Trapping and spectroscopic characterization of an FeIII-superoxo intermediate from a nonheme mononuclear iron-containing enzyme.
Fe(III)-O(2)*(-) intermediates are well known in heme enzymes, but none have been characterized in the nonheme mononuclear Fe(II) enzyme family. Many steps in the O(2) activation and reaction cycle of Fe(II)-containing homoprotocatechuate 2,3-dioxygenase are made detectable by using the alternative substrate 4-nitrocatechol (4NC) and mutation of the active site His200 to Asn (H200N). Here, the first intermediate (Int-1) observed after adding O(2) to the H200N-4NC complex is trapped and characterized using EPR and Mössbauer (MB) spectroscopies. Int-1 is a high-spin (S(1) = 5/2) Fe(III) antiferromagnetically (AF) coupled to an S(2) = 1/2 radical (J ≈ 6 cm(-1) in ). It exhibits parallel-mode EPR signals at g = 8.17 from the S = 2 multiplet, and g = 8.8 and 11.6 from the S = 3 multiplet. These signals are broadened significantly by hyperfine interactions (A((17)O) ≈ 180 MHz). Thus, Int-1 is an AF-coupled species. The experimental observations are supported by density functional theory calculations that show nearly complete transfer of spin density to the bound O(2). Int-1 decays to form a second intermediate (Int-2). MB spectra show that it is also an AF-coupled Fe(III)-radical complex. Int-2 exhibits an EPR signal at g = 8.05 arising from an S = 2 state. The signal is only slightly broadened by (< 3% spin delocalization), suggesting that Int-2 is a peroxo-Fe(III)-4NC semiquinone radical species. Our results demonstrate facile electron transfer between Fe(II), O(2), and the organic ligand, thereby supporting the proposed wild-type enzyme mechanism.
Mbughuni
Michael M
MM
Department of Biochemistry, Molecular Biology and Biophysics, and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, MN 55455, USA.
Chakrabarti
Mrinmoy
M
Hayden
Joshua A
JA
Bominaar
Emile L
EL
Hendrich
Michael P
MP
Münck
Eckard
E
Lipscomb
John D
JD
D002396:Catechols / Q000737:chemistry*; D003461:Crystallography; D049308:Dioxygenases / Q000737:chemistry* / Q000235:genetics; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D007501:Iron / Q000737:chemistry*; D009154:Mutation; D009574:Nitro Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D015204:Spectroscopy, Mossbauer; D013379:Substrate Specificity; D013481:Superoxides / Q000737:chemistry*
D002396:Catechols; D009574:Nitro Compounds; C506350:nitrocatechol; D013481:Superoxides; D007501:Iron; D049308:Dioxygenases; C020055:3,4-dihydroxyphenylacetate 2,3-dioxygenase