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K. V. Rajagopalan - One of the best experts on this subject based on the ideXlab platform.

  • Structure of the molybdenum site of Escherichia coli trimethylamine N-oxide Reductase.
    Inorganic chemistry, 2007
    Co-Authors: Limei Zhang, K. V. Rajagopalan, Kimberly Johnson Nelson, Graham N. George
    Abstract:

    We report a structural characterization of the molybdenum site of recombinant Escherichia coli trimethylamine N-oxide (TMAO) Reductase using X-ray absorption spectroscopy. The enzyme active site shows considerable similarity to that of dimethyl sulfoxide (DMSO) Reductase, in that, like DMSO Reductase, the TMAO Reductase active site can exist in multiple forms. Examination of the published crystal structure of TMAO oxidase from Shewanella massilia indicates that the postulated Mo coordination structure is chemically impossible. The presence of multiple active site structures provides a potential explanation for the anomalous features reported from the crystal structure.

  • Interaction of product analogues with the active site of rhodobacter sphaeroides dimethyl sulfoxide Reductase.
    Inorganic chemistry, 2007
    Co-Authors: Graham N. George, Kimberly Johnson Nelson, Hugh H. Harris, Christian J. Doonan, K. V. Rajagopalan
    Abstract:

    We report a structural characterization using X-ray absorption spectroscopy of Rhodobacter sphaeroides dimethyl sulfoxide (DMSO) Reductase reduced with trimethylarsine and show that this is structurally analogous to the physiologically relevant dimethyl sulfide reduced DMSO Reductase. Our data unambiguously indicate that these species should be regarded as formal MoIV species and indicate a classical coordination complex of trimethylarsine oxide, with no special structural distortions. The similarity of the trimethylarsine and dimethyl sulfide complexes suggests, in turn, that the dimethyl sulfide reduced enzyme possesses a classical coordination of DMSO with no special elongation of the S−O bond, as previously suggested.

  • The 1.3 A Crystal Structure of Rhodobacter sphaeroides Dimethylsulfoxide Reductase Reveals Two Distinct Molybdenum Coordination Environments
    2000
    Co-Authors: K Temple, K. V. Rajagopalan, H. Schindelin
    Abstract:

    During the past four years, a substantial amount of structural information has been accumulated on the molybdoenzyme dimethyl sulfoxide (DMSO) Reductase from purple bacteria. This enzyme contains a...

  • Re-design of Rhodobacter sphaeroides Dimethyl Sulfoxide Reductase ENHANCEMENT OF ADENOSINE N1-OXIDE Reductase ACTIVITY
    The Journal of biological chemistry, 1999
    Co-Authors: James C. Hilton, Carrie A. Temple, K. V. Rajagopalan
    Abstract:

    Abstract The periplasmic DMSO Reductase fromRhodobacter sphaeroides f. sp. denitrificanshas been expressed in Escherichia coli BL21(DE3) cells in its mature form and with the R. sphaeroides or E. coli N-terminal signal sequence. Whereas the R. sphaeroides signal sequence prevents formation of active enzyme, addition of a 6× His-tag at the N terminus of the mature peptide maximizes production of active enzyme and allows for affinity purification. The recombinant protein contains 1.7–1.9 guanines and greater than 0.7 molybdenum atoms per molecule and has a DMSO Reductase activity of 3.4–3.7 units/nmol molybdenum, compared with 3.7 units/nmol molybdenum for enzyme purified from R. sphaeroides. The recombinant enzyme differs from the native enzyme in its color and spectrum but is indistinguishable from the native protein after redox cycling with reduced methyl viologen and Me2SO. Substitution of Cys for the molybdenum-ligating Ser-147 produced a protein with DMSO Reductase activity of 1.4–1.5 units/nmol molybdenum. The mutant protein differs from wild type in its color and absorption spectrum in both the oxidized and reduced states. This substitution leads to losses of 61–99% of activity toward five substrates, but the adenosine N 1-oxide Reductase activity increases by over 400%.

