Polysaccharides branched-chain monosaccharides

Eur. J. Biochem. (1980) 111, 295-298 Nomenclature of unsaturated monosaccharides Eur. J. Biochem. (1981) 119, 1-3 Nomenclature of branched-chain monosaccharides Eur. J. Biochem. (1981) 119, 5-8 Abbreviated terminology of oligosaccharide chains /. Biol Chem. (1982) 257, 3347-3351 Polysaccharide nomenclature /. Biol Chem. (1982) 257, 3352-3354 Symbols for specifying the conformation of polysaccharide chains... 

Branched-chain monosaccharides have now been detected as components of bacterial polysaccharides. The known examples include yersiniose [3,6-dideoxy-4-C-(hydroxyethyl)-D-xy/o-hexose228] from Y. pseudotuberculosis, a 3-C-(hydroxymethyl)pentofuranose from Coxiella bumetti,229 and 6-deoxy-3-C-methylhexoses from the same organism and from Nitrobacter hamburgiensis.229 Several branched-chain monosaccharides were identified as components of antibiotics, and the pathways of their biosynthesis in bacteria were studied. These investigations were discussed in detail by Grisebach in this Series.230 The usual precursors for the formation of the monosaccharides of this group are the nucleoside 6-deoxyhexosyl-4-ulose diphosphates 7a and 7b. 

Polysaccharides that are formed from only one type of monosaccharide are called homo-glycans, while those formed from different sugar constituents are called heteroglycans. Both forms can exist as either linear or branched chains. 

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. 

Monomers and polymers of carbohydrates, (a) The most common carbohydrates are the simple six-carbon (hexose) and five-carbon (pentose) sugars. In aqueous solution, these sugar monomers form ring structures, (b) Polysaccharides are usually composed of hexose monosaccharides covalently linked together by glycosidic bonds to form long straight-chain or branched-chain structures. 

Naturally occurring polysaccharides can occur either as individual carbohydrate molecules or in combination with other naturally occurring substances, such as proteins (glycoproteins) and lipids (glycolipids). In all cases the polysaccharide section may have linear or branched chain structures, which often contain the derivatives of both monosaccharides and aminosugars (Figure 1.20). 

This group includes a set of anionic polysaccharides secreted by unrelated bacteria. The common theme, however, is that their main chains have the same tetrasaccharide repeat. Although (high acyl) native gellan (42) and (deacylated) gellan (40,41) are linear polymers, welan (43) is a branched polymer in which a monosaccharide side chain is regularly attached to each repeat. Other members of this family, such as S-657 and rhamsan, are also branched, like welan."0... 

The bulk of all carbohydrates in nature exists in the form of polysaccharides. These are very large molecules formed by linking together long chains of monosaccharide units. These chains may be linear, like polypeptides or polynucleotides, or branched. They may contain a single type of monosaccharide unit, similar to polyglycine or polyA for example, or two or more types of monosaccharide, like nucleic acids (four types of nucleotides) or proteins (20 types of amino acids). However, polysaccharides that contain more than two types of monosaccharide are rare in nature. 

In the case of branched polysaccharides, incorporation of side chains may take place through different mechanisms. In one of them, the assembly of the main chain is independent of the presence of side chains, and their incorporation into a polymeric molecule occurs as a modification of an initially formed, linear polysaccharide. Another situation is possible when incorporation of monosaccharide residues present in side chains is a necessary condition for elongation of the main chain, either through the monomeric or the block mechanism that is, intermediate formation of a linear, polysaccharide chain does not occur. Both mechanisms of incorporation of side chains were demonstrated to take place. 

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. 

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