Overview of carbohydrates

Overview of carbohydrates

The carbohydrates are a group of naturally

occurring carbonyl compounds (aldehydes

or ketones) that also contain several hydroxyl

groups. The carbohydrates include single sugars

(monosaccharides) and their polymers,

the oligosaccharides and polysaccharides.

A. Carbohydrates: overview

Polymeric carbohydrates–above all starch, as

well as some disaccharides–are important

(but not essential) components of food .

In the gut, they are broken down into

monosaccharides and resorbed in this form

. The form in which carbohydrates

are distributed by the blood of vertebrates is

glucose (“blood sugar”). This is taken up by the

cells and either broken down to obtain energy

(glycolysis) or converted into other metabolites

. Several organs (particularly

the liver and muscles) store glycogen as

a polymeric reserve carbohydrate .

The glycogenmolecules are covalently

bound to a protein, glycogenin. Polysaccharides

are used by many organisms as building

materials. For example, the cell walls of bacteria

contain murein as a stabilizing component

, while in plants cellulose and

other polysaccharides fulfill this role .

Oligomeric or polymeric carbohydrates

are often covalently bound to lipids or proteins.

The glycolipids and glycoproteins

formed in this way are found, for example,

in cell membranes (center). Glycoproteins

also occur in the blood in solute form(plasma

proteins; and, as components of

proteoglycans, form important constituents of

the intercellular substance .

B. Monosaccharides: structure 

The most important natural monosaccharide,

D-glucose, is an aliphatic aldehyde with six C

atoms, five of which carry a hydroxyl group

(1). Since C atoms 2 to 5 represent chiral

centers , there are 15 further

isomeric aldohexoses in addition to D-glucose,

although only a few of these are important in

nature. Most natural monosaccharides

have the same configuration at C-5 as

D-glyceraldehyde–they belong to the D series.

The open-chained form of glucose shown

in (1) is found in neutral solution in less than

0.1% of themolecules. The reason for this is an

intramolecular reaction in which one of the

OH groups of the sugar is added to the aldehyde

group of the same molecule (2). This

gives rise to a cyclic hemiacetal . In

aldohexoses, the hydroxy group at C-5 reacts

preferentially, and a six-membered pyran

ring is formed. Sugars that contain this ring

are called pyranoses. By contrast, if the OH

group at C-4 reacts, a five-part furan ring is

formed. In solution, pyranose forms and

furanose forms are present in equilibrium

with each other and with the open-chained

form, while in glucose polymers only the

pyranose form occurs.

The Haworth projection (2) is usually used

to depict sugars in the cyclic form, with the

ring being shown in perspective as viewed

from above. Depending on the configuration,

the substituents of the chiral C atoms are then

found above or below the ring. OH groups

that lie on the right in the Fischer projection

(1) appear under the ring level in the Haworth

projection, while those on the left appear

above it.

As a result of hemiacetal formation, an additional

chiral center arises at C-1, which can

be present in both possible configurations

(anomers) . To emphasize this, the

corresponding bonds are shown here using

wavy lines.

The Haworth formula does not take account

of the fact that the pyran ring is not

plain, but usually has a chair conformation. In

B3, two frequent conformations of D-glucopyranose

are shown as ball-and-stick models. In

the 1C4 conformation (bottom), most of the

OH groups appear vertical to the ring level, as

in the Haworth projection (axial or a position).

In the slightly more stable 4C1 conformation

(top), the OH groups take the equatorial

or e position. At room temperature, each

formcan change into the other, as well as into

other conformations.