Polysaccharides branched-chain sugars

Before 1983, branched-chain sugars had not been found in bacterial polysaccharides, but there are now five examples belonging to this class. The LPS from Coxiella burned phase I contains both 6-deoxy-3-C-methyl-L-gulose (L-virenose) as pyranoside (12) and 3-C-(hydroxymethyl)-L-lyxose as furan-oside (13). Another 6-deoxy-3-C-methylhexose, having the manno configuration, is a component of the Nitrobacter hamburgiensis 0-antigen. ... 

Incubation of D-[U-I4C]apiose with sterile Lemma minor (duckweed) produced less than 0.01% incorporation into the cell-wall polysaccharides.75 Most of the d-[U-i4C]apiose appeared as 14C02 some remained in solution in the medium and in the duckweed plants, primarily as degradation products of D-[U-14C]apiose, but not as the branched-chain sugar.75 There is an efficient synthesis of the [U-14C]apiose moiety of cell-wall polysaccharides from D-[U-14C]glucose under similar conditions.81 Of the plant tissues tested, only L. minor contained an enzyme system able to metabolize free apiose. Carrot, lettuce, and spinach tissues are unable to metabolize the free, branched-chain sugar.75... 

Some saccharides are branched and these types are found as constituents of various natural products. For example, D-apiose occurs widely in plant polysaccharides. Antibiotics produced by the microorganism Streptomyces are another rich source of branched chain sugars. 

In addition to sugars with straight chains, branched chain sugars also exist in nature, for example as components of pectin and other polysaccharides. Again, their preferred names are the trivial names. Aldopentose D-apiose with one chiral carbon atom has the systematic name 3-C-hydroxymethyl-D-g/ycero-tetrose (4-19). 

HemiceUulose is a mixture of amorphous branched-chain polysaccharides consisting of a few hundred sugar residues. They are easily hydrolyzed to monomeric sugars and uronic and acetic acids. Many different hemiceUuloses have been isolated from wood. 

Recent progress of basic and application studies in chitin chemistry was reviewed by Kurita (2001) with emphasis on the controlled modification reactions for the preparation of chitin derivatives. The reactions discussed include hydrolysis of main chain, deacetylation, acylation, M-phthaloylation, tosylation, alkylation, Schiff base formation, reductive alkylation, 0-carboxymethylation, N-carboxyalkylation, silylation, and graft copolymerization. For conducting modification reactions in a facile and controlled manner, some soluble chitin derivatives are convenient. Among soluble precursors, N-phthaloyl chitosan is particularly useful and made possible a series of regioselective and quantitative substitutions that was otherwise difficult. One of the important achievements based on this organosoluble precursor is the synthesis of nonnatural branched polysaccharides that have sugar branches at a specific site of the linear chitin or chitosan backbone [89]. 

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. 

D-Apiose can be isolated from parsley and is a component of the cell wall polysaccharide of various marine plants. Among its novel structural features is the presence of only a single chirality center. L-Vancosamine is but one portion of vancomycin, a powerful antibiotic that has emerged as one of only a few antibiotics that are effective against drug-resistant bacteria. L-Vancosamine is not only a branched-chain carbohydrate, it is a deoxy sugar and an amino sugar as well. 

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. 

Polysaccharide. A linear or branched-chain structure containing many sugar molecules linked by glycosidic bonds. 

A polysaccharide is a straight or branched chain of hundreds or thousands of sugar monomers, of the same or different kinds. Be able to give common examples and their functions. 

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