers and types of sugar incorporated are generally related to the number of enzymes in the array and a unit can only be introduced after the preceding enzyme has operated. 

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. 

Carbohydrates in algae and plants are often classified based on methodological discrimination. The structural carbohydrates are not water-soluble, whereas the other types of carbohydrates are water-soluble and typically extracted by hot water. In Phaeocystis five different pools of carbohydrates can be distinguished. Like all algal and plant cells, both solitary and colonial cells produce (1) structural carbohydrates, polysaccharides that are mainly part of the cell wall, (2) mono- and oligosaccharides, which are present as intermediates in the synthesis and catabolism of cell components, and (3) intracellular storage glucan. Colonial cells of Phaeocystis excrete (4) mucopolysaccharides, heteropolysaccharides that... 

---