Carbohydrates: Disaccharides and polysaccharides

Disaccharides:

Disaccharides are sugar molecules which are formed by linking two monosaccharides through glycosidic linkages.

• Maltose comprised of two glucose residues linked by an α-glycosidic linkage between C1 of one residue and C4 of the other residue. 
• In maltose, the second sugar residue has an unsubstituted anomeric carbon atom and therefore can function as a reducing agent as well as exhibit mutarotation. 
• In trehalose, two glucose residues are joined by an α-linkage via both anomeric carbon atoms; therefore, the disaccharide is not a reducing sugar and it does not show mutarotation. 
• Lactose, synthesized only by secretory cells of the mammary gland during lactation, is a disaccharide consisting of galactose and glucose. The glycosidic bond is a β-linkage between first carbon of galactose and fourth carbon of glucose. 
• Lactose is a reducing sugar and shows mutarotation by virtue of the anomeric C1 of the glucose residue. Lactulose is a disaccharide comprising of galactose and fructose linked through a β-linkage between C1 of galactose and C4 of fructose. 
• It is used in the treatment of few forms of chronic liver disease in which the ammonia content in the blood is elevated which is known as hyperammonemia. Normally, ammonia formed in the gastrointestinal tract, mainly in the colon by microbial action, is transported to the liver via the portal circulation and inactivated by conversion to urea. 
• Oral administration of lactulose cures hyperammonemia by microfloral conversion in the colon to diffenrent kinds of organic acids (e.g., lactate) that acidify the colonic contents. 
• Lactulose is neither catabolized nor absorbed in the small intestine. Reduction of the colonic luminal pH helps in formation of ammonium ion from ammonia, which is not easily absorbed, and thus its absorption is decreased. 
• Decreased luminal pH may additionally influence a microflora that causes a decrease in the production of ammonia as well as an increase in its utilization. The osmotic activity of the disaccharide and its metabolites leads to an osmotic diarrhea, which is useful in eliminating toxic waste products.
• Another nonabsorbable disaccharide, lacitol can be used in the treatment of hepatic encephalopathy. Compared to lactose, lactitol has the advantage of higher acceptability and fewer side effects. 
• Ammonia production in the colonic lumen by urease-producing bacteria can be decreased by providing antibiotics such as neomycin or metronidazole. 
• The therapeutic effect of the combined use of a nonabsorbable disaccharide and an antibiotic may result from the metabolism of the disaccharide by antibiotic-resistant bacteria. 
• Sucrose, a widely occurring disaccharide found in many plants, consists of glucose and fructose residues linked together through C1 of glucose and C2 of fructose. 
• Sucrose is not a reducing sugar and does not mutarotate. Because of its sweet taste sucrose is ingested in large amounts.

Trehalose

Maltose

Lactose

Lactulose

Sucrose

Polysaccharides:

• Polysaccharides, also called as glycans, contain many monosaccharide units linked together by glycosidic linkages. 
• They may be homopolysaccharides (e.g., glycogen, starch, and cellulose), which contain only single type of monomeric residue, or heteropolysaccharides, which comprise of two or more different types of monosaccharide units glycosidically linked in different ways. 
• The heteropolysaccharides contain complex structures, and they may also be found covalently linked with other biomolecules like proteins and lipids. 
• Starch and glycogen are energy storage forms of carbohydrate and are thus known as storage carbohydrates. 
• When the supply of carbohydrate is more than the needs of the cell, the excess is converted to storage forms. When the situation is reversed, the storage forms are converted to usable forms of carbohydrate. Therefore, a storage carbohydrate ought to be capable of fast synthesis as well as breakdown in response to the energy necessities of the cell. 
• As monosaccharide aggregate within the cell, its fast conversion to insoluble, high-molecular-weight polysaccharide prevents an osmotic imbalance and also keeps a favorable concentration gradient between the intra- and extracellular compartments, which facilitates sugar transport. 

