monosaccharide

Definition: Monosaccharides (from Greek monos: single, sacchar: sugar) are the simplest carbohydrates. They cannot be hydrolyzed into simpler sugars. They consist of one sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste.

Monosaccharides are the major source of fuel for metabolism, being used both as an energy source (glucose being the most important in nature) and in biosynthesis.

When monosaccharides are not immediately needed by many cells they are often converted to more space efficient forms, often polysaccharides.

In many animals, including humans, this storage form is glycogen, especially in liver and muscle cells. In plants, starch is used for the same purpose.

Examples of monosaccharides include glucose (dextrose), fructose, galactose, xylose and ribose.

Monosaccharides are the building blocks of disaccharides like sucrose (common sugar) and polysaccharides (such as cellulose and starch).

Further, each carbon atom that supports a hydroxyl group (except for the first and last) is chiral, giving rise to a number of isomeric forms all with the same chemical formula.

For instance, galactose and glucose are both aldohexoses, but they have different chemical and physical properties.

Classification

 trioses

• ketotriose (dihydroxyacetone)
• aldotriose (glyceraldehyde)

 tetroses

• Ketotetrose (erythrulose)
• Aldotetroses (erythrose, threose)

 pentoses

• Ketopentose (Ribulose, Xylulose)
• Aldopentose (Ribose, Arabinose, Xylose, Lyxose)
• Deoxy sugar (deoxyribose)

 hexoses

• ketohexose (Psicose, fructose, Sorbose, Tagatose)
• aldohexose (Allose, Altrose, glucose, mannose, Gulose, Idose, galactose, Talose)
• deoxy sugar (fucose, Fuculose, Rhamnose)

 heptose

• sedoheptulose

Monosaccharides are classified according to three different characteristics:
 the placement of its carbonyl group,
 the number of carbon atoms it contains,
 its chiral handedness.

If the carbonyl group is an aldehyde, the monosaccharide is an aldose; if the carbonyl group is a ketone, the monosaccharide is a ketose.

Monosaccharides with three carbon atoms are called trioses, those with four are called tetroses, five are called pentoses, six are hexoses, and so on.

These two systems of classification are often combined. For example, glucose is an aldohexose (a six-carbon aldehyde), ribose is an aldopentose (a five-carbon aldehyde), and fructose is a ketohexose (a six-carbon ketone).

Each carbon atom bearing a hydroxyl group (-OH), with the exception of the first and last carbons, are asymmetric, making them stereocenters with two possible configurations each (R or S).

Because of this asymmetry, a number of isomers may exist for any given monosaccharide formula.

The aldohexose D-glucose, for example, has the formula (C·H2O)6, of which all but two of its six carbons atoms are stereogenic, making D-glucose one of 2x2x2x2 = 16 possible stereoisomers.

In the case of glyceraldehyde, an aldotriose, there is one pair of possible stereoisomers, which are enantiomers and epimers.

1,3-dihydroxyacetone, the ketose corresponding to the aldose glyceraldehyde, is a symmetric molecule with no stereocenters).

The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group: in a standard Fischer projection if the hydroxyl group is on the right the molecule is a D sugar, otherwise it is an L sugar.

The "D-" and "L-" prefixes should not be confused with "d-" or "l-", which indicate the direction that the sugar rotates plane polarized light.

This usage of "d-" and "l-" is no longer followed in carbohydrate chemistry.

Ring-straight chain isomerism

The aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal, forming a heterocyclic ring with an oxygen bridge between two carbon atoms.

Rings with five and six atoms are called furanose and pyranose forms, respectively, and exist in equilibrium with the straight-chain form.

During the conversion from straight-chain form to the cyclic form, the carbon atom containing the carbonyl oxygen, called the anomeric carbon, becomes a stereogenic center with two possible configurations: The oxygen atom may take a position either above or below the plane of the ring.

The resulting possible pair of stereoisomers are called anomers.

In the α anomer, the -OH substituent on the anomeric carbon rests on the opposite side (trans) of the ring from the CH2OH side branch.

The alternative form, in which the CH2OH substituent and the anomeric hydroxyl are on the same side (cis) of the plane of the ring, is called the β anomer.

In a Fischer Projection, the α anomer is represented with the anomeric hydroxyl group trans to the CH2OH and cis in the β anomer.

See also

 carbohydrates