Performs the first step in the degradation of the flavonoid quercetin by a dioxygenase reaction. The enzyme catalyzes the cleavage of the O-heteroaromatic ring of the flavonol quercetin yielding the depside 2-protocatechuoyl-phloroglucinol carboxylic acid and carbon monoxide. This involves the remarkable dioxygenolytic cleavage of two carbon-carbon bonds. O2 + quercetin = 2-(3,4-dihydroxybenzoyloxy)-4,6-dihydroxybenzoate + CO Binds 1 copper ion per subunit. Inhibited by diethyldithiocarbamate and kojic acid. Flavonoid metabolism; quercetin degradation. Homodimer. The N-linked glycan at Asn-191 consists of Man(5)-GlcNAc(2).