Digestible Polysaccharides

Starch and glycogen are digestible polysaccharides of glucose. Starch is found in plant cells, in both linear and branched forms. Glycogen has a highly branched structure and is found in animal tissues, particularly muscle and liver.

Starch. Starch is one of the most abundant polysaccharides in plants, where it is stored in the seeds, tubers, roots, and some fruits. It is composed of two families of polymers, a mostly linear amylose [(1—»4)-oH>glucan] and the branched amylopectin [(l->4)-a-D-glucan with branches linked to C-6]. Starches from different sources vary in structure, but typically amylopectin has an average chain length between branch points of 20 to 25 glucose units. Typical starches contain 20% to 30% amylose and 70% to 80% amylopectin; however, high amylopectin (waxy corn, 98% amylopectin) and high amylose starches are also available. Starches for food processing are produced from many sources. The most important sources are corn (regular, waxy, and high amylose), potato, rice, tapioca, and wheat. The physical properties and, to some extent, the digestibility of the starches vary with their fine structure and reflect their source.

Amylose and amylopectin molecules are laid down during biosynthesis in highly organized particles called granules. The hydrogen-

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Figure 1-12. A cartoon illustrating branching structures of polysaccharides, (a) Linear polysaccharide. Circles represent sugar units linked by glycosidic bonds, (b) Alternating branches consisting of a single sugar unit, •; (c) blocks of consecutive single sugar unit branches, •; (d) ramified structure (branches on branches). Nonreducing end, •/ reducing end, 0/ one sugar of a sequence of sugar units, Q.

bonded structure of the starch components renders the granules insoluble in water below about 55°C. Above this temperature the granule imbibes water, swells, and eventually undergoes fragmentation, releasing the amylo-pectin and amylose. The swollen granules are responsible for the high viscosity of partially gelatinized starch dispersions. Pure amylose and amylopectin have limited solubility in water, and there is a tendency for the amylose chains, and to a lesser extent the amylopectin branches, to aggregate through hydrogen bonding and become insoluble in cold water. This can occur in processed foods, causing resistance to a-amylase digestion.

Hydrated (gelatinized) starch is readily hydrolyzed by the various amylases, whereas the native starch granule is more resistant to enzymatic digestion. Gelatinized starch is hydrolyzed to glucose in the gastrointestinal tract by the combined action of salivary and pancreatic a-amylase and the intestinal mucosal a-glucosidases (glucoamylase, sucrase/

a-dextrinase). The a-amylases, which cleave the (al—>4)-linkages only, catalyze hydrolysis of starch to maltose, maltotriose, and malto-tetraose and to oligosaccharides called a-limit dextrins, composed of a minimum of four glucose units and including an (al^6)-linked branch point. These disaccharides and oligosaccharides are then converted to glucose by the a-glucosidases. Figure 1-13 shows a fragment of the amylopectin structure and the (l-*4)- and (1—>6)-a-glucosidic linkages that can be hydrolyzed by human digestive enzymes. The human upper digestive tract does not possess an endogenous (1—>4)-|3-D-gluca-nase, and therefore cellulose, with its p-link-age, is not digestible.

Glycogen. Glycogen, like starch amylopectin, is a (1^4)-a-D-glucan with branches on branches that are (al—*6)-linked. The average length of the (1—»4)-linked chain between branch points is 10 to 14 glucose units. Because of this increased branching compared

Structure Cellulose

Figure 1-13. (a) A segment of starch amylopectin structure showing the a-d-glucosidic bonds and the branch points. Glycogen has a similar structure, lb) The conformational structure of cellulose shows that alternate /3-d-glucosyl units are flipped 180°, giving a flat, ribbon-like structure stabilized by hydrogen bonds (• • •). The glucosidic bonds are indicated by the small arrows.

Figure 1-13. (a) A segment of starch amylopectin structure showing the a-d-glucosidic bonds and the branch points. Glycogen has a similar structure, lb) The conformational structure of cellulose shows that alternate /3-d-glucosyl units are flipped 180°, giving a flat, ribbon-like structure stabilized by hydrogen bonds (• • •). The glucosidic bonds are indicated by the small arrows.

with amylopectin, glycogen is readily soluble in cold water and gives solutions of relatively low viscosity, which facilitate its use as a readily available endogenous energy source. The branching pattern interferes with intra-and intermolecular hydrogen bonding of the glycogen chains, thus permitting rapid solvation and easy access to the enzymes. A low viscosity facilitates diffusion of the substrate to the enzymes and diffusion of the products away from the active sites of the enzymes.

Glycogen is present in most animal tissues, with the highest content in liver and skeletal muscle. It may constitute up to 10% (wet weight) of the human liver. Mammalian tissue levels of glycogen are highly variable and affected by factors such as nutritional status and time of day. Glycogen has a high molecular mass, in the range of 106 to 109 Da.

