Fructose, ribulose and xylulose are the three main examples of ketose sugars. Other examples include erythrulose, tagatose, sorbose, psicose and dihydroxyacetone, the last of which is still considered a ketose, even though it is not a sugar.
Ketose sugars are defined as a monosaccharide that contains one ketone group in its chemical structure. A ketone group consists of one oxygen atom and one carbon atom that are joined together by a double bond. Ketose sugars differ from the other primary group of monosaccharides, aldose sugars, which contain one aldehyde group in place of the ketone group. These groups are just two examples of many types of carbohydrates.