The mechanism of the action of copper-dependent quercetin 2,3-dioxygenase （2,3QD） has been investigated by means of hy- brid density functional theory. The 2,3QD enzyme cleaves the O-heterocycle of a quercetin by incorporation of both oxygen atoms into the substrate and releases carbon monoxide. The calculations show that dioxygen attack on the copper complex is energetically favorable. The adduct has a possible near-degeneracy of states between [Cu2＋-（substrate H＋）] and [Cu＋-（sub- strate-H）. ], and in addition the pyramidalized C2 atom is ideally suited for forming a dioxygembridged structure. In the next step, the C3-C4 bond is cleaved and intermediate lnt5 is formed via transition state TS4. Finally, the Oa-Ob and C2-C3 bonds are cleaved, and CO is released in one concerted transition state （TS5） with the barrier of 63.25 and 61.91 k J/tool in the gas phase and protein environments, respectively. On the basis of our proposed reaction mechanism, this is the rate-limiting step of the whole catalytic cycle and is strongly driven by a relatively large exothermicity of 100.86 kJ/mol. Our work provides some valuable fundamental insights into the behavior of this enzyme.
