title: A density functional investigation of the extradiol cleavage mechanism in non-heme iron catechol dioxygenases.
abstract: 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 approximately 25 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 approximately 30-40 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 approximately 75 kcal mol(-1) on the U4 surface.
authors.affiliation: Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. r.j.deeth@warwick.ac.uk
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Deeth
authors.forename: Timothy D H
authors.initials: TD
authors.lastname: Bugg
mesh_terms: D002396:Catechols / Q000032:analysis / Q000737:chemistry*; D004958:Estradiol / Q000032:analysis / Q000737:chemistry*; D006418:Heme / Q000032:analysis / Q000737:chemistry*; D019159:Nonheme Iron Proteins / Q000032:analysis / Q000737:chemistry; D010105:Oxygenases / Q000032:analysis / Q000737:chemistry*
chemical_list: D002396:Catechols; D019159:Nonheme Iron Proteins; D006418:Heme; D004958:Estradiol; D010105:Oxygenases