EPR spectra of different forms of quercetin 2,4-dioxygenase from A. japonicus. A) Resting quercetin 2,4-dioxygenase at pH 6 (solid lines represent the major, tetrahedral form and dashed lines represent the minor, 5 coordinate form). B) Resting quercetin 2,4-dioxygenase at pH 10 (only the trigonal bipyramidal form contributes). C) quercetin 2,4-dioxygenase enzyme-substrate complex with 1 eq. quercetin (2.5% DMSO). D) quercetin 2,4-dioxygenase enzyme-product complex observed after exposure of (C) to air.