Pentoses

Pentoses are monosaccharides that contain five carbon atoms. They can be classified as aldopentoses, which have an aldehyde group at carbon 1, or ketopentoses, which have a ketone at carbon 2. The most important examples in biology are ribose, deoxyribose, xylose, and arabinose, as well as the ketopentoses ribulose and xylulose. In natural ribose, the sugars exist in both D- and L- forms, with the D-enantiomers predominating in biology. In solution, pentoses can form furanose rings, five-membered cyclic forms, as well as linear chains.

In nucleic acids, ribose is the sugar component of RNA, while 2'-deoxyribose is the sugar in DNA. The pentose moiety also occurs in essential cofactors such as ATP, NAD+, and FAD. Although pentoses are not major energy sources in humans, they are interconverted and utilized in metabolism.

A central hub for pentose metabolism is the pentose phosphate pathway (PPP). The oxidative phase generates NADPH and ribulose-5-phosphate, which can be converted into ribose-5-phosphate for nucleotide synthesis. The non-oxidative phase interconverts sugars to glyceraldehyde-3-phosphate and fructose-6-phosphate, linking pentose metabolism with glycolysis and lipid synthesis.

Pentoses are abundant in nature, particularly in plant polysaccharides such as xylans and arabinoxylans. Xylose and arabinose are released by hydrolysis and are used by some microbes as carbon sources. In nutrition, they contribute to the diet as minor sugars and, as reducing sugars, participate in browning reactions.