Oxygenolysis reaction mechanism of copper-dependent quercetin 2,3-dioxygenase: A density functional theory study

The mechanism of the action of copper-dependent quercetin 2,3-dioxygenase (2,3QD) has been investigated by means of hybrid 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)center dot], and in addition the pyramidalized C-2 atom is ideally suited for forming a dioxygen-bridged structure. In the next step, the C-3-C-4 bond is cleaved and intermediate Int(5) is formed via transition state TS4. Finally, the O-a-O-b and C-2-C-3 bonds are cleaved, and CO is released in one concerted transition state (TS5) with the barrier of 63.25 and 61.91 kJ/mol 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.