Disaccharides and Oligiosaccharides

This section is divided into the reducing and non-reducing types of disaccharide. If two monosaccharides are linked through their anomeric centers the disaccharide formed is a non-reducing disaccharide. If one monosaccharide is linked by one of its other hydroxyl groups, then the anomeric center is unsubstituted and a reducing disaccharide occurs. Two monosaccharides have many more ways of connecting to form a reducing disaccharide than a non-reducing disaccharide.

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Reducing Disaccharides

   Cellobiose (4-O-β-D-glucopyranosyl-D-glucose) results from the hydrolysis of cellulose by bacteria. Mammals lack the necessary enzymes—cellobiohydrolases and endo-cellulases—to hydrolyze cellulose. Cellobiose constitutes materials such as cotton and paper. Maltose is a homopolmyer of cellobiose.
Gentiobiose (6-O-β-D-glucopyranosyl-D-glucose) is found in many glycosides such as amygdalin.
Isomaltose (6-O-α-D-glucopyranosyl-D-glucose) is formed from two glucose monosaccharides. It is often found at the branching points of amylopectin and glycogen.
Lactose (4-O-β-D-galactopyranosyl-D-glucose) is the predominant disaccharide found in milk. Lactose intolerance is a condition in which there is a lack of the enzyme lactase. Galactosaemia is a condition that results from an inability to process the D-galactose after hydrolysis.
Laminaribiose is a polysaccharide building unit for laminarin (brown algae), pachyman (fungi), paramylon (unicellular algae), and callose.
Maltose (4-O-α-D-glucopyranosyl-D-glucose) results from hydrolysis of starch by enzymes (amylases) in the mammalian digestive tract. Notice that the glycosidic linkage is α and that of the homopolymer cellobiose is a β linkage. Maltose is used as a sweetner and as a substrate for fermentation. It is also the constituent of the polymer amylose.
Mannobiose is the unit for the plant polysaccharide mannan.
Xylobiose is the unit found in many polysaccharides such as the xylans that constitute plant cell walls.
 

Non-Reducing Disaccharides

Sucrose (β-D-fructofuranosyl α-D-glucopyranoside) is the predominant disaccharide found in sugar cane and sugar beet. It is a well known sweetner and has a five-membered furanosyl unit. 108 tons of sucrose are produced annually for consumption.
Trehalose (α-D-glucopyranosyl α-D-glucopyranoside) is found in microbes, fungi, and certain insects. One isomer is neotrehalose with an α,β link. Another isomer is isotrehalose which has a β,β link.
 

Oligiosaccharides

Many oligiosaccharides are not found as isolated molecules. Instead, they may be attached to other biomolecules (glycoconjugates). Glycoconjugates of proteins and various lipids are especially likely. For example, the lipids on the surface of erythrocytes are conjugated with various oligiosaccharides. The oligiosaccharides are involved in cellular recognition and are responsible for the blood group serotype (A, B, AB, and O).
α-, β-, and γ-cyclodextrins are a class of cyclic oligosaccharides that contain from six to eight monosaccharides. Since they are derived from starch, the units are all D-glucopyranose saccharides. They are held together by an α-1,4 linkage. Ligands are able to attach by partitioning into the hydrophobic interior of the cyclodextrin.
Raffinose is a D-galactosylated version of sucrose: α-D-galactopyranosyl-(1→6)-α-D-glucopyranosyl β-D-fructofuranoside. It is more convenient to abbreviate this long name as α-D-Galp-(1→6)-α-D-Glcp-(1↔2)-β-D-Fruf.
 

