Pentose
23640
209384492
2008-05-01T03:25:28Z
Webridge
4860466
/* See also */
A '''pentose''' is a [[monosaccharide]] with five [[carbon]] [[atom]]s.
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 [[stereoisomer]]s are possible.
The 4 <small>D</small>-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
| | | |
CH<sub>2</sub>OH CH<sub>2</sub>OH CH<sub>2</sub>OH CH<sub>2</sub>OH
<small>D</small>-[[Ribose]] <small>D</small>-[[Arabinose]] <small>D</small>-[[Xylose]] <small>D</small>-[[Lyxose]]
A [[mnemonic]] suggested that can be used to remember the four <small>D</small>-aldopentoses is "'''rib'''s '''ar'''e e'''x'''tra '''l'''ean."
The ketopentoses have 2 chiral centres and therefore 4 possible stereoisomers — [[ribulose]] (<small>L</small>- and <small>D</small>-form) and [[xylulose]] (<small>L</small>- and <small>D</small>-form).
The <small>D</small>-isomers of both are known to occur naturally as is the <small>L</small>-isomer of [[xylulose]]:
CH<sub>2</sub>OH CH<sub>2</sub>OH
| |
C=O C=O
| |
HC-OH HO-CH
| |
HC-OH HC-OH
| |
CH<sub>2</sub>OH CH<sub>2</sub>OH
<small>D</small>-[[Ribulose]] <small>D</small>-[[Xylulose]]
The aldehyde and ketone functional groups in these carbohydrates react with neighbouring [[hydroxyl]] functional groups to form [[intramolecular]] [[hemiacetal]]s or [[hemiketal]]s, 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]].
== See also ==
* [[Triose]]
* [[Tetrose]]
* [[Hexose]]
* [[Heptose]]
* [[Pentose phosphate pathway]]
{{biochem-stub}}
{{Carbohydrates}}
[[Category:Monosaccharides]]
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