 The bacterial quercetinase metal centers typically have a trigonal bipyramidal or octahedral coordination environment when there are four protein ligands; the metal centers of the copper-dependent QueD enzymes possesses a distorted tetrahedral geometry in which only the three conserved histidine residues provide coordination ligands.Empty coordination sites in all metal centers are occupied by aqua ligands until these are displaced by the incoming substrate.The ability of these dioxygenases to retain activity in the presence of other metal cofactors with wide ranges of redox potentials suggests the metal center does not play an active role in the activation of dioxygen.