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 2 Fe(2+) ions per subunit. Flavonoid metabolism; quercetin degradation. Homodimer.