Quercetin 2,3-dioxygenase is a copper-dependent enzyme that catalyses the reaction of dioxygen with quercin (3,5,7,3',4'-pentahydroxy flavone). Dioxygenases are enzymes that catalyses the incorporation of both atoms of molecular oxygen into organic substrates, most notably during the degradation of aromatic compounds. They typically use a metal ion to circumvent the spin barrier that prevents the direct reaction of the triplet ground state of dioxygen with singlet-state organic compounds. In most of the well-studied dioxygenases, the metal used is iron; quercetin 2,3-dioxygenase is the only known member of this family to use copper.