09/24/2017

indian medical pg entrance exam subject-biochemistry

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CARBOHYDRATES OF PHYSIOLOGIC SIGNIFICANCE / 107

fructose which is called “invert sugar” because the

strongly levorotatory fructose changes (inverts) the previous

dextrorotatory action of sucrose.

POLYSACCHARIDES SERVE STORAGE

& STRUCTURAL FUNCTIONS

Polysaccharides include the following physiologically

important carbohydrates.

Starch is a homopolymer of glucose forming an a-

glucosidic chain, called a glucosan or glucan. It is the

most abundant dietary carbohydrate in cereals, potatoes,

legumes, and other vegetables. The two main constituents

are amylose (15–20%), which has a nonbranching

helical structure (Figure 13–12); and amylopectin

(80–85%), which consists of branched chains

composed of 24–30 glucose residues united by 1 . 4

linkages in the chains and by 1 . 6 linkages at the

branch points.

Glycogen (Figure 13–13) is the storage polysaccharide

in animals. It is a more highly branched structure

than amylopectin, with chains of 12–14 a-D-glucopyranose

residues (in a[1 . 4]-glucosidic linkage), with

branching by means of a(1 . 6)-glucosidic bonds.

Table 13–4. Disaccharides.

Sugar Source Clinical Significance

Maltose Digestion by amylase or hydrolysis of starch.

Germinating cereals and malt.

Lactose Milk. May occur in urine during pregnancy. In lactase deficiency, malabsorption leads to diarrhea and flatulence.

Sucrose Cane and beet sugar. Sorghum. Pineapple. In sucrase deficiency, malabsorption leads to diarrhea and flatulence.

Carrot roots.

Trehalose1 Fungi and yeasts. The major sugar of insect

hemolymph.

1O-a-D-Glucopyranosyl-(1 .1)-a-D-glucopyranoside.

HO

H

HOCH2

H

OH

H

H

OH

O

H

O-a-D-Glucopyranosyl-(1 . 4)-a-D-glucopyranose

O-a-D-Glucopyranosyl-(1 . 2)-ß-D-fructofuranoside

3

5

6

4

H

HOCH2

H

OH

H

H

OH

O

OH

H

5

6

4

O

Maltose

1* 1*

2 32

HO

H

HOCH2

H

OH

H

H

OH

O

H

3

5

6

4

O

Sucrose

1

2

*

HOCH2

1

O

OH H

H

COH

H2

H HO

2 5

6

3 4

O-ß-D-Galactopyranosyl-(1 . 4)-ß-D-glucopyranose

H

HO

HOCH2

H

OH

H

H

OH

O

3

5

6

4

HOCH2

H

OH

H

H

OH

O

H

OH

5

6

4

Lactose

1 * 1 *

2 32

H

H

O

*

Figure 13–11. Structures of important disaccharides. The a and ß

refer to the configuration at the anomeric carbon atom (asterisk). When

the anomeric carbon of the second residue takes part in the formation

of the glycosidic bond, as in sucrose, the residue becomes a glycoside

known as a furanoside or pyranoside. As the disaccharide no longer has

an anomeric carbon with a free potential aldehyde or ketone group, it

no longer exhibits reducing properties. The configuration of the

ß-fructofuranose residue in sucrose results from turning the ß-fructofuranose

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