By electron microscopy, glycogen appears as uniform spherical particles and higher molecular weight aggregates of these particles ((3 and a particles, respectively). The a-particles are composed of a few (3-particle carbohydrate chains covalently linked to protein, which is aggregated by disulfide linkages.

Nondigestible Plant Polysaccharides

Polysaccharides represent the major components of plant cell walls and interstitial spaces.

Plants also synthesize storage polysaccharides other than starch, including galactomannans, the (l-*3)(l-»4)-|3-D-glucans of cereal grains, and the fructans of grasses and some tubers. All of these nonstarch polysaccharides as well as those added during food processing constitute dietary fiber (Table 1-2).

Cellulose is a linear (l->4)-3-D-glucan with a flat, ribbon-like conformation in which alternate glucose units are flipped 180°, and hydrogen bonded intramolecularly (Fig. 1-13). These ribbon-like chains are aligned in parallel arrays called microfibrils in which the chains are strongly hydrogen bonded to each other. The microfibrils are similarly packed together into strong fibers, which are very insoluble and which provide rigidity to the plant cell wall. Associated with cellulose in the cell wall are several other insoluble polysaccharides, the hemicelluloses. These include the xyloglucans, which have a celluloselike backbone with ct-D-xylose units linked to C-6 of the glucosyl unit, and arabinoxylans, in which the (1—>4)-p-D-xylan chain has a-L-arabinofuranose and D-glucuronic acid branches at C-2 or C-3.

Pectic polysaccharides [(1—>4)-a-D-galac-turonan with occasional a-L-rhamnose units] and other associated polysaccharides (galac-tans and arabinans) are present in the cell

TABLE 1-2

Nondigestible Food Polysaccharides of Plant, Algal, and Bacterial Origin

Polysaccharide Main Chain or Repeat Unit*

Branches, Other Substituents *

Plants

Cellulose Arabinoxylan Xyloglucan Pectin

Cereal ß-glucan Galactomannan Arabinogalactan

Algae

Alginic acid Carrageenan Bacteria Xanthan gum

  • Glc(ßl-4)Glc--Xyl(ßl-l)Xyl--Glc(ßl-4)Glc-
  • GalA(a 1 -4) ] „GaIA(a 1-2) L-Rha(a 1-4)--[Glc(ß 1-4) ]„Glc(ß 1-3)--Man(ßl-4)Man--Gal(ßl-4)GaI-
  • ManA(ß 1-4) ] „ [L-GulA(a 1-4) ] „--[GaI(ßl-4)3,6-anhydroGal(al-3)]-
  • Glc(ßl-4)Glcf none

Xyl(al-6)-ß-Gal-; a-L-Ara/or ct-L-Fuc linked to Xyl

Gal- and L-Ara/7-; methyl ester of GalA

none

none

Pyr = pyruvic acid CHjCCOQ- at C-4 and C-6 of Man

  • Sugars are d in pyranose form unless indicated otherwise.
  • The substituent group replaces the proton of the —OH of a sugar unit in the main chain.

f Alternating glucose units are substituted at C-3 by the trisaccharide branch. The nonreducing terminal Man carries a pyruvic acid substituent, and the other Man is substituted at C-6 by an acetyl group.

walls of immature plant tissues and in the interstitial spaces. Native pectic galacturonan in the plant tissue is relatively insoluble, but isolated commercial pectin is soluble in hot water. Calcium ions form complexes with the galacturonic acid units of pectin, cross-linking the chains into a gel network. This is thought to account partially for the insolubility of native pectin in the plant tissue. The calcium-pectin complex is also the basis for dietary low-sugar, low-calorie fruit jams and jellies, whereas jellies prepared without calcium require a high sugar content to form a gel structure. More comprehensive descriptions of the structures and properties of the polysaccharides constituting dietary fiber, and their organization in the plant tissue, are provided in the reviews by Carpita (1990) and Selven-dran (1984).

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Responses

  • LONGO
    Is startch a polysaccharide found in animal tissue?
    4 years ago
  • Arabella
    What are the digestible polysaccharides?
    4 years ago
  • Ines
    What are the digestible polysaccharide?
    2 years ago
  • salvia
    Which polysaccharides are not digestible by animal enzymes?
    2 years ago
  • Maik
    Why is glycogen a digestible ploysaccharide?
    7 months ago
  • antonio struble
    How is glycogen a digestible Polysaccharides?
    7 months ago
  • krystian
    How is glucose a digestible poly saccharide?
    7 months ago
  • gruffo
    Which of these fibres are not digestible pectin starch carrageenan gum none?
    2 months ago
  • natale lucchesi
    Are digestible polysaccharides while _______ are not digestible polysaccharides.?
    11 days ago

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