Inchemistry, apentose is amonosaccharide (simple sugar) with fivecarbonatoms.[1] Thechemical formula of many pentoses isC
5H
10O
5, and theirmolecular weight is 150.13 g/mol.[2]
Pentoses are very important inbiochemistry.Ribose is a constituent ofRNA, and the related molecule,deoxyribose, is a constituent ofDNA.Phosphorylated pentoses are important products of thepentose phosphate pathway, most importantlyribose 5-phosphate (R5P), which is used in the synthesis ofnucleotides andnucleic acids.
Like some other monosaccharides, pentoses exist in two forms, open-chain (linear) or closed-chain (cyclic), that easily convert into each other in water solutions.[3] The linear form of a pentose, which usually exists only in solutions, has an open-chain backbone of five carbons. Four of these carbons have onehydroxylfunctional group (–OH) each, connected by a singlebond, and one has an oxygen atom connected by a double bond (=O), forming acarbonyl group (C=O). The remaining bonds of the carbon atoms are satisfied by sixhydrogen atoms. Thus the structure of the linear form is H–(CHOH)x–C(=O)–(CHOH)4-x–H, wherex is 0, 1, or 2.
The term "pentose" sometimes is assumed to includedeoxypentoses, such asdeoxyribose: compounds with general formulaC
5H
10O
5-y that can be described as derived from pentoses by replacement of one or more hydroxyl groups with hydrogen atoms.
Thealdopentoses are a subclass of the pentoses which, in the linear form, have the carbonyl at carbon 1, forming analdehyde derivative with structure H–C(=O)–(CHOH)4–H. The most important example isribose. Theketopentoses instead have the carbonyl at positions 2 or 3, forming aketone derivative with structure H–CHOH–C(=O)–(CHOH)3–H (2-ketopentose) or H–(CHOH)2–C(=O)–(CHOH)2–H (3-ketopentose). The latter is not known to occur in nature and are difficult to synthesize.
In the open form, there are eight aldopentoses and four 2-ketopentoses,stereoisomers that differ in the spatial position of the hydroxyl groups. These forms occur in pairs ofoptical isomers, generally labelled "D" or "L" by conventional rules (independently of theiroptical activity).
The aldopentoses have threechiral centers; therefore, eight (23) differentstereoisomers are possible.
 D-Arabinose |  D-Lyxose |  D-Ribose |  D-Xylose |
 L-Arabinose |  L-Lyxose |  L-Ribose |  L-Xylose |
Ribose is a constituent ofRNA, and the related molecule,deoxyribose, is a constituent ofDNA. Phosphorylated pentoses are important products of thepentose phosphate pathway, most importantlyribose 5-phosphate (R5P), which is used in the synthesis ofnucleotides and nucleic acids, anderythrose 4-phosphate (E4P), which is used in the synthesis ofaromatic amino acids.
The 2-ketopentoses have two chiral centers; therefore, four (22) different stereoisomers are possible. The 3-ketopentoses are rare.
 D-Ribulose |  D-Xylulose |
 L-Ribulose |  L-Xylulose |
The closed or cyclic form of a pentose forms when thecarbonyl group reacts with ahydroxyl in another carbon, turning the carbonyl into a hydroxyl and creating anether bridge –O– between the two carbons. Thisintramolecular reaction yields acyclic molecule, with a ring consisting of one oxygen atom and usually four carbon atoms; the cyclic compounds are then calledfuranoses, for having the same rings as thecyclic ethertetrahydrofuran.[3]
The ring closure converts the carbonyl carbon into achiral center, which may adopt either of two configurations, depending on the position of the new hydroxyl. Therefore, each linear form can produce two distinct closed forms, identified by prefixes "α" and "β".
The one deoxypentose has two total stereoisomers.
 D-Deoxyribose |
 L-Deoxyribose |
In the cell, pentoses have a highermetabolic stability thanhexoses.
Apolymer composed of pentose sugars is called apentosan.
The most important tests for pentoses rely on converting the pentose to furfural, which then reacts with achromophore. InTollens' test for pentoses (not to be confused withTollens' silver-mirror test forreducing sugars), thefurfural ring reacts withphloroglucinol to produce a colored compound;[4] in theaniline acetate test with aniline acetate;[5] and inBial's test, withorcinol.[6] In each of these tests, pentoses react much more strongly and quickly than hexoses.
Types ofcarbohydrates | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| General | |||||||||||||||
| Geometry | |||||||||||||||
| Monosaccharides |
| ||||||||||||||
| Multiple |
| ||||||||||||||
| International | |
|---|---|
| National | |
| Other | |
