Disaccharides And Oligosaccharides And Their Properties

Oligosaccharides are composed of monosaccharides covalently linked by glycosidic bonds. They are either reducing or nonreduc-ing. An oligosaccharide terminating with a residue that has an unsubstituted anomeric —OH is reducing. Reducing oligosaccharides undergo all the chemical reactions of the aldose sugars, including reduction, oxidation, cho hoch

hcoh

hcoh

hoch

L-Fucose (Fuc)

H CH

HCOH

HCOH

CH2OH

COOH

HCOH

AcHNCH

HOCH

HCOH

HCOH

CH2OH

A/-Acetyl-Neuraminic Acid (NeuAc or NANA)

Figure 1-10. Examples of deoxy and amino sugars that are constituents of important biological compounds such as DNA, glycoproteins, and glycoconjugates.

2-Deoxy-D-Ribose (dRib)

HCNHAc

HOCH HCOH

HCOH

CH2OH

N- Acetyl-D-Giucosamine (GlcNAc)

HCNHAc

HOCH

hoch

hcoh

ch2oh

A/-Acetyl-D-Gaiactosamine (GalNAc)

and base-catalyzed epimerization and isomer-ization at their reducing end. Oligosaccharides are readily hydrolyzed to their constituent sugars by acid or enzyme catalysis, with the enzymes showing strong specificity for the sugar units and their anomeric linkage. As a result of this specificity humans digest primarily two types of oligosaccharides: those containing a-D-glucose or p-D-galactose at the non-reducing end. The structures of the three major dietary disaccharides (sucrose, lactose, and maltose) are shown in Figure 1-11.

Sucrose (table sugar), a nonreducing di-saccharide, is composed of a-D-glucopyrano-syl and p-D-fructofuranosyl units covalently linked through the anomeric carbon of each sugar unit to form a-D-glucopyranosyl-(l—>2)-(3-D-fructofuranoside. Sucrose is widely distributed in plants and produced commercially from sugar cane and sugar beets. It is easily hydrolyzed to glucose and fructose in acid solution and rapidly digested by sucrase, an a-glucosidase of the intestinal villi. Sucrose is the major caloric sweetener for commercial or home use.

Lactose ((3-D-galactopyranosyl-(l->4)-D-glucopyranose, milk sugar) is synthesized in the mammary glands of mammals. The concentration in milk varies with species and constitutes about 4 g/100 mL of bovine milk compared with 6.4 g/100 mL of human milk (Newburg and Neubauer, 1995). Lactose has about one third the sweetness of sucrose. It is readily digested to glucose and galactose by a (3-galactosidase (lactase) of the intestinal villi. Lactose is a reducing disaccharide and therefore susceptible to reactions of the glucose carbonyl group, including the Maillard reaction. Alkaline isomerization of lactose gives lactulose, in which the glucose unit has been isomerized to fructose. This isomerization also occurs to some extent during heating of milk. Lactulose is not digested or absorbed in the body and it appears to promote growth of Bifidobacterium and Lactobacillus species in the colon. Colonization by these bacteria is effective in preventing acute diarrhea. Production of short-chain fatty acids from the lactulose and dietary fiber polysaccharides leads to a decrease in colonic pH and limits potential growth of pathogenic bacteria. Lactulose is also used as a therapeutic agent in the treatment of hepatic encephalopathy

Only small amounts of maltose (a-D-glu-

CH2OH

CH2OH

Glucosyl Group Lactose

Lactose A: p-D-Galp-(1 ->4)-D-Glcp B: Gal(|31-4)Glc

CH2OH

Lactose A: p-D-Galp-(1 ->4)-D-Glcp B: Gal(|31-4)Glc

CH2OH

Sucrose Nonreducing

Maltose A: a-D-Glcp-(1->4)-D-Glcp B: Glc(a1-4)Glc

Figure 1-11. Reducing (lactose and maltose) and nonreducing (sucrose) disaccharides. The free anomeric —OH of the glucosyl unit of lactose and of maltose indicates the reducing nature of these disaccharides. In sucrose, both anomeric carbons are involved in the glycosidic bond, and sucrose is nonreducing. The small arrows point to the glycosidic bonds. Two abbreviated structural notations are shown for these disaccharides. Notation A defines the complete structure, whereas B assumes the more common ring form and d-configuration for each sugar unit.

Abbreviated Structure Galactose

CH2OH

OH H Sucrose A: a-D-Glcp-(1 <->2)-(3-D-Fru/ B: Glc(a1-2fi)Fru

CH2OH

OH H Sucrose A: a-D-Glcp-(1 <->2)-(3-D-Fru/ B: Glc(a1-2fi)Fru

Maltose A: a-D-Glcp-(1->4)-D-Glcp B: Glc(a1-4)Glc

Figure 1-11. Reducing (lactose and maltose) and nonreducing (sucrose) disaccharides. The free anomeric —OH of the glucosyl unit of lactose and of maltose indicates the reducing nature of these disaccharides. In sucrose, both anomeric carbons are involved in the glycosidic bond, and sucrose is nonreducing. The small arrows point to the glycosidic bonds. Two abbreviated structural notations are shown for these disaccharides. Notation A defines the complete structure, whereas B assumes the more common ring form and d-configuration for each sugar unit.

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