Quercetin 2,4-Dioxygenase Activates Dioxygen in a Side-On O2-Ni Complex

Quercetin 2,4-dioxygenase (quercetinase) from Streptomyces uses nickel as the active-site cofactor to catalyze oxidative cleavage of the flavonol quercetin. How this unusual active-site metal supports catalysis and O-2 activation is under debate. We present crystal structures of Ni-quercetinase in three different states, thus providing direct insight into how quercetin and O-2 are activated at the Ni2+ ion. The Ni2+ ion is coordinated by three histidine residues and a glutamate residue (E-76) in all three states. Upon binding, quercetin replaces one water ligand at Ni and is stabilized by a short hydrogen bond through E-76, the carboxylate group of which rotates by 90 degrees. This conformational change weakens the interaction between Ni and the remaining water ligand, thereby preparing a coordination site at Ni to bind O-2. O-2 binds side-on to the Ni2+ ion and is perpendicular to the C2-C3 and C3-C4 bonds of quercetin, which are cleaved in the following reaction steps.