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 &mdash; [[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 &mdash; yielding two distinct configurations (&alpha; and &beta;). 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]] [[ar:بنتوز]] [[cs:Pentosa]] [[cy:Pentos]] [[da:Pentose]] [[de:Pentosen]] [[es:Pentosa]] [[eo:Pentozo]] [[fr:Pentose]] [[id:Pentosa]] [[it:Pentosi]] [[mk:Пентоза]] [[nl:Pentose]] [[oc:Pentòsa]] [[pl:Pentozy]] [[pt:Pentose]] [[ru:Пентозы]] [[sr:Пентоза]] [[sh:Pentoza]] [[su:Péntosa]] [[fi:Pentoosi]] [[sv:Pentoser]] [[uk:Пентоза]] [[zh:戊糖]]