Carbohydrates

Chapter 7

Carbohydrates

In sheer amount carbohydrates make up the bulk of organic matter on the earth. They are predominantly of plant origin, one of the most abundant forms being the structural carbohydrate cellulose. In higher animals, however, protein rather than carbohydrate is the principal structural material although special types of carbohydrate, namely the glycosaminoglycans, are important constituents of skeletal and other connective tissues. Other types of polysaccharide play a structural role in the cell walls of bacteria. These may act as antigens so that, when invaded, the body is able to defend itself by producing specific antibodies that will lead to destruction of the bacteria.

Apart from their structural importance, carbohydrates provide energy for synthetic processes and other work undertaken by cells and also provide many of the simple starting materials that are required for the synthesis of more complex substances. Furthermore, certain sugars and their phosphate esters act as key compounds in the storage and transfer of energy. Sugars are also constituents of many coenzymes as well as of the nucleic acids.

The carbohydrates of the diet are of special interest to the dental profession since, as discussed later, their type and quality is an important factor affecting the health of the oral tissues.

A carbohydrate may be defined as a polyhydroxyaldehyde, polyhydroxyketone or a substance that can be hydrolysed to give these compounds. The carbohydrates are usually classified as follows:

In addition, carbohydrate-containing polymers are known which contain an appreciable amount of non-carbohydrate material. These include glycoproteins, glycopeptides, glycolipids and nucleic acids.

The sugars, of which the most important are the mono- and di-saccharides, have been given trivial names characteristically ending in the suffix -ose. They are crystalline substances which are soluble in water and dilute ethanol but insoluble in most organic solvents. Usually they have a sweet taste.

Monosaccharides

The monosaccharides contain between three and seven carbon atoms and are given group names which indicate the number as shown below:

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Sugars differ from the polyalcohols from which they are theoretically derived in that one of the alcohol groups is oxidized to a carbonyl group. Thus removal of two H atoms from glycerol results in either an aldehyde or a ketone according to the position of the carbonyl group formed as shown below:

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Sugars that contain an aldehyde grouping are known as aldoses and those with a ketone grouping as ketoses. Thus the best known of the monosaccharides, glucose, which contains six carbon atoms and an aldehyde group, is an aldohexose whereas fructose is a ketohexose. Since the sugars possess one or more asymmetric carbon atoms they exist in various stereoisomeric forms. As with the amino acids the L and D forms of glyceraldehyde are used as reference compounds. Their molecular asymmetry, which is best appreciated by examining solid models, can be represented by projection formulae

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If the carbon chain is lengthened by the addition of another –CHOH unit between the asymmetric carbon atom and the aldehyde group this extra carbon atom will also be asymmetric and two new isomeric compounds will be formed. Thus the number of possible aldose sugars increases by a factor of 2 with each additional C atom so that the number of possible aldose sugars is 2n where n = the number of asymmetric C atoms in the molecule. It follows that there are four different aldotetroses and eight aldopentoses. In each case half the sugars belong to the D-series and half to the L-series.

Of the eight aldopentoses D-ribose is the most important from the biochemical point of view. It is a component of various coenzymes as well as the ribonucleic acids. Closely related to it is D-2-deoxyribose which differs only in the absence of an O atom from the second carbon of the sugar chain.

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The naturally occurring sugars almost always belong to the D-series of compounds.

When sugars are represented by linear projection formulae the aldehyde, or other more highly oxidized group, is shown at the top and this C atom is designated C-1, while the asymmetric C atom nearest the bottom of the chain determines whether the sugar belongs to the D or L series. Such formulae clearly demonstrate the presence of any asymmetric C atoms but are misleading in suggesting that the bonds joining the C atoms are at an angle of 180°. When the bond angles of a pentose sugar are correctly shown at 110° the aldehyde group involving C-1 and the hydroxyl group in C-5 can be seen to be situated quite close together.

This proximity facilitates the formation of an oxygen-containing intramolecular ring which may be either a five-membered (C4 + O) furanose ring or a six-membered (C5 + O) pyranose ring. Compounds of this type which result from the condensation of an aldehyde group and a hydroxyl group are known as hemiacetals (page 10). Projection formulae such as the following, may be

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drawn showing these ring structures but they give a very distorted picture of the molecule and Haworth suggested that they should be replaced by perspective formulae. The perspective formulae for the pyranose and furanose forms of ribose are shown below with D-glucopyranose included for comparison:

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Dec 10, 2015 | Posted by in General Dentistry | Comments Off on Carbohydrates
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