Glycosides glycosidic link

By far the majority of carbohydrate material in nature occurs in the form of polysaccharides. By our definition, polysaccharides include not only those substances composed only of glycosidically linked sugar residues but also molecules that contain polymeric saccharide structures linked via covalent bonds to amino acids, peptides, proteins, lipids, and other structures. 

In this Section, ether and acetal substituents will be discussed. In some polysaccharides, the terminal reducing sugar is glycosidically linked to a non-sugar aglycon, and this will be discussed in a special part. 

Cylodextrins (CDs) are a class of chiral cyclic oligosaccharides that have molecule-sized cavities. They commonly comprise between six and eight D-glucopyranoside units that are linked via a-l,4-glycosidic links. Their bowl-shaped form is generally represented as a cylindrical funnel by analogy to the calixarenes family. There is a large number of cyclodextrin derivatives in the... 

A bacterial phosphatidylinositol specific phospholipase C (PI-PLC) had been available for many years before it was demonstrated to strip a number of membrane-bound proteins from eukaryotic cell surfaces [1], Such proteins are anchored by a PI moiety in which the 6 position of inositol is glycosidically linked to glucosamine, which in turn is bonded to a polymannan backbone (Fig. 3-10). The polysaccharide chain is joined to the carboxyl terminal of the anchored protein via amide linkage to ethanolamine phosphate. The presence of a free NH2 group in the glucosamine residue makes the structure labile to nitrous acid. Bacterial PI-PLC hydrolyzes the bond between DAG and phosphati-dylinositols, releasing the water-soluble protein polysac charide-inositol phosphate moiety. These proteins are tethered by glycosylphosphatidylinositol (GPI) anchors. 

One example in which the C-4 hydroxyl group of lipid A is substituted is known. Thus, in R. tenue (90), nonacylated GlcpN is glycosidically linked to position 4 of GlcN(I). 

As an example of the carbohydrate component of a glycoprotein, the structure of one of the oligosaccharide chains of immunoglobulin G (IgG see p.300) is shown here. The oligosaccharide has an N-glycosidic link to the amide group of an asparagine residue in the Fc part of the protein. Its function is not known. 

On the cell surface of certain glycoproteins, O-glycosidic links are found between the carbohydrate part and a serine or threonine residue, instead of N-glycosidic links to asparagine residues. This type of link is less common than the N-glycosidic one. 

When a nucleic acid base is N-glycosidically linked to ribose or 2-deoxyribose (see p.38), it yields a nucleoside. The nucleoside adenosine (abbreviation A) is formed in this way from adenine and ribose, for example. The corresponding derivatives of the other bases are called guanosine (G), uridine (U), thymidine (T) and cytidine (C). When the sugar component is 2-deoxyribose, the product is a deoxyribonucleoside—e. g., 2 -deoxyadeno-... 

L-arabinose, have also been reported to be in direct linkage with a protein. L-Arabinose may also be linked to L-serine and L-threonine in a hyaluronic acid-polypeptide complex from ox vitreous-humor.750 D-Mannose is reported to be glycosidically linked in a glucoamylase [(l->4)-a-D-glucan glucohydrolase, EC 3.2.1.3] from Aspergillus... 

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