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pentose

 
Dictionary: pen·tose   (pĕn'tōs', -tōz') pronunciation
 
n.

Any of a class of monosaccharides having five carbon atoms per molecule and including ribose and several other sugars.


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A sugar that has five carbon atoms per molecule. See monosaccharide.



 
Food and Nutrition: pentoses
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Monosaccharide sugars with five carbon atoms. The most important is ribose.

 

A monosaccharide containing five carbon atoms in a molecule.

  • p. cycle — see pentose phosphate pathway (below).
  • p. phosphate pathway — called also pentose cycle; see pentose phosphate pathway.
 
Wikipedia: Pentose
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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 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.

See also


 
 

 

Copyrights:

Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Chemistry Dictionary. A Dictionary of Chemistry. Sixth Edition. Copyright © Market House Books Ltd, 2008. All rights reserved.  Read more
Food and Nutrition. A Dictionary of Food and Nutrition. Copyright © 1995, 2003, 2005 by A. E. Bender and D. A. Bender. All rights reserved.  Read more
Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.  Read more
Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pentose" Read more