A density functional investigation of the extradiol cleavage mechanism in non-heme iron catechol dioxygenases

The mechanism for extradiol cleavage in non-heme iron catechol dioxygenase was modelled theoretically via density functional theory. Based on the Fe-II-His,His,Glu motif observed in enzymes, an active site model complex, [Fe(acetate)(imidazole)(2)(catecholate)(O-2)](-), was optimized for states with six, four and two unpaired electrons (U6, U4 and U2, respectively). The transfer of the terminal atom of the coordinated dioxygen leading to "ferryl" Fe = O intermediates spontaneously generates an extradiol epoxide. The computed barriers range from 19 kcal mol(-1) on the U6 surface to similar to25 kcal mol(-1) on the U4 surface, with overall reaction energies of + 11.6, 6.3 and 7.1 kcal mol(-1) for U6, U4 and U2, respectively. The calculations for a protonated process reveal the terminal oxygen Of O-2 to be the thermodynamically favoured site but subsequent oxygen transfer to the catechol has a barrier of similar to30-40 kcal mol(-1), depending on the spin state. Instead, protonating the acetate group gives a slightly higher energy species but a subsequent barrier on the U4 surface of only 7 kcal mol(-1) relative to the hydroperoxide complex. The overall exoergicity increases to 13 kcal mol(-1). The favoured proton-assisted pathway does not involve significant radical character and has features reminiscent of a Criegee rearrangement which involves the participation of the aromatic ring pi-orbitals in the formation of the new carbon-oxygen bond. The subsequent collapse of the epoxide, attack by the coordinated hydroxide and final product formation proceeds with an overall exoergicity of similar to75 kcal mol(-1) on the U4 surface.