Homopolysaccharides

Homopolysaccharides contain only one type of monosaccharide, whereas... 

Three main types of homopolysaccharides are known (Figure 7.2). 

Figure 7.2 Three main types of homopolysaccharides, a) Single linkage type, eg curdlan. b) Side chain type, eg scleroglycan. c) Branched chain type, eg dextran. The figure also show various ways of illustrating exopolysaccharida structure.

Exopolysaccharides, in almost every case, are synthesised at tire cell membrane and then exported from tire cell. The only exceptions that have been recorded to date are the homopolysaccharides levan and dextran which are synthesised extracellularly. 

Homopolysaccharides are synthesised by relatively few spedfic enzymes and are not constructed from subunits. The commerdally important homo polymer dextran is synthesised extracellularly by the enzyme dextransucrase. In Leuconostoc mesenteroides the enzyme is induced by the substrate sucrose. This is deaved to release free fructose and link the glucose to the redudng end of the acceptor dextran chain, which is bound to the enzyme. The product from this bacterium is composed almost exdusively of... 

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.)... 

Deoxy-L-mannose (L-rhamnose) is common, but D-rhamnose has only been found in some 0-antigens, as in that of Pseudomonas cepacia IMV 3181, which is a homopolysaccharide containing both a- and y -pyranosyl residues. 

The 0-antigen from Yersinia enterocolitica serotype 2 is a homopolysaccharide composed of 6-deoxy-)8-L-altropyranosyl residues. In the LPS from... 

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... 

Scheme 5.11 Polycondensation of a monosaccharide unit leading to a homopolysaccharide.

Figure 9.16 Structures of glucopyranose homopolysaccharides. Cellulose is a linear structure of the glucopyranose units linked /3( 1 —>4). Starch consists of amylose, which has a linear < (1—>4) structure, and amylopectin, which has a(l—>6) branch points on the linear a(l—>4) chains. Glycogen has a similar structure to amylopectin, but with a greater degree of < ( 1—>6) branching.

The best known homopolysaccharides are derived from o-glucose and known as glucans. Glucose has a number of reactive sites and a wide variety of polymers formed utilizing combinations of these reactive sites are found in nature. We have already visited the two most well-known members of this group—cellulose and starch containing amylose and amylopectin. Here we will visit some other important members. 

Figure 9.1. Dextran is a homopolysaccharide composed exclusively of D-glucose monomers linked predominantly by 1-6 glycosidic bonds (as in the fragment of dextran Ijackbone shown above). Different dextran preparations vary only in molecular mass and their degree of branching...

Pomin, V. H. (2008). Review An overview about the structure-function relationship of marine sulfated homopolysaccharides with regular chemical structures. Biopolymers 91, 601-609. 

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