ketose

A ketose is a type of monosaccharide in which the carbonyl functional group is a ketone rather than an aldehyde. In most ketoses the carbonyl lies at the second carbon of the chain (C2), although variations occur in longer sugars. Ketoses are distinguished from aldoses, whose carbonyl group is at the first carbon. Like aldoses, ketoses have multiple hydroxyl groups and exist in both D- and L- configurations.

Ketoses can cyclize in aqueous solution to form ring structures, typically five-membered (furanose) or six-membered (pyranose) rings. Fructose, the best-known ketose, is a ketohexose and commonly adopts a furanose form in solution. Other ketoses include dihydroxyacetone (the simplest ketose, a triose) and ketopentoses such as xylulose and ribulose. Rare examples include allulose (D-psicose) and tagatose.

In biology and nutrition, ketoses occur as metabolic intermediates and dietary sugars. Ribulose and xylulose participate in the pentose phosphate pathway, providing ribose-5-phosphate and related intermediates for nucleotide synthesis. Fructose is a major dietary sugar, metabolized primarily in the liver and used as an energy source. Allulose and tagatose have gained interest as low-calorie sweeteners.

Reactivity and chemistry: ketoses are reducing sugars because they can tautomerize under alkaline conditions to aldoses, enabling reactions that typically require an aldehyde carbonyl. They can also be chemically reduced to corresponding sugar alcohols (alditols). In various contexts, ketoses serve as biomarkers, intermediates in biosynthetic pathways, and industrial feedstocks.