A pentose is a monosaccharide with five carbon atoms.

They either have an aldehyde functional group in position 1 (aldopentoses), or a ketone functional group in position 2 (ketopentoses).

The aldopentoses have three chiral centres ("asymmetric carbon atoms") and so 8 different stereoisomers are possible.

The 4 D-aldopentoses, in the Fischer projection, are:

  CH=O       CH=O          CH=O         CH=O  |           |             |            | HC-OH     HO-CH           HC-OH      HO-CH  |           |             |            | HC-OH       HC-OH       HO-CH        HO-CH  |           |             |            | HC-OH       HC-OH         HC-OH        HC-OH  |           |             |            |  CH2OH       CH2OH         CH2OH        CH2OH D-Ribose    D-Arabinose   D-Xylose    D-Lyxose 

The ketopentoses have 2 chiral centres and therefore 4 possible stereoisomers — ribulose (L- and D-form) and xylulose (L- and D-form).

The D-isomers of both are known to occur naturally as is the L-isomer of xylulose:

   CH2OH        CH2OH     |            |        C=O          C=O      |            |       HC-OH      HO-CH       |            |       HC-OH        HC-OH     |            |        CH2OH        CH2OH  D-Ribulose    D-Xylulose   

The aldehyde and ketone functional groups in these carbohydrates react with neighbouring hydroxyl functional groups to form intramolecular hemiacetals or hemiketals, respectively. The resulting ring structure is related to furan, and is termed a furanose. The ring spontaneously opens and closes, allowing rotation to occur about the bond between the carbonyl group and the neighbouring carbon atom — yielding two distinct configurations (α and β). This process is termed mutarotation.

Ribose is a constituent of RNA, and the related deoxyribose of DNA.

A polymer composed of pentose sugars is called a pentosan.

[edit] See also

• Triose
• Tetrose
• Hexose
• Heptose
• Pentose phosphate pathway
• The aniline acetate test for distinguishing pentoses from other carbohydrates

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v • d • e Types of Carbohydrates General: Aldose · Ketose · Pyranose · Furanose Geometry Cyclohexane conformation · Anomer · Mutarotation Monosaccharides 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) >6 Heptose (Sedoheptulose) · Octose · Nonose (Neuraminic acid) Multiple Disaccharides Sucrose · Lactose · Maltose  · Trehalose · Turanose · Cellobiose Trisaccharides Raffinose · Melezitose · Maltotriose Tetrasaccharides Acarbose · Stachyose Other oligosaccharides Fructooligosaccharide (FOS) · Galactooligosaccharides (GOS) · Mannan-oligosaccharides (MOS) Polysaccharides Glucose/Glucan: Glycogen · Starch (Amylose, Amylopectin) · Cellulose · Dextrin/Dextran · Beta-glucan (Zymosan, Lentinan, Sizofiran)  · Maltodextrin Fructose/Fructan: Inulin · Levan beta 2→6 Mannose/Mannan Galactose/Galactan N-Acetylglucosamine: Chitin Major families of biochemicals Saccharides/Carbohydrates/Glycosides · Amino acids/Peptides/Proteins/Glycoproteins · Lipids/Terpenes/Steroids/Carotenoids · Alkaloids/Nucleobases/Nucleic acids · Cofactors/Phenylpropanoids/Polyketides/Tetrapyrroles

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