Polysaccharides homopolysaccharides

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.)... 

Three 3-deoxynonulosonic acids containing amino groups are known. The most abundant of these is 5-amino-3,5-deoxy-D- /yc ro-D-ga/acto-nonulosonic acid (neuraminic acid, 27), which occurs in different extracellular polysaccharides. Some of these, like colominic acid from E. coli K1, are homopolysaccharides. Neuraminic acid is generally A-acetylated and, as in the animal glycoconjugates, has only been found in the a-pyranosyl form (27). It also occurs in some LPS, for example those from some Rhodobacter... 

Unperturbed dimensions are calculated for several 1,3- and 1,4-linked homopolysaccharides. The characteristic ratio depends strongly on the bonding geometry and on the degree of rotational freedom about the glycosidic bonds. It appears that unperturbed dimensions can be predicted qualitatively from a knowledge of the regular conformations of the polysaccharide. 

Chitin is a linear homopolysaccharide composed of Af-acetylglucosamine residues in /3 linkage (Fig. 7-18). The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acety-lated amino group. Chitin forms extended fibers similar to those of cellulose, and like cellulose cannot be digested by vertebrates. Chitin is the principal component of the hard exoskeletons of nearly a million species of arthropods—insects, lobsters, and crabs, for example— and is probably the second most abundant polysaccharide, next to cellulose, in nature. 

The monomeric mechanism of chain assembly is characteristic for homopolysaccharides, which constitute the most difficult case for biosynthetic studies, as accumulation of intermediates could not be induced by removal of one of the glycosyl donors required for chain elongation. Among these polymers, the most extensive information has been obtained for polymers of JV-acetylneuraminic acid, namely, the capsular polysaccharides (36 and 37) of E. coli K1 (Ref. 354) and Neisseria meningitidis type c (Ref. 355), respectively. CMP-NeuAc serves as the glycosyl donor in the formation... 

Figure 1 represents the general structure of Salmonella lipopolysaccharides. They contain an external polysaccharide, the O-antigenic chain, and an innermost component, termed lipid A. O-chain and lipid A are linked to each other by an oligosaccharide referred to as the core. O-Specific Chains. As indicated in Figure 1, O chains are in general made up of repeating units of di-, tri-, or higher oligosaccharides. In rare cases the O-chain is a homopolysaccharide. The structure of the O-chain is unique to each bacterial serotype great diversity is encountered in the structures of O-chains.

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]. 

Many studies have been conducted to investigate the in vitro antiviral activity of various SPs. Studies have generally concentrated on synthetic dex-tran sulfates (DSs), pentosan sulfates, clinically used heparins, and seaweed-derived carrageenans. Some reviews reported that sulfated homopolysaccharides are more potent than sulfated heteropolysaccharides [2,3]. In general, polysaccharides exhibiting antiviral potential are highly sulfated [4]. 

In order to provide access here to information about other microbial polysaccharides, there follows a short review of reviews. The Chemical Nature of Bacterial Antigens is a source of information on the earlier work, and this was followed by two reviews - in the present Series in one of these, the bacterial homopolysaccharides were discussed and some of the more complex products were mentioned. A wide review of mucopolysaccharides and mucoproteins included references to many bacterial polysaccharides, and, subsequently, an account of the specific polysaccharides of the Gram-positive Pneumococcus, and of dextrans, levans, and products of Gram-negative forms appeared. There followed a comprehensive review of pneumococcal polysaccharides, and an account restricted to products of Mycobacterium tuberculosis appeared in 1948. A broad coverage was provided in 1950, in this case the products of pathogenic species being dealt with more particularly this work encompassed the basis of the more... 

Homopolysaccharides have a single type of residue. Most common polysaccharides contain glucose which is used for energy (food), storage (starches and glycogen) and structure (cellulose). 

Hemicelluloses are a family of four basic types of polysaccharides, composed of two or more monosaccharide residues. All have structural features similar to cellulose in that they have their main chains that are -(1 4) linked, with the exception of the arabinoglactans that are /3-(l—>3) linked. The main chains are homopolysaccharides composed of a single monosaccharide residue, but they are highly branched by one or two different kinds of monosaccharides that are linked for the most part to give single monosaccharide branches. 

Some natural polysaccharides are homopolysaccharides and they consist of a unique monomeric unit interconnected by identical links. Among these are cellulose, amylose, amylopectin, chitin, and glycogen, which are very common in nature. Polysaccharides can be formed from pentoses or hexoses with different types of ether links. Table 7.1.5 shows the type of links in some natural homopolysaccharides. However, many natural polysaccharides are formed from two or more types of residues and they are heteropolysaccharides. 

Fungal polysaccharides include homopolysaccharides as well as heteropolysaccharides including D-galacto-D-mannans, D-xylo-D-mannans, etc. Scleroglucan is used as an industrial gum. Several homopolysaccharides found in fungi are shown in Table 7.11.1. 

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