Oligo-Saccharides

Oligo-Saccharides are those carbohydrates that yield 2 to 10 monosaccharide units (monomers) on hydrolysis. Examples: Sucrose, Trehalose, Raffinose, Stachyose.

1.0Classification of Oligosaccharides

Oligosaccharides have been classified based on number of monosaccharide units.

Types of Oligosaccharide

Disaccharides :

· Composed of two monosaccharide units. e.g. Maltose, Sucrose, Lactose, Trehalose. 

· All disaccharides are water soluble and sweet, so they are known as sugar.

Ø Maltose– Maltose is commonly called malt sugar. It is an intermediate compound in starch digestion. Maltose is more abundant in germinating starchy seeds.

Ø Lactose– Lactose is milk sugar. Lactose is the least sweet sugar. Maximum % of lactose in human milk » 7

Ø Sucrose:  In plants, the transport of sugars mainly occurs in the form of sucrose. Sucrose is also known as Invert sugar. Sucrose is called cane sugar, or Table Sugar or Commercial Sugar. 

Ø Trehalose: Trehalose is present in the haemolymph of insects.

Trisaccharides :

· Composed of three monosaccharide units. Example : Raffinose = Glucose + Galactose + Fructose

Tetrasaccharides :

· Composed of four sugar units. Example : Stachyose = Glucose + Fructose + 2 molecules of Galactose

Pentasaccharides : 

· Composed of five sugar units. Example: Verbascose = 3 molecules of Galactose + Fructose + Glucose.

Similarly, Hexasaccharides and Heptasaccharides are composed of six and seven sugar units, respectively.

2.0Glycosidic Bond Formation

· In oligosaccharides, monosaccharides are linked together by glycosidic bonds.

· Aldehyde or ketone group of one monosaccharide reacts with the alcoholic group of another monosaccharide to form a glycosidic bond.

· One molecule of H2O is eliminated during glycosidic bond formation (dehydration synthesis).

· The direction of the glycosidic bond is mostly 1'-4".

· When another monosaccharide unit is fructose, the direction of linkage is 1'-2". (Non-reducing sugars). E.g. Sucrose

Glycosylation:

Glycosylation is the addition of protein, lipid or another organic molecule to a glycan (In certain glycoconjugates, oligosaccharides may serve as glycans)

(a) N-linked glycosylation: In this glycosylation, glycan forms the bond with the nitrogen atom of the Asparagine or Arginine residue of the protein.

(b) O-linked glycosylation: In this glycosylation, glycan forms the bond with hydroxyl Oxygen of Serine, Threonine, Tyrosine, Hydroxylysine or Hydroxyproline.

3.0Types of Oligosaccharides

(1) N-linked oligosaccharides: When he oligosaccharide is attached to Asparagine.

(2) O-linked oligosaccharides: When the oligosaccharides are attached to the Serine or Threonine of a protein.

4.0Significance of Oligosaccharides

Glycoproteins present between phospholipids of plasma membrane provide structure ability to cell membrane. Glycolipids are also associated with cell recognition, cell adhesion, immunological role and cell signalling.

Human blood groups (A, B, AB, O) are based on the presence of antigens on the surface of RBC. Antigens present on RBCs are generally glycolipids. 

Dietary oligosaccharides are the source of energy as well as have a prebiotic effect.