Carbohydrates

CARBOHYDRATES Carbohydrates are the most abundant organic constituents of plants. They serve as the major source of chemical energy for living organisms (e.g. sugars and starch), as well as important constituents of supporting tissues (e.g. cellulose in wood, cotton and flax). The general name "Carbohydrates" is derived from that most of these compounds have the formula Cx (H2O)y and thus, appear to be "hydrates of carbon". They usually contain hydrogen and oxygen in the same proportion as in water (2:1).

Metabolism Stored carbohydrates such as the starch of plant and glycogen of animals are made available for energy production through oxidation to carbon dioxide and water. The energy released is converted to heat, and much of it is conserved in a new chemical form .

Monosaccharides These consist of only one saccharide or sugar unit and they are non-hydrolysable. They are subclassified according to: 1-The number of carbon atoms present in their molecule and, 2-The type of carbonyl group they contain. Thus, a monosaccharide containing three carbon atoms is called a triose and that containing five is called a pentose and so on. A monosaccharide containing an aldehyde group is called an aldose and one containing a keto group is called a ketose. These two classifications are frequently combined: e.g. a five-carbon aldose, for example, is called an aldopentose, a six-carbon ketose is called a ketohexose

Oligosaccharides These consist of 2 and up to 10 molecules of simple sugars and are hydrolysable. They are sometimes called compound or complex sugars. They are subclassified into di-, tri- and tetrasaccharides etc…, according to the number of molecules of simple sugars they yield on hydrolysis. Polysaccharides Polysaccharides are high molecular weight polymers of monosaccharides of very complex nature. They are hydrolysable and yield a large number of monosaccharides.

Configuration of Monosaccharides D and L designations of monosaccharides The simplest monosaccharide, which contains only one stereocenter, is glyceraldehyde.

These two forms of glyceraldehyde have been chosen as arbitrary standards for the D and L-series of monosaccharides in the sugar chemistry A monosaccharide in which the OH group attached to the carbon atom next to the CH2OH (farthest asymmetric carbon atom from the carbonyl group) is always to the right is designated as a “D-sugar” and that with the same OH to the left as “L -sugar”. D and L designations are like (R) and (S) designations in that they are not necessarily related to the optical rotations of the sugars to which they are applied. Thus, one may encounter sugars that are D (+) or D (-) and others that are L (-) or L (+).

Reactions of monosaccharides 1-Reactions similar to alcohols Ether Formation: As alcohols, simple sugars form ethers. The methyl derivatives (ethers) formed by reaction with methylating agents are useful in determination of both the ring size of monosaccharides and site of linkage in oligo- and polysaccharides.

2-Reactions similar to carbonyl compounds Glycoside formation ( acetal formation ) Each carbonyl group (aldehydic or ketonic) reacts with two molecules of alcohol to give an acetal. Oxidation These reactions produce different products according to the reagent used. i-Bromine water; is a mild oxidizing reagent. It selectively oxidizes the -CHO group into -COOH, and converts aldoses to the corresponding aldonic acids, e.g. glucose is transformed to gluconic acid. ii-Nitric acid; is a strong oxidizing agent. It oxidizes both the -CHO and terminal -CH2OH of an aldose to -COOH groups, and these dicarboxylic acids are known as aldaric acids. Example, oxidation of glucose into glucaric acid (or saccharic acid) and galactose into mucic acid.

2-Reactions similar to carbonyl compounds Glycoside formation ( acetal formation ) Each carbonyl group (aldehydic or ketonic) reacts with two molecules of alcohol to give an acetal. Oxidation These reactions produce different products according to the reagent used. i-Bromine water; is a mild oxidizing reagent. It selectively oxidizes the -CHO group into -COOH, and converts aldoses to the corresponding aldonic acids, e.g. glucose is transformed to gluconic acid. ii-Nitric acid; is a strong oxidizing agent. It oxidizes both the -CHO and terminal -CH2OH of an aldose to -COOH groups, and these dicarboxylic acids are known as aldaric acids. Example, oxidation of glucose into glucaric acid (or saccharic acid) and galactose into mucic acid.

Reaction with oxidising cations All monosaccharides and reducing disaccharides (i.e. all sugars containing free hemiacetal or hemiketal groups) are readily oxidized by metal ions such as Cu+2 (Fehling’s and Benedict’s reagents), Bi+3 and Hg+2 in alkaline medium. These reactions are used for identification and quantification of reducing sugars.

β-D-Fructose (levulose or fruit sugar) Ketohexoses β-D-Fructose (levulose or fruit sugar) Source It is found free in honey and in fruits juices, or as constituent of polysaccharide e.g. inulin. Preparation The best large-scale source of fructose is the “inversion” (hydrolysis) of sucrose by acid or invertase enzyme. Properties Fructose occurs as colourless crystals with intense sweet taste. It is freely soluble in water. It reduces Fehling's, and Barfoed's solutions It forms an osazone similar to that of glucose It gives a positive Seliwanoff’s test for ketoses (rapid furfural). Uses Fructose is used as food for diabetics (in emergencies of diabetic acidosis) and in infant feeding formulae (more easily digested than glucose).

The Mucopolysaccharides Mucopolysaccharides are polysaccharides which on hydrolysis yields amino sugar units. Amino sugars are derived from monosaccharides by replacement of a hydroxyl group by an amino group. ex. Heparin Source Heparin is the powerful blood anticoagulant; it can be isolated from the mast cells of liver, heart and lung.

Biologically Active Carbohydrates According to recent researches carbohydrates promise to be a major source of drug discovery. The diversity and complexity of carbohydrates explain their wide range of biological functions. There are several established carbohydrates-based products with “biopharmaceutical” application, as well as, other new products with potential application in medicine, e.g., development of specific cancer vaccines, new non-steroidal anti-inflammatory drugs, and many other examples.

Sweeteners Carbohydrates play an important factor in increasing the incidence of diabetes, obesity and dental caries. There is increasing need for other alternatives to sucrose as sweetening agents for medical purposes, especially in case of diabetes, and for diet improvement. These agents should have high solubility in water, good stability, and a relative sweetness close or equivalent to that of sucrose. They should be safe, low caloric, non-cariogenic and non-carcinogenic.

Bulk sweeteners These are the traditional sweeteners such as sucrose, glucose, fructose and the polyols or sugar alcohols e.g. sorbitol, mannitol, xylitol and lactitol. Intense sweeteners These are either synthetic e.g. saccharin, aspartame, cyclamate and or natural e.g. steviol glycosides. .

Artificial sweeteners These are synthetic sweeteners, non-caloric, non-nutritive and must be used at very low concentrations. Examples: Cyclamate: 300 times sweeter than sucrose. Aspartame: 1500 times sweeter than sucrose. Saccharin: 3500 times sweeter than sucrose