In enzymology, a 4-deoxy-L-threo-5-hexosulose-uronate ketol-isomerase (EC 5.3.1.17) is an enzyme that catalyzes the chemical reaction

4-deoxy-L-threo-5-hexosulose uronate ${\displaystyle \rightleftharpoons }$ 3-deoxy-D-glycero-2,5-hexodiulosonate

Hence, this enzyme has one substrate, 4-deoxy-L-threo-5-hexosulose uronate, and one product, 3-deoxy-D-glycero-2,5-hexodiulosonate.

This enzyme belongs to the family of isomerases, specifically those intramolecular oxidoreductases interconverting aldoses and ketoses. The systematic name of this enzyme class is 4-deoxy-L-threo-5-hexosulose-uronate aldose-ketose-isomerase. This enzyme is also called 4-deoxy-L-threo-5-hexulose uronate isomerase. This enzyme participates in pentose and glucuronate interconversions.

As of late 2007, 3 structures have been solved for this class of enzymes, with PDB accession codes 1X8M, 1XRU, and 1YWK.

• Preiss J (1966). "4-Deoxy-L-threo-5-hexosulose uronic acid isomerase". Carbohydrate Metabolism. Methods in Enzymology. Vol. 9. pp. 602–604. doi:10.1016/0076-6879(66)09121-3. ISBN 978-0-12-181809-8.