  • Crystal structure of DMSO Reductase: redox-linked changes in molybdopterin coordination.
    Science (New York N.Y.), 1996
    Co-Authors: Hermann Schindelin, K. V. Rajagopalan, Caroline Kisker, James C. Hilton, Douglas C. Rees
    Abstract:

    The molybdoenzyme dimethylsulfoxide (DMSO) Reductase contributes to the release of dimethylsulfide, a compound that has been implicated in cloud nucleation and global climate regulation. The crystal structure of DMSO Reductase from Rhodobacter sphaeroides reveals a monooxo molybdenum cofactor containing two molybdopterin guanine dinucleotides that asymmetrically coordinate the molybdenum through their dithiolene groups. One of the pterins exhibits different coordination modes to the molybdenum between the oxidized and reduced states, whereas the side chain oxygen of Ser147 coordinates the metal in both states. The change in pterin coordination between the Mo(VI) and Mo(IV) forms suggests a mechanism for substrate binding and reduction by this enzyme. Sequence comparisons of DMSO Reductase with a family of bacterial oxotransferases containing molybdopterin guanine dinucleotide indicate a similar polypeptide fold and active site with two molybdopterins within this family.

Graham N. George - One of the best experts on this subject based on the ideXlab platform.

  • X-ray absorption spectroscopy of a quantitatively Mo(V) dimethyl sulfoxide Reductase species.
    Inorganic chemistry, 2013
    Co-Authors: M. Jake Pushie, Russ Hille, Julien J. H. Cotelesage, Ganna Lyashenko, Graham N. George
    Abstract:

    Molybdenum K-edge X-ray absorption spectroscopy (XAS) has been used to probe the structure of a Mo(V) species that has been suggested to be a catalytic intermediate in the reaction of dimethyl sulfoxide (DMSO) Reductase with the alternative substrate trimethylamine N-oxide (Bennet et al. Eur. J. Biochem. 1994, 255, 321-331; Cobb et al. J. Biol. Chem. 2005, 280, 11007-11017; Mtei, et al. J. Am. Chem. Soc. 2011, 133, 9672-9774). The oxidized Mo(VI) state of DMSO Reductase has previously been structurally characterized as being six coordinate, with four sulfurs from pyranopterin dithiolene molybdenum cofactors, a terminal oxygen ligand, and an additional oxygen coordination from a serine residue. We find the most plausible structure for the Mo(V) active site is a five-coordinate species with four sulfur donors from the two pyranopterin dithiolene ligands, with an average Mo-S bond-length of 2.35 A, plus a single oxygen donor at 1.99 A, very likely from an Mo-OH ligand. Our results thus suggest that the oxygen of the serine residue has dissociated from the metal ion, suggesting hitherto unsuspected flexibility of the active site, and calling into question whether this putative intermediate is catalytically relevant. The relevance to previous Mo(V) electron paramagnetic resonance and other spectroscopic studies on DMSO Reductase is discussed. XAS of an extensively studied Mo(V) form of Rhodobacter sphaeroides DMSO Reductase (the high-g split species) shows that previously suggested structures for the active site are likely incorrect.

  • Structure of the molybdenum site of Escherichia coli trimethylamine N-oxide Reductase.
    Inorganic chemistry, 2007
    Co-Authors: Limei Zhang, K. V. Rajagopalan, Kimberly Johnson Nelson, Graham N. George
    Abstract:

    We report a structural characterization of the molybdenum site of recombinant Escherichia coli trimethylamine N-oxide (TMAO) Reductase using X-ray absorption spectroscopy. The enzyme active site shows considerable similarity to that of dimethyl sulfoxide (DMSO) Reductase, in that, like DMSO Reductase, the TMAO Reductase active site can exist in multiple forms. Examination of the published crystal structure of TMAO oxidase from Shewanella massilia indicates that the postulated Mo coordination structure is chemically impossible. The presence of multiple active site structures provides a potential explanation for the anomalous features reported from the crystal structure.

  • Interaction of product analogues with the active site of rhodobacter sphaeroides dimethyl sulfoxide Reductase.
    Inorganic chemistry, 2007
    Co-Authors: Graham N. George, Kimberly Johnson Nelson, Hugh H. Harris, Christian J. Doonan, K. V. Rajagopalan
    Abstract:

    We report a structural characterization using X-ray absorption spectroscopy of Rhodobacter sphaeroides dimethyl sulfoxide (DMSO) Reductase reduced with trimethylarsine and show that this is structurally analogous to the physiologically relevant dimethyl sulfide reduced DMSO Reductase. Our data unambiguously indicate that these species should be regarded as formal MoIV species and indicate a classical coordination complex of trimethylarsine oxide, with no special structural distortions. The similarity of the trimethylarsine and dimethyl sulfide complexes suggests, in turn, that the dimethyl sulfide reduced enzyme possesses a classical coordination of DMSO with no special elongation of the S−O bond, as previously suggested.

