DISACCHARIDES

4th lec.1st year MBBS.IMDC

disaccharide• consist of two monosaccharides joined covalently by an O-

glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other. This reaction represents the formation of an acetal from a hemiacetal (such as glucopyranose) and an alcohol (a hydroxyl group of the second sugar molecule).

• N-glycosyl bonds join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins and nucleotides

• Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base.

• General formula is C12H22O11

maltose

maltose

• Malt sugar• Reducing• Shows mutarotation• Osazone formation(sun flower like)• Sweet sugar , soluble in water• Sources = starch, baby foods, cereals

isomaltose

Trehalose

Trehalose

• a disaccharide of D-glucose that, like sucrose, is a nonreducing sugar.

• No mutarotation• is a major constituent of the circulating fluid

(hemolymph) of insects, serving as an energy-storage compound.

• Also found in fungi & yeast

lactose

• -D- Galactopyranosyl (1-4)α-D- glucopyranoside

• Milk sugar, about 5%.• At body temp-, a mixture

of both α and β form in 2:3 ratio

• Synthesized by secr- cells• 16 % sweet • Not very soluble in H2O

lactose

• Dextrorotatory• Reducing• Can form osazone (powder puff or hedge

hoge)• Shows mutarotation• Hydrolyzed by “Lactase”• Lactose intolerance

lactulose

• Galactose + Fructose• Through β C1 of galactose and C4 of fructose• Neither absorbed nor broken down• NH3 intoxication

sucrose

sucrose

• Table sugar• Sources• Very soluble & very sweet• Non reducing• No osazone formation• No mutarotation• Dextrorotatory• Invert sugar

Cleavage of Sucrose(a-glucosidase or invertase)

OO

CH2OH

HHO

H

HO OH

H

H

OH

H HOCH2

H

OH H

H

CH2OHO

H2O

OCH2OH

HH

HO OH

H

H

OH

H

OH

OHO

HOCH2

H

OH H

H

CH2OHH

Glucose Fructose

Reducing end of sugars• The oxidation of a sugar’s anomeric carbon by cupric or

ferric ion (the reaction that defines a reducing sugar) occurs only with the linear form, which exists in equilibrium with the cyclic form(s).

• When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a nonreducing sugar.

• In describing disaccharides or polysaccharides, the end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end.

Artificial sweetners

• Secharine• Aspartame(L-aspartyl L-phenylalanine)

Saccharin(benzoic sulfimide) C7H5NO3S

oligosaccharides• Oligosaccharides are short polymers of several

monosaccharides joined by glycosidic bonds. At one end of the chain, the reducing end, is a monosaccharide unit whose anomeric carbon is not involved in a glycosidic bond

• contain from 3 to roughly 12 monosaccharides linked together.• They are often found attached through N- or O-glycosidic

bonds to proteins to form glycoproteins. (O2 atom of serine or threonine by o- linkage, nitrogen of aspargine by N- glycosidic)

• Integral membrane proteins contains covalently attached units on their extracellular face.

• Many secreted proteins e.g. antibodies, coagulation factors contain .

• Helps in molecular targeting & cell-cell recognition• Required for endocytosis.

POLYSACCHARIDES• Most carbohydrates found in nature occur as

polysaccharides, polymers of medium to high molecular weight.

• also called glycans, differ from each other in the identity of their recurring monosaccharide units, in the length of their chains, in the types of bonds linking the units, and in the degree of branching.

• Homopolysaccharides contain only a single type of monomer;• heteropolysaccharides contain two or more different kinds

Complex Carbohydrates

• CelluloseMost abundant carbohydrate on the planet!

– Component of plant cell walls– Indigestible by animals

• β 1-4 bonds

• Starch– Energy storage molecule in plants – Can be digested by animals

• α 1-6 bonds

Cellulose• Cellulose is a linear

polysaccharide in which some 1500 glucose rings link together. It is the chief constituent of cell walls in plants.

• Human digestion cannot break down cellulose for use as a food, animals such as cattle and termites rely on the energy content of cellulose. They have protozoa and bacteria with the necessary enzymes in their digestive systems. Only animals capable of breaking down cellulose are tunicates.

Starches • Starches are carbohydrates in which

300 to 1000 glucose units join together. It is a polysaccharide used to store energy for later use. Starch forms in grains with an insoluble outer layer which remain in the cell where it is formed until the energy is needed. Then it can be broken down into soluble glucose units. Starches are smaller than cellulose units, and can be more readily used for energy. In animals, the equivalent of starch is glycogen, which can be stored in the muscles or in the liver for later use.

• α-1,6 bonds

• Glycogen

polysaccharides can be linked to other molecules to form glyco-proteins and glyco-lipids

GlycoproteinsSome examples

• Polysaccharide component of antibodies has major effect on antibody function

• Polysaccharides attached to proteins on surface of red blood cells (RBC) determine blood type (A,B,O)

• Polysaccharides are attached to proteins in the Golgi apparatus through a process of post-translational modification

• Different types of cells do different post-tranlational modifications– More about this later

Glycolipids• Polysaccharides can also be attached to lipid molecules

•An outer-membrane constituent of gram negative bacteria, LPS, which includes O-antigen, a core polysaccharide and a Lipid A, coats the cell surface and works to exclude large hydrophobic compounds such as bile salts and antibiotics from invading the cell. O-antigen are long hydrophilic carbohydrate chains (up to 50 sugars long) that extend out from the outer membrane while Lipid A (and fatty acids) anchors the LPS to the outer membrane.

Glycolipids

• Polysaccharides (blue) are also used in animal cells to link surface proteins and lipid anchors to the membrane.