Polysaccharides Serve Storage Structural Functions

Polysaccharides include the following physiologically important carbohydrates.

Starch is a homopolymer of glucose forming an a-glucosidic chain, called a glucosan or glucan. It is the most abundant dietary carbohydrate in cereals, pota toes, legumes, and other vegetables. The two main constituents are amylose (15-20%), which has a non-branching helical structure (Figure 13-12); and amy-lopectin (80-85%), which consists of branched chains composed of 24-30 glucose residues united by 1 ^ 4 linkages in the chains and by 1 ^ 6 linkages at the branch points.

Glycogen (Figure 13-13) is the storage polysaccharide in animals. It is a more highly branched structure than amylopectin, with chains of 12-14 a-D-glucopyra-nose residues (in a[1 ^ 4]-glucosidic linkage), with branching by means of a(1 ^ 6)-glucosidic bonds.

Maltose

hoch2 5 J-O

hoch2

hoch2

OH H/OH

0La-d-Glucopyranosyl-(1 ^ 4)-a-d-glucopyranose

Lactose hoch2

HO/H

Lactose

O-ß-d-GalactopyranosyKI ^ 4)-ß-d-glucopyranose

hoch2

5j-O

O-ß-d-GalactopyranosyKI ^ 4)-ß-d-glucopyranose

Sucrose hoch2

-O

H

OH

H

il 3

2 ii

HO/COH

OH H

HO/COH

OH H

0-a-d-Glucopyranosyl-(1 ^ 2)-ß-d-fructofuranoside

Figure 13-11. Structures of important disaccharides. The a and p refer to the configuration at the anomeric carbon atom (asterisk). When the anomeric carbon of the second residue takes part in the formation of the glycosidic bond, as in sucrose, the residue becomes a glycoside known as a furanoside or pyranoside. As the disaccharide no longer has an anomeric carbon with a free potential aldehyde or ketone group, it no longer exhibits reducing properties. The configuration of the p-fructofuranose residue in sucrose results from turning the p-fructofu-ranose molecule depicted in Figure 13-4 through 180 degrees and inverting it.

Figure 13-12. Structure of starch. A: Amylose, showing helical coil structure. B: Amylopectin, showing 1 ^ 6 branch point.

HOCH,

Figure 13-13. The glycogen molecule. A: General structure. B: Enlargement of structure at a branch point. The molecule is a sphere approximately 21 nm in diameter that can be visualized in electron micrographs. It has a molecular mass of 107 Da and consists of polysaccharide chains each containing about 13 glucose residues. The chains are either branched or unbranched and are arranged in 12 concentric layers (only four are shown in the figure). The branched chains (each has two branches) are found in the inner layers and the unbranched chains in the outer layer. (G, glycogenin, the primer molecule for glycogen synthesis.)

Chitin

Chitin

Hyaluronic acid
ß-Glucuronic acid N-Acetylglucosamine

Chondroitin 4-sulfate

(Note: There is also a 6-sulfate)

Chondroitin 4-sulfate

(Note: There is also a 6-sulfate)

P-Glucuronic acid N-Acetylgalactosamine sulfate

Heparin

Heparin

Figure 13-14. Structure of some complex polysaccharides and glycosaminoglycans.

Inulin is a polysaccharide of fructose (and hence a fruc-tosan) found in tubers and roots of dahlias, artichokes, and dandelions. It is readily soluble in water and is used to determine the glomerular filtration rate. Dextrins are intermediates in the hydrolysis of starch. Cellulose is the chief constituent of the framework of plants. It is insoluble and consists of P-D-glucopyranose units linked by P(1 ^ 4) bonds to form long, straight chains strengthened by cross-linked hydrogen bonds. Cellulose cannot be digested by mammals because of the absence of an enzyme that hydrolyzes the P linkage. It is an important source of "bulk" in the diet. Microorganisms in the gut of ruminants and other herbivores can hydrolyze the P linkage and ferment the products to short-chain fatty acids as a major energy source. There is limited bacterial metabolism of cellulose in the human colon. Chitin is a structural polysaccharide in the exoskeleton of crustaceans and insects and also in mushrooms. It consists of V-acetyl-D-glucosamine units joined by P (1 ^ 4)-glycosidic linkages (Figure 13-14).

Glycosaminoglycans (mucopolysaccharides) are complex carbohydrates characterized by their content of amino sugars and uronic acids. When these chains are attached to a protein molecule, the result is a pro-teoglycan. Proteoglycans provide the ground or packing substance of connective tissues. Their property of holding large quantities of water and occupying space, thus cushioning or lubricating other structures, is due to the large number of —OH groups and negative charges on the molecules, which, by repulsion, keep the carbohydrate chains apart. Examples are hyaluronic acid, chondroitin sulfate, and heparin (Figure

Glycoproteins (mucoproteins) occur in many different situations in fluids and tissues, including the cell membranes (Chapters 41 and 47). They are proteins

Table 13-5. Carbohydrates found in glycoproteins.

Hexoses

Mannose (Man) Galactose (Gal)

Acetyl hexosamines

N-Acetylglucosamine (GlcNAc) N-Acetylgalactosamine (GalNAc)

Pentoses

Arabinose (Ara) Xylose (Xyl)

Methyl pentose

L-Fucose (Fuc; see Figure 13-15)

Sialic acids

N-Acyl derivatives of neuraminic acid, eg, N-acetylneuraminic acid (NeuAc; see Figure 13-16), the predominant sialic acid.

Glucopyranose

Figure 13-15. ß-L-Fucose (6-deoxy-ß-L-galactose).

containing branched or unbranched oligosaccharide chains (see Table 13-5). The sialic acids are N- or O-acyl derivatives of neuraminic acid (Figure 13-16). Neuraminic acid is a nine-carbon sugar derived from mannosamine (an epimer of glucosamine) and pyruvate. Sialic acids are constituents of both glycoproteins and gangliosides (Chapters 14 and 47).

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