xylulose

Xylulose is a five-carbon monosaccharide of the ketose class. It exists in two enantiomeric forms, D-xylulose and L-xylulose, with D-xylulose being the more biologically prominent form in humans. In its open-chain form the molecule bears a ketone group at carbon 2 and five carbon atoms, giving the formula C5H10O5. In solution it equilibrates with cyclic furanose forms, as is typical for ketopentoses.

Biological occurrence: In human metabolism the most relevant form is D-xylulose-5-phosphate, a phosphorylated sugar produced in the pentose phosphate pathway. It is generated by the isomerization of ribulose-5-phosphate to xylulose-5-phosphate via ribulose-5-phosphate-3-epimerase and participates in non-oxidative PPP steps through transketolase reactions.

Role in metabolism: Xylulose-5-phosphate serves as a donor of two-carbon units to ribose-5-phosphate or as a substrate for the production of glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate, linking carbohydrate metabolism with nucleotide synthesis and glycolysis. Xylulose can also be formed from xylose, the aldopentose substrate, by microbial and some plant enzymes during xylose metabolism or oxidation.

Clinical notes: L-xylulose appears in urine in essential pentosuria, a rare benign inherited condition. The disorder reflects impaired metabolism of xylulose with accumulation and urinary excretion, but it typically has no serious health consequences.

Other notes: Free xylulose is typically present at low levels in foods and is not a major dietary component; its significance is mainly as an intermediate in the pentose phosphate pathway and in microbial metabolism.