Quercetin 2,3-dioxygenase () is an enzyme that catalyzes the chemical reaction
The two substrates of this enzyme are quercetin and oxygen. Its products are 2-(3,4-dihydroxybenzoyloxy)-4,6-dihydroxybenzoic acid and carbon monoxide.
This enzyme belongs to the family of oxidoreductases, specifically those acting on single donors with O<sub>2</sub> as oxidant and incorporation of two atoms of oxygen into the substrate (oxygenases). The oxygen incorporated need not be derived from O<sub>2</sub>. The systematic name of this enzyme class is quercetin:oxygen 2,3-oxidoreductase (decyclizing). Other names in common use include quercetinase and flavonol 2,4-oxygenase. It has two cofactors: iron and copper.
As of late 2007, six crystal structures have been solved for this class of enzymes, with PDB accession codes , , , , , and .