Showing papers in "Advances in Carbohydrate Chemistry and Biochemistry in 1982"

Chemistry, Metabolism, and Biological Functions of Sialic Acids

Abstract: Publisher Summary This chapter discusses the chemistry, metabolism, and biological functions of sialic acids. Interest in the sialic acids has rapidly increased in recent years, especially because of the reorganization of their involvement in the regulation of a great variety of biological phenomena. The occurrence of sialic acids in plants has never unequivocally been established, although some positive reports exist. The acylneuraminic acids can be released from their glycosidic linkages either by dilute (aqueous or methanolic) acids or sialidases. The unequivocal determination of sialic acids occurring in low concentrations in many biological materials is rather difficult, and this explains the many errors that have been made in this field. The biosynthesis of N-acetylneuraminic acid (Neu5Ac) is briefly reported in this chapter and more attention is given to the enzyme reactions modifying this compound. Little information is available about the role of the modifications of sialic acid resulting in a species- and tissue-specific distribution of different N,O-acylated and O-methylated sialic acids.

The Lipid Pathway of Protein Glycosylation and its Inhibitors: The Biological Significance of Protein-Bound Carbohydrates

Abstract: Publisher Summary This chapter focuses on the inhibitors of proteins glycosylation acting at the level of lipid-linked saccharides formation, compounds that interfere with stages after the transfer of oligosaccharides to the protein. Dolichols are long-chain poly(isoprenol)s found in eukaryotes only and consist of 13–22 isoprene units. The inhibitors of lipid-dependent glycosylation of proteins are also antiviral and antibacterial agents. Glycoproteins are widely distributed in nature, being found in the cytoplasm of cells and in plasma membranes, cell walls, secretions, mucins, and body fluids. Information on the significance of carbohydrate chains of interferon—the protective agent against a number of virus diseases—has been obtained by using the inhibitors of glycosylation during its synthesis. Many aspects of the social behavior of cells are determined by the composition, arrangement, and interaction of cell-surface molecules. Although 2-deoxy- d -arabinose-hexose has been shown mainly to affect glycosylation (in the systems studied so far), its mode of action in more complex, biological systems may not always depend on its well known property.

The Synthesis of Sugars from Non-Carbohydrate Substrates

Abstract: Publisher Summary This chapter describes the syntheses of sugar-type compounds containing four or more carbon atoms, an aldehyde or a ketone group, and a minimum of two hydroxyl groups (or their equivalents, such as amino or thiol groups) with at least one of them being bound to a center of chirality. Among the variety of noncarbohydrate precursors, acetylenes and alkenes have found wide application as substrates for the synthesis of monosaccharides. The parent compounds of all pyranoses—the 2H - and 4H -pyrans—and their derivatives are impractical as substrates for the synthesis of sugars. The formation of furan derivatives in acid-catalyzed dehydrations of carbohydrate substrates is a well known reaction. 1,2-Ethenediyl carbonate (1,3-dioxol-2-one; vinylene carbonate) is a readily available, versatile synthon having pronounced dienophilic properties. For the synthesis of optically active sugars, a number of natural compounds have been successfully employed. The syntheses of sugars described in this chapter demonstrate the general utility of starting materials employed as the precursors of various sugars of desired structure.

Biosynthesis and catabolism of glycosphingolipids.

Abstract: Publisher Summary This chapter focuses on the advances made in the biosynthesis and catabolism of sugar chains in glycosphingolipids during the past two decades. Glycosphingolipids in animal cells have been shown to reflect tissue and species specificity. The incubation mixture for the biosynthesis of glycosphingolipids usually includes an enzyme preparation, a glycosyl donor, a glycolipid acceptor, a detergent, and metal ions. In vitro detection of the transfer of a glycosyl group from a sugar nucleotide to an exogenous, glycosphingolipid acceptor by an enzyme preparation does not necessarily imply that such a reaction actually occurs in vivo . The accumulation of partially degraded, complex carbohydrates in a variety of storage diseases has drawn attention to the roles of glycosidases in the catabolism of glycoconjugates. However, very little is known about the specificities of various glycosidases toward different glycoconjugates. Also, very little information has been obtained about the control and regulation of the catabolism of glycosphingolipids. The discovery of protein activators that stimulate the hydrolysis of glycosphingolipids is one of the important developments in this field.

Bibliography of crystal structures of polysaccharides 1977-1979.

Abstract: Publisher Summary This chapter presents the bibliography of crystal structures of polysaccharides. In the past decade, many advances in the methods for building theoretical models were witnessed, not only for the conformational analysis of isolated chains, but also for the subsequent or simultaneous packing of the chains in the crystallographic lattice. Many complex polysaccharides, having disaccharide to hexasaccharide repeating units, are being studied. Crystal structure refinement of a polysaccharide requires not only definition of the conformation of the backbone but also substituent orientation with respect to the backbone. The structural features published for polysaccharides and information from the papers presented at the American Chemical Society Centennial Symposium in 1976 are presented in the chapter. Knowledge of the chemical repeating pattern is a prerequisite to analysis of the diffraction patterns from polysaccharides. The patterns from the calcium salts of heparin are different from those from the sodium salt form.