  • X-ray absorption spectroscopic characterization of the molybdenum site of Escherichia coli dimethyl sulfoxide Reductase.
    Inorganic chemistry, 2007
    Co-Authors: Graham N. George, Christian J. Doonan, Richard A. Rothery, Nasim Boroumand, Joel H. Weiner
    Abstract:

    Structural studies of dimethyl sulfoxide (DMSO) Reductases were hampered by modification of the active site during purification. We report an X-ray absorption spectroscopic analysis of the molybdenum active site of Escherichia coli DMSO Reductase contained within its native membranes. The enzyme in these preparations is expected to be very close to the form found in vivo. The oxidized active site was found to have four Mo-S ligands at 2.43 A, one Mo=O at 1.71 A, and a longer Mo-O at 1.90 A. We conclude that the oxidized enzyme is a monooxomolybdenum(VI) species coordinated by two molybdopterin dithiolenes and a serine. The bond lengths determined for E. coli DMSO Reductase are very similar to those determined for the well-characterized Rhodobacter sphaeroides DMSO Reductase, suggesting similar active site structures for the two enzymes. Furthermore, our results suggest that the form found in vivo is the monooxobis(molybdopterin) species.

  • The Active Site of Arsenite Oxidase from Alcaligenes faecalis
    Journal of the American Chemical Society, 2002
    Co-Authors: Thomas P. Conrads, Graham N. George, Craig Hemann, Ingrid J. Pickering, Roger C. Prince, Russ Hille
    Abstract:

    Arsenite oxidase, a member of the DMSO Reductase family of molybdenum enzymes, has two molecules of guanosine dinucleotide molybdenum cofactor coordinating the molybdenum at the active site. X-ray ...

Alastair G. Mcewan - One of the best experts on this subject based on the ideXlab platform.

  • Maturation of molybdoenzymes and its influence on the pathogenesis of non-typeable Haemophilus influenzae
    Frontiers in Microbiology, 2015
    Co-Authors: Rabeb Dhouib, Dk Seti Maimonah Pg Othman, Ama-tawiah Essilfie, Phil Hansbro, Jeffrey O. Hanson, Alastair G. Mcewan, Ulrike Kappler
    Abstract:

    Mononuclear molybdenum enzymes of the dimethylsulfoxide (DMSO) Reductase family occur exclusively in prokaryotes, and a loss of some these enzymes has been linked to a loss of bacterial virulence in several cases. The MobA protein catalyzes the final step in the synthesis of the molybdenum guanine dinucleotide (MGD) cofactor that is exclusive to enzymes of the DMSO Reductase family. MobA has been proposed as a potential target for control of virulence since its inhibition would affect the activities of all molybdoenzymes dependent upon MGD. Here, we have studied the phenotype of a mobA mutant of the host-adapted human pathogen Haemophilus influenzae. H. influenzae causes and contributes to a variety of acute and chronic diseases of the respiratory tract., and several enzymes of the DMSO Reductase family are conserved and highly expressed in this bacterium. The mobA mutation caused a significant decrease in the activities of all Mo-enzymes present, and also resulted in a small defect in anaerobic growth. However, we did not detect a defect in in vitro biofilm formation nor in invasion and adherence to human epithelial cells in tissue culture compared to the wild-type. In a murine in vivo model, the mobA mutant showed only a mild attenuation compared to the wild-type. In summary, our data show that MobA is essential for the activities of molybdenum enzymes, but does not appear to affect the fitness of H. influenzae. These results suggest that MobA is unlikely to be a useful target for antimicrobials, at least for the purpose of treating H. influenzae infections.

  • Electrochemically mediated enantioselective reduction of chiral sulfoxides.
    Journal of biological inorganic chemistry : JBIC : a publication of the Society of Biological Inorganic Chemistry, 2014
    Co-Authors: Kuan-i Chen, Ulrike Kappler, Alastair G. Mcewan, Victoria L. Challinor, Linda Kielmann, Philip C. Sharpe, James J. De Voss, Paul V. Bernhardt
    Abstract:

    The respiratory DMSO Reductase from Rhodobacter capsulatus catalyzes the reduction of dimethyl sulfoxide to dimethyl sulfide. Herein, we have utilized this Mo enzyme as an enantioselective catalyst to generate optically pure sulfoxides (methyl p-tolyl sulfoxide, methyl phenyl sulfoxide and phenyl vinyl sulfoxide) from racemic starting materials. A hexaaminecobalt coordination compound in its divalent oxidation state was employed as the mediator of electron transfer between the working electrode and DMSO Reductase to continually reactivate the enzyme after turnover. In all cases, chiral HPLC analysis of the reaction mixture revealed that the S-sulfoxide was reduced more rapidly leading to enrichment or isolation of the R isomer.

