ketohexoses

Ketohexoses are a subset of hexose sugars characterized by a ketone functional group on their carbon backbone, typically at the second carbon in the linear form. Like other sugars, they can exist in open-chain form and in cyclic forms such as pyranoses (six-membered) and furanoses (five-membered). They exhibit D- and L- configurations and form a range of stereoisomers.

The most well-known ketohexose is fructose, a naturally occurring sugar found in fruits, honey, and many plants. Other ketohexoses include sorbose, tagatose, and psicose (also known as allulose). These sugars differ in the arrangement of substituents around their carbon centers, producing various enantiomers and diastereomers.

In solution, ketohexoses can cyclize to form ring structures; for fructose, beta-D-fructofuranose is a common predominant form, while pyranose forms also exist. Ketohexoses participate in disaccharide formation, for example glucose–fructose linkage in sucrose, which combines glucose and fructose units.

Biological and practical aspects: fructose is absorbed and metabolized differently from glucose, primarily in the liver, where it is phosphorylated by fructokinase to fructose-1-phosphate and, after cleavage by aldolase B, yields dihydroxyacetone phosphate and glyceraldehyde. These intermediates enter glycolysis downstream of a key regulatory step, influencing metabolic pathways. Ketohexoses are used as nutrients and, in some cases, as low-calorie sweeteners (e.g., allulose).