Formation of Glycosidic Linkages

Sugars react intermolecularly with alcohols under appropriate catalysis, forming a- and (3-glycosides as shown in Figure 1-5 for the acid-catalyzed synthesis of methyl a-d-glucopyrano-side. The alcohol may be aliphatic, aromatic, or another sugar. When properly activated, sugars react with each other to form specific

CH?OH CH20H

CH?OH CH20H

OH OH

Methyl a-D-Glucopyranoside

Figure 1-5. Reactions of sugars with alcohols. Acid-catalyzed synthesis of the glycoside methyl a-d-glucopyranoside. This reaction is reversible. The glycosidic bond is hydrolyzed by cleavage between the anomeric carbon of the glucosyl group and the oxygen of the bond.

OH OH

Methyl a-D-Glucopyranoside

Figure 1-5. Reactions of sugars with alcohols. Acid-catalyzed synthesis of the glycoside methyl a-d-glucopyranoside. This reaction is reversible. The glycosidic bond is hydrolyzed by cleavage between the anomeric carbon of the glucosyl group and the oxygen of the bond.

oligosaccharides and polysaccharides. Thus, the sugar units in oligosaccharides and polysaccharides are linked by O-glycosidic bonds. Sugars also react with amines or thiols to give N- or S-glycosides, respectively. Thus, (3-d-ribose and 2-deoxy-(3-d-ribose in nucleic acids are bonded to purines and pyrimidines by N-glycosidic bonds. In uridine diphosphate (UDP)-glucose, the f$-d-ribose is linked to uracil by an /V-glycosidic bond, and the a-d-glu-cose and p-d-ribose units are each ester-linked to phosphate, as shown in Figure 1-6. The sugar nucleotides are used extensively in vivo for enzymatic synthesis of carbohydrates, including lactose and glycogen. In glycoproteins, the oligosaccharide chains are linked to the (3-carboxamide nitrogen of asparagine (an yV-glycosidic bond) or to the hydroxyl of serine/threonine (an O-glycosidic bond). Plants use the glycosidic bond extensively in synthesizing different glycosides, many of

Structure Uridine

HO OH

Figure 1-6. The structure of uridine diphosphate (UDP)-glucose IUDPG), an activated form of glucose and an intermediate in the synthesis of glycogen in vivo. In this structure, p-d-ribose is linked to the amine uracil by an N-glycosidic bond, and the a-d-glucose unit is esterified to phosphate.

HO OH

Figure 1-6. The structure of uridine diphosphate (UDP)-glucose IUDPG), an activated form of glucose and an intermediate in the synthesis of glycogen in vivo. In this structure, p-d-ribose is linked to the amine uracil by an N-glycosidic bond, and the a-d-glucose unit is esterified to phosphate.

which are physiologically active. Various hy-droxylated compounds are glycosylated in the liver and excreted as glucuronic acid glycosides ((B-d-glucuronides), which is a major means of detoxification and excretion.

Glycosides are more stable than aldoses and ketoses in several respects. The carbonyl/ hemiacetal carbon is protected from base-cat-alyzed reactions and from reduction and oxidation. The pyranose and furanose ring structures and the anomeric configuration are also stabilized and do not undergo the interconversions shown in Figure 1-3. However, the glycosidic bonds can be hydrolyzed by acid or enzyme catalysis releasing the free sugar and alcohol. Glycosidases, which catalyze hydrolysis of glycosides, have high specificity for the sugar and the anomeric linkage (a or (3) but lower specificity for the alcohol unit (the agly-cone).

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