aldopentoses

Aldopentoses are a class of aldose monosaccharides that contain five carbon atoms. They have an aldehyde group at carbon 1 and three stereogenic centers at C2–C4, with C5 bearing a primary alcohol (CH2OH). In Fischer projections they are depicted with the aldehyde at the top and the hydroxyl pattern on the remaining carbons, giving multiple stereoisomers. There are eight possible stereoisomers in total, consisting of four pairs of enantiomers. The best known D-aldopentoses are D-ribose, D-arabinose, D-xylose, and D-lyxose; each has an L-counterpart.

In solution, aldopentoses readily form cyclic structures through intramolecular hemiacetal formation, predominantly five-membered rings called furanoses. These rings can exist in alpha or beta anomeric forms at C1 as they interconvert (mutarotation).

Biological relevance varies among the aldopentoses. Ribose is a component of RNA and participates in metabolism via ribose-5-phosphate in the pentose phosphate pathway. Xylose and arabinose occur in plant hemicelluloses and can be metabolized by certain microorganisms through pathways that yield intermediates such as xylulose-5-phosphate or ribulose-5-phosphate. Aldopentoses are related to deoxypentoses, such as deoxyribose in DNA, though deoxyribose lacks an oxygen on C2. Enzymes such as aldose isomerases and various dehydrogenases mediate interconversion and catabolism of aldopentoses in biological systems.

Sources and applications include the breakdown of plant polysaccharides; commercially, xylose is used in fermentation processes and for the production of sugar substitutes like xylitol.