Monosaccharide components

Sections Disaccharides are carbohydrates in which two monosaccharides are 25.14-25.15 joined by a glycoside bond. Polysaccharides have many monosaccharide units connected through glycosidic linkages. Complete hydrolysis of disaccharides and polysaccharides cleaves the glycoside bonds, yielding the free monosaccharide components. 

FIGURE 7.21 Amylose and amylopectin are the two forms of starch. Note that the linear linkages are o (1 4), but the branches in amylopectin are o (1 6). Branches in polysaccharides can involve any of the hydroxyl groups on the monosaccharide components. Amylopectin is a highly branched structure, with branches occurring every 12 to 30 residues. 

Many are neutral glucans, being composed of the monosaccharide component... 

Brand-Miller, J., Bull, S., Miller, J., and McVeagh, P. (1994). The oligosaccharide composition of human milk Temporal and individual variations in monosaccharide components. J. Pediatr. Gastroenterol. Nutr. 19, 371-376. 

Osmotic laxatives (e.g., lactulose, sorbitol) are poorly absorbed or nonabsorbable compounds that draw additional fluid into the GI tract. Lumen osmolality increases, and fluid movement occurs secondary to osmotic pressure. Lactulose is a synthetic disaccharide that is poorly absorbed from the GI tract, since no mammalian enzyme is capable of hydrolyzing it to its monosaccharide components. It therefore reaches the colon unchanged and is metabolized by colonic bacteria to lactic acid and to small quantities of formic and acetic acids. Since lactulose does contain galactose, it is contraindicated in patients who require a galactose-free diet. Metabolism of lactulose by intestinal bacteria may result in increased formation of intraluminal gas and abdominal distention. Lactulose is also used in the treatment of hepatic encephalopathy. 

A characteristic of this stepwise approach is that the chemical transformations on the growing oligosaccharide chain are kept to a minimum. Thus, once the protected monosaccharide components have been prepared, the synthetic manipulations are restricted to coupling followed by deprotection, coupling-deprotection. .. and so on [24],... 

III. Biosynthesis of Monosaccharide Components, and Their Activation for Polymeric-Chain Formation... 

A great diversity of monosaccharide structures is a characteristic feature of bacterial polysaccharide chains, especially of O-specific chains of lipopolysaccharides from Gram-negative bacteria. Several surveys of monosaccharide components of bacterial polysaccharides have been published.95 98... 

Other monosaccharide components (of bacterial polysaccharides) that are structurally related to D-ribose include D-riburonic acid,232 identified in the exocellular polysaccharide produced by a strain of Rhizobium meliloti, and D-arabinose, frequently present as the furanose, in polysaccharides of mycobacterial cell-wall.233,234 L-Xylose235,236 should probably be included in the group, as it may be derived from D-arabinose through epimerization at C-4. Biosynthesis of these monosaccharides was not investigated. 

In this group of monosaccharide components of bacterial polysaccharides, a primary glycosyl nucleotide is ADP-D-g/ycero-D-manno-heptose (9), identified in extracts of a mutant strain of Shigella sonnei.231... 

Two of the most frequent monosaccharide components of bacterial polymers belonging to this group have been the subjects of articles in this Series. They are 3-deoxy-D-manno-2-octulosonic acid,247 a normal constituent of the core region of bacterial lipopolysaccharides that is also present in some other polymers, and N-acetylneuraminic acid,248 found in several capsular polysaccharides. Enolpyruvate phosphate serves as the precursor of the C-l-C-3 fragment of the monosaccharides, with D-arabinose 5-phosphate or 2-acetamido-2-deoxy-D-mannose 6-phosphate being an acceptor for transfer of the three-carbon unit. Characteristic, activated forms of these monosaccharides are the CMP derivatives. 

Table IV, which briefly summarizes the material described in previous sub-sections, shows the total of the different monosaccharide components identified in bacterial polysaccharides, and our present knowledge about their activated forms. It may be seen that identification of the activated forms has been achieved for only approximately half of the monosaccharides known to be involved. The most striking gap in the information available is the lack of data about the activated forms of D-ribose-derived monosaccharides and of most of the D-fructose-derived aldoses having configurations other than gluco, galacto, and manno.

The list of the most common monosaccharide components of bacterial polysaccharides, shown in Table VI, includes the sugars that were found at the nonreducing end of nine or more different disaccharide fragments of the polymers. The right-hand column of the Table, which shows the total number of such fragments, gives an estimate of the number of different glycosyltransferases that use the activated form of the monosaccharides as... 

Abbreviations for biosynthetic types are composed from abbreviations of nucleoside residues of activated forms of the monosaccharide components. 6 See footnote a to Table VI lin. and br. mean linear and branched polysaccharides. c Including other bacterial amphi-philes of Gram-negative, outer membranes. d Including amphiphiles of Gram-positive cell-membranes. 

N. K. Kochetkov, Unusual monosaccharides Components of O-antigenic polysaccharides of microorganisms, Russian Chem. Rev., 65 (1996) 735-768. 

Extracellular polysaccharides are produced by some species of the genus Rhodella, but their composition seems to be even more complex than that of Porphyridium. In addition to monosaccharide components just mentioned, rhamnose and 3-O-methyl-xylose were found in the mucilage of R. maculata.723 724 An extremely complex proteoglycan was shown to be produced by Rhodella grisea.725... 

Of theoretical and practical value has been the use of anti-carbohydrate antibodies to determine the d or l configurations of monosaccharides. Antibodies generated with the d enantiomer of the sugar in the antigen are inhibited by the d enantiomer of the monosaccharide, but not by the l. The reverse holds for antibodies generated with the l enantiomer of the monosaccharide component in the antigen. Some representative... 

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