Methane monooxygenase EXAFS

Some preparations of iron exchanged into zeolite H-MFI by vapor-phase FeCL are known to be active and selective catalysts for the reduction of NO, with hydrocarbons or ammonia in the presence of excess oxygen and water vapor (45,46). The active centers in Fe/MFI are assumed to be binuclear, oxygen-bridged iron complexes, as follows from H2-TPR, CO-TPR, and ESR data (45,47) and EXAFS and XANES results (48,49). These complexes are structurally similar to the binuclear iron centers in methane monooxygenase enzymes that are employed by methanotrophic bacteria in utilization of methane as their primary energy source (50). It is believed that molecular oxygen reacts with these centers to form peroxide as the initial step in this chemistry (50). 

Ericson, A., Hedman, B., Green, J., Bentsen, J. G., Beer, R. H., Lippard, S. J., Dalton, H., and Hodgson, K. O., 1988, Structural characterization by EXAFS spectroscopy of the binuclear iron center in protein A of methane monooxygenase from Methylococcus capsulatus (Bath), J. Am. Chem. Soc. 110 2330n2332. 

Shu, L., Liu,Y., Lipscomb, J. D., and Que, L., Jr., 1996, EXAFS studies of the methane monooxygenase hydroxylase component from Methylosinus trichosporium OB3b, JBIC l 297n304. 

ACP = acyl carrier protein ACPA D = ACPA desat-urase AlkB = octane 1-monooxygenase AOX = alternative oxidase DMQ hydroxylase = 5-demethoxyquinone hydroxylase EXAFS = extended X-ray absorption fine structure spectroscopy FMN = flavin mononucleotide FprA = flavoprotein A (flavo-diiron enzyme homologue) Hr = hemerythrin MCD = magnetic circular dichroism MME hydroxylase = Mg-protophorphyrin IX monomethyl ester hydroxylase MMO = methane monooxygenase MMOH = hydroxylase component of MMO NADH = reduced nicotinamide adenine dinucleotide PAPs = purple acid phosphatases PCET = proton-coupled electron transfer, PTOX = plastid terminal oxidase R2 = ribonucleotide reductase R2 subunit Rbr = rubrerythrin RFQ = rapid freeze-quench RNR = ribonucleotide reductase ROO = rubredoxin oxygen oxidoreductase XylM = xylene monooxygenase. 

A report has highlighted the delicate interplay between Fe "Fe distances in such systems and the chemistry that results with the point made that this distance has been found to be particularly short at 2.53 A in the peroxo intermediate of ferroxidase activity as deduced from X-ray absorption studies.Although other techniques have been applied and compared to support this result, caution in placing too much reliance on metrical data from EXAFS should be exercised. Nonetheless, the hypothesis that this short distance and the strain that is introduced into the system might account for the fact that in ferroxidase activity the peroxo intermediate is a substrate intermediate rather than a catalytic cofactor as is the case in the related diferric enzymes such as methane monooxygenase. 

Costas et al. have reported spectroscopic evidence for an Fe Fe complex that can be considered a structural model for the putative Fe Fe (/x-0)2 core of methane monooxygenase intermediate The synthetic complex was prepared at —80 °C in CH2CI2 by decay of a mononuclear low-spin Fe peroxo precursor. The Mossbauer spectra showed that all iron in the sample is intermediate spin (5 = 1) Fe, but the data were compatible with either a mononuclear site or a weakly coupled ( J <5cm ) symmetric dimer. Combination of the Mossbauer technique with resonance Raman and EXAFS spectroscopies provided evidence for a bis-/x-oxo bridged diiron(IV) complex. The complex of Costas et al however, is not an electronic model for intermediate Q, as the latter contains high-spin Fe sites. 

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