In vivo self-hydroxylation of an iron-substituted manganese-dependent extradiol cleaving catechol dioxygenase

The homoprotocatechuate 2,3-dioxygenase from Arthrobacter globiformis (MndD) catalyzes the oxidative ring cleavage reaction of its catechol substrate in an extradiol fashion. Although this reactivity is more typically associated with non-heme iron enzymes, MndD exhibits an unusual specificity for manganese(II). MndD is structurally very similar to the iron(II)-dependent homoprotocatechuate 2,3-dioxygenase from Brevibacterium fuscum (HPCD), and we have previously shown that both MndD and HPCD are equally active towards substrate turnover with either iron(II) or manganese(11) (Emerson et al. in Proc. Natl. Acad. Sci. USA 105:7347-7352, 2008). However, expression of MndD in Escherichia coli under aerobic conditions in the presence of excess iron results in the isolation of inactive blue-green iron-substituted MndD. Spectroscopic studies indicate that this form of iron-substituted MndD contains an iron(III) center with abound catecholate, which is presumably generated by in vivo self-hydroxylation of a second-sphere tyrosine residue, as found for other self-hydroxylated non-heme iron oxygenases. The absence of this modification in either the native manganese-containing MndD or iron-containing HPCD suggests that the metal center of iron-substituted MndD is able to bind and activate 02 in the absence of its substrate, employing a high-valence oxoiron oxidant to carry out the observed self-hydroxylation chemistry. These results demonstrate that the active site metal in MndD can support two dramatically different 02 activation pathways, further highlighting the catalytic flexibility of enzymes containing a 2-His-1-carboxylate facial triad metal binding motif

X-ray absorption spectroscopic studies of high-spin Nonheme (Alkylperoxo)iron(III) intermediates

The reactions of iron(II) complexes [Fe(Tp t-Bu,i-Pr)(OH)] (1a, Tp t-Bu,i-Pr = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)-borate) , [Fe(B-Me 2BPMCN)(OTf) 2] (1b, 6-Me 2BPMCN = N,N-bis((2-methylpyridin-6-yl)methyl)-N,N???-dimethyl-trans-1, 2-diaminocyclohexane), and [Fe(L 8Py 2)(OTf)](OTf) (1c, L 8Py 2 = 1,5-bis(pyridin-2-ylmethyl)-1,5-diazacyclooctane) with tert-BuOOH give rise to high-spin Fe III-OOR complexes. X-ray absorption spectra (XAS) of these high-spin species show characteristic features, distinct from those of low-spin Fe-OOR complexes (Rohde, J.-U.; et al. J. Am. Chem. Soc. 2004, 126, 16750-16761). These include (1) an intense 1s ??? 3d preedge feature, with an area around 20 units, (2) an edge energy, ranging from 7122 to 7126 eV, that is affected by the coordination environment, and (3) a 1.86-1.96 Å Fe-OOR bond, compared to the 1.78 Å Fe-OOR bond in low-spin complexes. These unique features likely arise from a flexible first coordination sphere in those complexes. The difference in Fe-OOR bond length may rationalize differences in reactivity between low-spin and high-spin Fe III-OOR species. © 2007 American Chemical Society.close161