Hexose

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A hexose is a monosaccharide with six carbon atoms having the chemical formula C₆H₁₂O₆.

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

The aldohexoses have four chiral centres ("asymmetric carbon atoms") and so 16 different stereoisomers are possible. Of these, only 3 occur in nature - D-glucose, D-galactose and D-mannose. The D/L configuration is based on the orientation of the hydroxyl group at position 3, and does not refer to the direction of optical activity.

The eight D-aldohexoses 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
   |              |             |               |
  HC-OH          HC-OH         HC-OH           HC-OH
   |              |             |               |
   CH2-OH         CH2-OH        CH2-OH          CH2-OH
D-Allose      D-Altrose     D-Glucose      D-Mannose

   CH=O        CH=O            CH=O            CH=O
   |           |               |               |
  HC-OH     HO-CH             HC-OH         HO-CH
   |           |               |               |
  HC-OH       HC-OH         HO-CH           HO-CH
   |           |               |               |
HO-CH       HO-CH           HO-CH           HO-CH
   |           |               |               |
  HC-OH       HC-OH           HC-OH           HC-OH
   |           |               |               |
   CH2-OH      CH2-OH          CH2-OH          CH2-OH
D-Gulose    D-Idose       D-Galactose        D-Talose

Mnemonic: all altruists gladly make gum in gallon tanks.

The ketohexoses have 3 chiral centres and therefore 8 possible stereoisomers. Of these, only the 4 D-isomers are known to occur naturally - D-Fructose, D-Psicose, D-Sorbose and D-Tagatose.

Only the naturally occurring hexoses are capable of being fermented by yeasts.

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 pyran, and is termed a pyranose. 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. Hexose sugars can form dihexose sugars with a condensation reaction to form a 1,6 glycosidic bond

See also

• Triose
• Tetrose
• Pentose
• Heptose

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