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Query: UMLS:C0016632 (Fox)
 1,461 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Trichloroethylene was oxidized by purified toluene dioxygenase obtained from recombinant E. coli strains. The major oxidation products were formic acid and glyoxylic acid. Other potential products, dichloroacetic acid, chloral, phosgene, carbon monoxide, and carbon dioxide, were not detected. [14C]trichloroethylene became covalently attached to protein components and NADPH suggesting non-specific alkylation by reactive products. Oxidation of deuterated trichloroethylene yielded 50.2% deuterated formate. Oxidation of trichloroethylene in D2O yielded 43.7% deuterated formate. These data indicate that both carbon atoms are giving rise to formic acid. The results are consistent with a mechanism of TCE oxygenation not involving epoxide, dioxetane, or dihydroxy intermediates and indicate significant differences from those previously proposed for cytochrome P-450 (Miller, R.E. & Guengerich, F.P. (1982) Biochemistry 21, 1090-1097) or methane monooxygenase (Fox, B.G., Borneman, B.G., Wackett, L.P., & Lipscomb, J.D. (1990) Biochemistry 29, 6419-6227).
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PMID:Trichloroethylene oxidation by toluene dioxygenase. 159 83

Electron spin echo envelope modulation spectroscopy identified two ligand 14N interactions with the mixed-valence, Fe(II/III) diiron center of methane monooxygenase hydroxylase from Methylococcus capsulatus (Bath). Characteristic features of the spectra obtained at 9 and 10 GHz were analyzed and fit by simulation. One of the nitrogens possessed superhyperfine parameters (Aiso = 0.8 MHz, reff = 3.2 A, e2Qq = 1.8 MHz, eta = 0.35) consistent with a non-coordinating amino nitrogen of a histidine imidazole ligand to a Fe(III). The second, more strongly interacting nitrogen (Aiso = 5.0 MHz, reff = 2.2 A, e2Qq = 3.0 MHz, eta = 0.3) corresponds to the N delta directly bound to the effective Fe(II). These findings extend the previous electron nuclear double resonance results on the Methylosinus trichosporium hydroxylase (Hendrich, M.P., Fox, B.G., Andersson, K.K., Debrunner, P.G., and Lipscomb, J.D. (1992) J. Biol. Chem. 267, 261-269), which identified the N delta-Fe(II) interaction but failed to quantify its magnitude. Measurement of the linear electric field g shift of this mixed-valence species indicated that the site is charge-polarized on to one of the iron atoms, and its symmetry suggests that either charge is shifted away from the Fe-Fe axis (if gmax is defined by the Fe-Fe axis) or that gmid and gmax are perpendicular to the Fe-Fe axis (charge strongly localized at Fe(III) and axis taken as gmin).
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PMID:Nuclear hyperfine coupling of nitrogen in the coordination sphere of the diiron center of methane monooxygenase hydroxylase. 820 95

Non-heme diiron clusters occur in a number of enzymes (e.g., ribonucleotide reductase, methane monooxygenase, and Delta9-stearoyl-ACP desaturase) that activate O2 for chemically difficult oxidation reactions. In each case, a kinetically labile peroxo intermediate is believed to form when O2 reacts with the diferrous enzyme, followed by O-O bond cleavage and the formation of high-valent iron intermediates [formally Fe(IV)] that are thought to be the reactive oxidants. Greater kinetic stability of a peroxodiiron(III) intermediate in protein R2 of ribonucleotide reductase was achieved by the iron-ligand mutation Asp84 --> Glu and the surface mutation Trp48 --> Phe. Here, we present the first definitive evidence for a bridging, symmetrical peroxo adduct from vibrational spectroscopic studies of the freeze-trapped intermediate of this mutant R2. Isotope-sensitive bands are observed at 870, 499, and 458 cm-1 that are assigned to the intraligand peroxo stretching frequency and the asymmetric and symmetric Fe-O2-Fe stretching frequencies, respectively. Similar results have been obtained in the resonance Raman spectroscopic study of a peroxodiferric species of Delta9-stearoyl-ACP desaturase [Broadwater, J. A., Ai, J., Loehr, T. M., Sanders-Loehr, J., and Fox, B. G. (1998) Biochemistry 37, 14664-14671]. Similarities among these adducts and transient species detected during O2 activation by methane monooxygenase hydroxylase, ferritin, and wild-type protein R2 suggest the symmetrical peroxo adduct as a common intermediate in the diverse oxidation reactions mediated by members of this class.
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PMID:O2 activation by non-heme diiron proteins: identification of a symmetric mu-1,2-peroxide in a mutant of ribonucleotide reductase. 977 40

The diiron enzyme toluene 4-monooxygenase from Pseudomonas mendocina KR1 catalyzes the NADH- and O(2)-dependent hydroxylation of toluene. A combination of sequence alignments and spectroscopic studies indicate that T4MO has an active site structure closely related to the crystallographically characterized methane monooxygenase hydroxylase. In the methane monooxygenase hydroxylase, active site residue T213 has been proposed to participate in O(2) activation by analogy to certain proposals made for cytochrome P450. In this work, mutagenesis of the comparable residue in the toluene 4-monooxygenase hydroxylase, T201, has been used to investigate the role of an active site hydroxyl group in catalysis. Five isoforms (T201S, T201A, T201G, T201F, and T201K) that retain catalytic activity based on an in vivo indigo formation assay were identified, and detailed characterizations of the purified T201S, T201A, and T201G variants are reported. These isoforms have k(cat) values of 1.2, 1.0, and 0.6 s(-)(1), respectively, and k(cat)/K(M) values that vary by only approximately 4-fold relative to that of the native isoform. Moreover, these isoforms exhibit 80-90% coupling efficiency, which also compares favorably to the >94% coupling efficiency determined for the native isoform. For the T201S, T201A, and T201G isoforms, the regiospecificity of toluene hydroxylation was nearly identical to that of the natural isoform, with p-cresol representing 90-95% of the total product distribution. In contrast, the T201F isoform caused a substantial shift in the product distribution, and gave o- and p-cresol in a 1:1 ratio. In addition, the amount of benzyl alcohol was increased approximately 10-fold with the T201F isoform. For reaction with p-xylene, previous studies have shown that the native isoform reacted to give 4-methybenzyl alcohol and 2, 5-dimethylphenol in a 4:1 ratio [Pikus, J. D., Studts, J. M., McClay, K., Steffan, R. J., and Fox, B. G. (1997) Biochemistry 36, 9283-9289]. For comparison, the T201S, T201A, and T201F isoforms gave a slightly relaxed 3:1 ratio of these products, while the T201G isoform gave a dramatically relaxed 1:1 ratio. On the basis of these studies, we conclude that the hydroxyl group of T201 is not essential to maintaining the turnover rate or the coupling of the toluene 4-monooxygenase complex. However, changing the volume occupied by the side chain at the position of T201 can lead to alterations in the regiospecificity of the hydroxylation, presumably by producing different orientations for substrate binding during catalysis.
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PMID:Threonine 201 in the diiron enzyme toluene 4-monooxygenase is not required for catalysis. 1065 45