Starch:

• Starch, the storage polysaccharide of most plants and particularly of tubers and seeds, consists of a mixture of amylose and amylopectin. 
• Amylose is an unbranched polymer of glucose in which the glucosyl residues are linked in α (1 --->4) glycosidic linkages. 
• The conformation of amylose has been elucidated by the use of stable amylose complexes prepared by reacting amylose with iodine. 
• X-ray diffraction studies of such complexes have revealed a helical conformation with six glucose residues per turn of the helix. 
• The amylose-iodine complex has an intense blue color, which provides the basis for the iodine test for starch. 
• Amylopectin contains glucosyl units joined together in both α (1 --> 4) and α (1 --> 6) linkages, the latter linkages being responsible for branch points. Unlike amylose, amylopectin is unable to form a stable helical structure because of the branching. 
• Amylopectin complexes with iodine to a much lesser extent than amylose; therefore, the amylopectin-iodine complex has a redviolet color that is much less intense than the blue of the
• amylose-iodine complex. 
• Starch from various sources contains various quantities of amylose and amylopectin. 
• In most plants, amylopectin is the more abundant form (about 75-80%). Virtually no amylose is found in starch obtained from some waxy varieties of maize (corn) and rice. 
• Starch is easily digested by humans. 

Starch

Glycogen:

• Glycogen is the polysaccharide which is abundant in animals and functions as the main storage polysaccharide in humans. 
• It is a branched polysaccharide of D-glucose and, like amylopectin, contains both α (1 --> 4) and α (1 --> 6) linkages, the latter forming branch points. 
• Each molecule of glycogen contains one reducing glucose molecule, which is the terminal unit on one of the chains. 
• At each of the other termini, the glucose molecule has a free hydroxyl group at C4, while the C1 hydroxyl group involve in the glycosidic linkage. Synthesis and breakdown take place at these termini. 
• A glycogen molecule contains about 105 glucose units. It has no discrete molecular weight, since its size varies considerably depending on the tissue of origin and its physiological state. In the human body, high amount of glycogen appear in liver and muscle. 
• The functional roles of glycogen in these two tissues are completely different: in muscle, glycogen serves as an energy reserve mostly for contraction, whereas liver glycogen supplies glucose to other tissues through blood circulatory system. 

Glycogen

Cellulose:

• Cellulose, the most abundant carbohydrate on earth, is an unbranched polymer with glucosyl residues joined together in β(1 --> 4) linkages. 
• Cellulose from variety of sources varies in molecular weight, and the number of glucose units lies in the range of 2,500-14,000. 
• Cellulose is the structural polysaccharide in plants. Cellulose microfibrils are closely packed aggregates of cellulose molecules, which are chemically inert and insoluble and possess vital mechanical strength. 
• Because of its β linkages, the preferred conformation of cellulose is one in which the ring oxygen of one residue forms a hydrogen bond with the C3 hydroxyl group of the next residue. The 40 or more individual cellulose molecules that aggregate to form microfibrils are held together by intermolecular hydrogen bonds. 
• In humans, cellulose is not digested in the small intestine but is digested in the large intestine to some extent by the microflora to yield short-chain fatty acids, hydrogen, carbon dioxide, and methane. 
• Undigested cellulose forms a part of the indigestible compound of the diet, called as dietary fiber. 
• Ruminants and termites are capable of digesting cellulose, which is a primary energy source for them, because they host microorganisms in their intestinal tract that elaborate cellulase, which catalyzes cleavage of the β (1 --> 4) glucosidic linkages.

Cellulose

Chitin:

In many fungal cell walls and invertebrates (shells of crustaceans and exoskeletons of insects), the main structural polysaccharide is chitin, which is a polymer of N-acetylglucosamine linked in β (1 --> 4) glycosidic linkage. 

Chitin

Hemicellulose:

Hemicellulose is heterogenous group of polysaccharides which can be easily hydrolyzed by hemicellulase enzyme. Various pentose polymers (xylans, arabinosylans), hexose polymers (galactans, mannans) and uronic acid polymers (galacturonic or glucronic acid) come under this group. These are not digested by small intestine but digested by microbial flora of the large intestine.

Pectins:

Pectins occur widely in apples, citrus fruits, and strawberries. It is a heteropolysaccharide comprises galactouronan and galactan. Carboxylate groups of the uronic acids are either free or esterified with methyl groups.  It acts as intercellular cementing material in plant tissues. Pectins cannot be digested by small intestine but digested to a small extent by microflora of large intestine.

Pectin

Lignin:

Lignin is nonpolysaccharide polymer found in woody plant tissues. This is totally indigestible even by ruminants.

Lignin

Gums and alginates like guar gum and polymannuronic acids respectively are heteropolysaccharides. They act as food stabilizers and thickening agents.