past.paper.5 - Published on Web 10/30/2007 Characterization...

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Characterization of the Arene-Oxidizing Intermediate in ToMOH as a Diiron(III) Species Leslie J. Murray, ² Sunil G. Naik, Danilo O. Ortillo, Ricardo Garcı ´ a-Serres, Jessica K. Lee, ² Boi Hanh Huynh,* ,‡ and Stephen J. Lippard* Contribution from the Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Physics, Emory Uni V ersity, Atlanta, Georgia 30322 Received August 14, 2007; E-mail: lippard@mit.edu; vhuynh@physics.emory.edu Abstract: We report the generation and characterization of a diiron(III) intermediate formed during reaction with dioxygen of the reduced hydroxylase component of toluene/ o -xylene monooxygenase from Pseudomo- nas sp. OX1. The decay rate of this species is accelerated upon mixing with phenol, a substrate for this system. Under steady-state conditions, hydrogen peroxide was generated in the absence of substrate. The oxidized hydroxylase also decomposed hydrogen peroxide to liberate dioxygen in the absence of reducing equivalents. This activity suggests that dioxygen activation may be reversible. The linear free energy relationship determined from hydroxylation of para-substituted phenols under steady-state turnover has a negative slope. A value of F< 0 is consistent with electrophilic attack by the oxidizing intermediate on the aromatic substrates. The results from these steady and pre-steady-state experiments provide compelling evidence that the diiron(III) intermediate is the active oxidant in the toluene/ -xylene monooxygenase system and is a peroxodiiron(III) transient, despite differences between its optical and Mo ¨ssbauer spectroscopic parameters and those of other peroxodiiron(III) centers. Introduction Dioxygen activation at structurally homologous active sites in metalloproteins often leads to transient species having similar spectroscopic properties. For example, heme prosthetic groups bind dioxygen in their ferrous forms to generate η 1 -peroxo- iron(III) intermediates. 1,2 Protonation of the distal oxygen atom in this intermediate facilitates O - O bond cleavage and forma- tion of an (oxo)iron(IV) - porphyrin cation radical (Scheme 1). The reactivity of the heme iron unit is modulated by the proximal ligand trans to the O 2 -binding site on the metal center and by the surrounding protein matrix, dictating dioxygen binding or activation depending on the requisite enzyme function. Similar phenomena occur for the non-heme carboxyl- ate-bridged diiron (CBDI) protein superfamily, in which a reduced, diiron(II) form reacts with dioxygen (Scheme 1). 3,4 The choice of nonbridging ligands at the dimetallic center, histidine vs aspartate or glutamate, and the availability of an open coordination site determine whether O 2 will bind reversibly or be activated for substrate oxidation. For dioxygen activation, the diiron(II) form of the protein typically reacts with dioxygen to form a peroxodiiron(III) center. In the hydroxylase component of methane monooxygenase (MMOH), either homolytic
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past.paper.5 - Published on Web 10/30/2007 Characterization...

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