Introduction to Agricultural Biochemistry: Carbohydrates – Part 1

Carbohydrates are defined chemically as the aldehyde or ketone derivatives of a higher polyhydroxy alcohol or as compounds which may yield these derivatives on Hydrolysis.

They are the single most abundant class of organic compounds found in nature. The term carbohydrate arises from the basic molecular formula (CH₂O)n which can be rewritten as (C.H₂O)n to show that they are hydrates of carbon.

In the general classification of foods, carbohydrates constitute a class of energy giving food. They rank next to lipids in terms of energy yield. They are widely distributed in plants and animals. In plants, they are produced by the process of photosynthesis. Energy from the sun captured by green plants, algae and some bacteria during photosynthesis is stored in the form of carbohydrates.

Carbohydrates are the metabolic precursors of virtually all other biomolecules. Breakdown down of carbohydrates provides the energy that sustains life. Carbohydrates are covalently linked to lipids (forming glycolipids) or to proteins (forming glycoproteins). These two classes of biomolecules (together called glycoconjugates) are indispensable components of cell walls and extra cellular structures in plants, animals and bacteria.

Classification of Carbohydrates

Carbohydrates are classified into three broad groups:

a) Monosaccharides

b) Oligosaccharides

c) Polysaccharides

Monosaccharides are called simple sugars, with the general formula (CH₂O)n, cannot be broken down into small sugars under mild conditions. They usually have 3-7 carbon atoms and contain only one aldehyde or ketone group.

Oligosaccharides derive their name from the Greek word “oligo” meaning few. They consist of 2-10 simple sugar molecules. They are oligomers of monosaccharides linked by the formation of glucosidic bonds.

Polysaccharides are the polymers of the simple sugars and their derivatives. They may be either linear or branched polymers and may contain hundreds or even thousands of monosaccharide units. Their molecular weights range up to one million or more. They include such important substances as glycogen, cellulose and starch.

Monosaccharides

They consist of 3 or 7 carbon atoms and are described as either aldoses or ketoses depending on whether the molecule in question contains an aldehyde functional group (–CHO) or ketone group (–CO). The simplest aldose is the glyceraldehyde and the simplest ketose is dihydroxyacetone.

They are also further classified by the number of carbon atoms they contain, viz:

Triose – 3 carbon atoms

Tetrose – 4 carbon atoms

Pentose – 5 carbon atoms

Hexose – 6 carbon atoms

Heptose – 7 carbon atoms

Aldoses

Aldotriose:                  Glyceraldehydes

Aldotetrose:                Erythrose, Threose

Aldopentose:               Ribose, Arabinose, Xylose, Lyxose

Aldohexose:                Allose, Altrose, Glucose, Mannose, Gulose, Idose, Galactose, Talose.

NB: Hexoses are the most abundant sugar in nature.

Ketoses

Ketotrioses:                 Dihydroxyacetone

Ketotetrose:                Erythrulose

Ketopentose:               Ribulose, Xylulose

Ketohexoses:               Fructose, Psicose, Sorbose, Tagatose

Ketoheptose:               Sedoheptulose

Trioses

These are important compounds in muscle metabolism and are the basic sugars to which all monosaccharides are referred. The polyhydric alcohol from which they are derived is glycerol. Oxidation on the end carbon atom produces the aldose sugar known as glyceraldehydes, whereas oxidation on the centre carbon atom yields the ketotriose known as dihydroxyacetone.

Pentoses

Pentoses occur in nature combined with polysaccharides from which they may be obtained by hydrolysis with acids. Arabinose, for example is obtained from gum, Arabic. Xylose is obtained from the hydrolysis of wood, corn cobs or straws. Ribose and deoxyribose are constituents of the ribose nucleic acid (RNA), and deoxyribose nucleic acid (DNA), respectively.

Hexoses

These are the most important monosacchrides from a nutritional and physiological point of view. Glucose, fructose, and galactose are the hexoses commonly occurring in foods, whereas mannose is a constituent of a vegetable polysaccharide.

Glucose is the normal sugar of the blood and tissue fluids and is utilized by the cells as a source of energy. Fructose is the sweetest sugar of all monosaccharides and often occurs in fruits. Galactose is a constituent of milk sugar and is found in the brain and nervous tissues.

Heptoses

Sedoheptulose is an important example of a monosaccharide containing seven carbon atoms and occur, as the phosphate, as an intermediate in the pentose phosphate metabolic pathway.

Structure of Monosaccharides

Monosaccharides’ formula may be written in the form of a straight chain or ring structure.

Monosaccharide occurs in a number of isomeric forms. Glucose and fructose (hexoses) are structural isomers. However, glucose has an aldehyde group and fructose a ketose group.

Under physiological conditions, sugars exist mainly in ring or cyclic structure, rather than straight chains. Glucose exists in the form of a pyranose ring and fructose a furanose ring. Each ring structure can occur in two isomeric forms—designated alpha (α) and beta (β).

In the structure of glyceraldehydes above, it is apparent that a different group is attached to each of the 4 bonds in carbon atom number 2 (–CHO, –H, –OH, –CH₂OH). A carbon atom to which 4 different groups are so attached (chiral carbon) is said to be asymmetrical and therefore, optically active. The presence of asymmetrical carbon atom in a compound makes possible the formation of isomers of that compound.

Compounds which are identical in composition but differ only in spatial configuration are called stereoisomers. Examples of such isomers are isomers of glucose, one of which is a mirror image of the other, viz: D-glucose and L-glucose.

The factor that distinguishes the D- and L- forms of sugars is the penultimate carbon atom. Any sugar in which the penultimate carbon atom has the –OH on the right is a D-sugar, whereas any sugar in which the penultimate carbon has the –OH on the left is an L-sugar. The designation of an isomer as a D- form or its mirror image, as L- form is determined by the spatial relationship to the parent substance of the carbohydrate family.

The number of possible isomers of any sugar depends on the number of asymmetric or chiral carbon atoms in the molecule and is derived from the formula 2ⁿ, where n= number of asymmetric or chiral carbon atoms. Aldohexoses (glucose, mannose, galactose, etc.) have 4 chiral carbon atoms. It follows that the number of isomers is sixteen, of which 8 are mirror images of the other 8. These two groups form members of the D- and L- forms of sugars.