Monosaccharides and disaccharides

Monosaccharides and disaccharides

A. Important monosaccharides 

Only the most important of the large number

of naturally occurring monosaccharides are

mentioned here. They are classified according

to the number of C atoms (into pentoses,

hexoses, etc.) and according to the chemical

nature of the carbonyl function into aldoses

and ketoses.

The best-known aldopentose (1), D-ribose,

is a component of RNA and of nucleotide

coenzymes and is widely distributed. In these

compounds, ribose always exists in the furanose

form (see p. 34). Like ribose, D-xylose and

L-arabinose are rarely found in free form.

However, large amounts of both sugars are

found as constituents of polysaccharides in

the walls of plant cells (see p. 42).

The most important of the aldohexoses (1)

is D-glucose. A substantial proportion of the

biomass is accounted for by glucose polymers,

above all cellulose and starch. Free D-glucose

is found in plant juices (“grape sugar”) and as

“blood sugar” in the blood of higher animals.

As a constituent of lactose (milk sugar), Dgalactose

is part of the human diet. Together

with D-mannose, galactose is also found in

glycolipids and glycoproteins .

Phosphoric acid esters of the ketopentose

D-ribulose (2) are intermediates in the pentose

phosphate pathway and in

photosynthesis . The most widely

distributed of the ketohexoses is D-fructose. In

free form, it is present in fruit juices and in

honey. Bound fructose is found in sucrose (B)

and plant polysaccharides (e. g., inulin).

In the deoxyaldoses (3), an OH group is

replaced by a hydrogen atom. In addition to

2-deoxy-D-ribose, a component of DNA

that is reduced at C-2, L-fucose is shown

as another example of these. Fucose, a sugar

in the λ series is reduced at C-6.

The acetylated amino sugars N-acetyl-Dglucosamine

and N-acetyl-D-Galactosamine

(4) are often encountered as components of

glycoproteins.

N-acetylneuraminic acid (sialic acid, 5), is a

characteristic component of glycoproteins.

Other acidic monosaccharides such as D-glucuronic

acid, D-galacturonic acid, and liduronic

acid, are typical constituents of the glycosaminoglycans

found in connective tissue.

Sugar alcohols (6) such as sorbitol and

mannitol do not play an important role in

animal metabolism.

B. Disaccharides 

When the anomeric hydroxyl group of one

monosaccharide is bound glycosidically with

one of the OH groups of another, a disaccharide

is formed. As in all glycosides, the glycosidic

bond does not allow mutarotation. Since

this type of bond is formed stereospecifically

by enzymes in natural disaccharides, they are

only found in one of the possible configurations

(α or β).

Maltose (1) occurs as a breakdown product

of the starches contained in malt (“malt

sugar”; and as an intermediate in

intestinal digestion. In maltose, the anomeric

OH group of one glucose molecule has an α-

glycosidic bond with C-4 in a second glucose

residue.

Lactose (“milk sugar,” 2) is themost important

carbohydrate in the milk of mammals.

Cow’s milk contains 4.5% lactose, while human

milk contains up to 7.5%. In lactose, the

anomeric OH group of galactose forms a β-

glycosidic bond with C-4 of a glucose. The

lactose molecule is consequently elongated,

and both of its pyran rings lie in the same

plane.

Sucrose (3) serves in plants as the formin

which carbohydrates are transported, and as a

soluble carbohydrate reserve. Humans value

it because of its intensely sweet taste. Sources

used for sucrose are plants that contain particularly

high amounts of it, such as sugar

cane and sugar beet (cane sugar, beet sugar).

Enzymatic hydrolysis of sucrose-containing

flower nectar in the digestive tract of bees—

catalyzed by the enzyme invertase—produces

honey, a mixture of glucose and fructose. In

sucrose, the two anomeric OH groups of glucose

and fructose have a glycosidic bond; sucrose

is therefore one of the non-reducing

sugars.