Polysaccharides

Amylopectin is a high molecular weight (~108 a.u.) polymer consisting of repeating units of maltose. For about every ten maltose units, branching [(1→6)-α-] occurs. Because of this branching, amylopectin is globular-shaped and non-crystalline.
Amylose has a lower molecular weight (~106 a.u.) than amylopectin. Maltose is the repeating unit with (1→4)-α-D linkages. This causes the semi-crystalline polysaccharide to assume a helical tube-like shape. The shape of amylose is able to accept iodine as a ligand. This interaction causes the blue coloration of starch in the presence of iodine.
Cellulose is the most abundant polymer on Earth. Cellulose is a major structural constituent of plant cell walls. It is a polymer of cellobiose units. Because of the β linkage, humans cannot hydrolize cellulose. In contrast, the α linkage of starch is succeptible to hydrolysis by enzymes in humans. The polymer shape of cellulose is similar to a twisted ribbon and is influenced by hydrogen bonding. This hydrogen bonding may be parallel or anti-parallel thus giving cellulose I and cellulose II. As a result, these "twisted ribbons" form fibers that give form and structure. Because of its fibrous structure, cellulose is used extensively in the paper industry.
Chitin is to the animal kingdom that cellulose is to the plant kingdom. Chitin is found in shells of crustaceans and in the exoskeleton of insects. Chitin is a polymer of N-acetyl-D-glucosamine with NI,NII-diacetylchitobiose being the repeating unit. Like cellulose, intramolecular hydrogen bonding is important for the structure of chitin.
Glycogen is the mammalian version of starch. Its structure is similar to amylopectin except for glycogen's more frequent branching (five instead of every eight). Its molecular weight is therefore usually higher than amylopectin. Glycogen phosphorylase and other various debranching enzymes combine to produce the D-glucose 1-phosphate needed for the glycolysis sequence.
Heparin is a heteropolysaccharide with anti-clotting properties. It has medicinal value for surgury and is used to treat thrombosis. Heparin is found in arterial walls where it facillitates interactions between antithrombin (an inhibitor of blood coagulation) and thrombin (a clot-forming protein).
Starch is produced by plants and is stored as granules for later use as energy. Starch is a 1 to 4 mixture of amylose and amylopectin.
 

O-Glycosides

The disaccharides shown above are all O-glycosides. A glycosidic bond may also involve non-saccharide compounds such as phenols.
Arbutin is found in the leaves of Rosaceae and, when hydrolyzed, produces hydroquinone. This compound therefore has strong reducing properties that have biochemical consequences.
Amygdalin is isolated as one of the components of the seeds of Rosaceae. Upon enzymatic hydrolysis hydrogen cyanide is evolved. It also has strong biochemical consequences.
Erythromycin A is a macrolide that has whithin its structure the amino saccharide D-desosamine and the branched saccharide L-cladinose. It inhibits protein synthesis by bactereial ribosomes.
Streptomycin also intereferes with bacterial peptide synthesis by binding to sites on the ribosomes. It is part of the aminoglycoside family of antibiotics. The saccharide derivatives in streptomycin include the formylpentose L-streptose as well as the rarely seen 2-deoxy-2-methylamino-L-glucose. One of the uses of streptomycin is to treat tuberculosis
Adriamycin along with the related daunomycin are anthracyclines possessing anti-Gram-positive activity. They also have anticancer properties towards soft-tissue sarcomas and leukaemias. This is probably due to their intercalating behavior towards DNA which results in the inhibition of RNA and ultimately protein synthesis. L-duanosamine is the saccharide unit found in each.
 

N-Glycosides

Glycosylamines have important roles in biochemistry since most peptide-carbohydrate linkages are of the N-glycosidic type. These linkages usually involve D-glucosamine bonding to the L-asparagine amido group. D-ribose and 2-deoxy-D-ribose can form N-glycosidic bonds with purine and pyrimidine bases to create nucleotides. This is very significant because nucleotides are components of both DNA and RNA.
Streptothricin F displays activity against both Gram-negative and Gram-positive bacteria. The aminosaccharide is derived from 2-amino-2-deoxy-D-gulose.

C-Glycosides

In a strict sense these compounds do not have glycosidic bonds. Substituting carbon for the anomeric oxygen of the saccharide destroys the anomeric acetal and renders the molecule stable to acids. This is different than the usual glycosidic bonds involving heteroatoms. The two natural products shown below each have a C-glycosidic bond.

aquayamycin showdomycin
  

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