Pentose
Pentoses (from Greek pente five) are monosaccharides, the carbon backbone contains five carbon atoms. They all have the molecular formula C5H10O5 and differ by the nature of the carbonyl function. If it is a ketone group, one speaks of keptopentoses with an aldehyde group they are called aldopentoses.
- 2.1 All ketopentoses in Fischer projection
The aldopentoses
The aldopentoses have three chiral centers, there is therefore eight stereoisomers. These occur only D-ribose, D -xylose, D- arabinose and L-arabinose in nature. Lyxose occurs in nature not in free form. D / L-configuration applies in the aldopentoses always to the position of the OH group at the 4 carbon atom and has no relation to the rotation direction of the optical activity.
All aldopentoses in Fischer projection
Arabinose
Xylose
Lyxose
Through an intramolecular hemiacetal formation, these open-chain molecules are in equilibrium with their pyranose or furanose forms. This leads to the oxygen bridge between the 1st and the 5th carbon atom ( pyranose ) and the first and fourth carbon atom ( furanose ), and the oxygen atom of the hydroxyl group is Aldhydgruppe.
The ketopentoses
The ketopentoses possess two chiral centers, there are therefore four stereoisomers. The D / L- configuration refers - as above - to the position of the OH group on the fourth carbon atom and has no relation to the direction of the optical activity. In nature, only the two D- isomers occur.
All ketopentoses in Fischer projection
Xylulose
Pentoses and their biological significance
The D- ribose and its reduced form, the D -deoxyribose are essential components of RNA and DNA.
D -ribulose is used as a CO2 acceptor in photosynthesis.
The metabolism of pentoses and their formation by decarboxylation of hexoses is summarized in the pentose phosphate cycle. Key reaction is the direct oxidation of glucose-6 -phosphate by the enzyme glucose-6 -phosphate dehydrogenase, whose gene is located in humans on the X chromosome. Defects can cause to certain drugs, a number of disease states (eg, in the formation of blood ) and overreactions.