Quercetin 2,3-dioxygenase (2,3QD) is a copper-containing dioxygenase that catalyses the oxidation of the flavonol quercetin to 2-protocatechuoylphloroglucinol carboxylic acid with concomitant production of carbon monoxide. In contrast to iron dioxygenases, very little is known about copper dioxygenases. We have characterized 2,3QD from the fungus Aspergillus japonicus by electron paramagnetic resonance spectroscopy (EPR). At pH , 2,3QD shows a mixture of two EPR species. The major form has parameters typical of type 2 Cu sites (g// ¼ , A// ¼ mT), the minor one has a more distorted geometry (g// ¼ , A// ¼ mT)