  • Cobalt hexaamine mediated electrocatalytic voltammetry of dimethyl sulfoxide Reductase: driving force effects on catalysis
    JBIC Journal of Biological Inorganic Chemistry, 2011
    Co-Authors: Kuan-i Chen, Alastair G. Mcewan, Paul V. Bernhardt
    Abstract:

    The bacterial molybdoenzyme dimethyl sulfoxide (DMSO) Reductase from Rhodobacter capsulatus catalyzes the reduction of DMSO to dimethyl sulfide in anaerobic respiration. In its native state, DMSO Reductase is reduced to its active state by a pentaheme cytochrome (DorC). Alternatively, we show that DMSO Reductase catalysis may be driven electrochemically using a series of homologous coordination compounds as mediating synthetic electron donors. All mediators are macrocyclic hexaaminecobalt(II) complexes in their active form, differing principally in their redox potentials over a range of about 250 mV. Thus, each complex presents a different reductive driving force to DMSO Reductase and this leads to pronounced differences in the electrocatalytic behavior as measured by cyclic voltammetry. Digital simulation of the experimental voltammetry enables the critical features of the catalytic cycle to be extracted.

  • Mediated electrochemistry of dimethyl sulfoxide Reductase from Rhodobacter capsulatus
    JBIC Journal of Biological Inorganic Chemistry, 2009
    Co-Authors: Kuan-i Chen, Alastair G. Mcewan, Paul V. Bernhardt
    Abstract:

    Electrochemically driven catalysis of the bacterial enzyme dimethyl sulfoxide (DMSO) Reductase ( Rhodobacter capsulatus ) has been studied using the macrocyclic complex ( trans -6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine)cobalt(III) as a mediator. In the presence of both DMSO and DMSO Reductase, the normal transient Co^III/II voltammetric response of the complex is transformed into an amplified and sigmoidal (steady-state) waveform characteristic of a catalytic EC′ mechanism. At low concentrations of DMSO (approximately K _M) or high mediator concentrations (more than the concentration of DMSO Reductase), the steady-state character of the voltammetric response disappears and is replaced by more complicated waveforms that are a convolution of transient and steady-state behavior as different steps within the catalytic cycle become rate limiting. Through digital simulation of cyclic voltammetry performed under conditions where the sweep rate, DMSO concentration, DMSO Reductase concentration and mediator concentration were varied systematically, we were able to model all voltammograms with a single set of rate and equilibrium constants which provide new insights into the kinetics of the DMSO Reductase catalytic mechanism that have hitherto been inaccessible from steady state or stopped flow kinetic studies.

  • the DMSO Reductase family of microbial molybdenum enzymes
    2004
    Co-Authors: Alastair G. Mcewan, Ulrike Kappler
    Abstract:

    Vol 35 No 3 December 2004 AUSTRALIAN BIOCHEMIST Page 17 Molybdenum is the only element in the second row of transition metals which has a defined role in biology. It exhibits redox states of (VI), (V) and (IV) within a biologically-relevant range of redox potentials and is capable of catalysing both oxygen atom transfer and proton/electron transfer. Apart from nitrogenase, all enzymes containing molybdenum have an active site composed of a molybdenum ion coordinated by one or two ene-dithiolate (dithiolene) groups that arise from an unusual organic moiety known as the pterin molybdenum cofactor or pyranopterin (1,2). The mononuclear molybdenum enzymes exhibit remarkable diversity of function and this is in part due to variations at the Mo active site that are additional to the common core structure. Prior to the appearance of X-ray crystal structures of molybdenum enzymes, EPR spectroscopy, X-ray absorption fine structure spectroscopy (EXAFS) and biochemical analysis had identified differences between the molybdenum hydroxylases, exemplified by xanthine dehydrogenase, and those enzymes that mostly functioned as 'oxotransferases' (3). The former possess a cyanolysable sulfido group at the Mo active site, while the latter are insensitive to cyanide and may possess oxo ligands (Fig. 1). Since 1995, X-ray crystal s t r u c t u r e s h a v e r a i s e d t h e u n d e r s t a n d i n g o f molybdenum enzymes to a much higher level and have led to a division of the oxotransferases into the sulfite dehydrogenase and the dimethylsulfoxide (DMSO) Reductase families (Fig. 1) (4). The Mo hydroxylases and oxotransferases can act either as dehydrogenases or Reductases in catalysis. This reaction can be summarised by the general scheme: X + H2O D X=O + 2H+ + 2eDuring this process the Mo ion cycles between the (IV) and (VI) oxidation states with electrons being transferred to or from an electron transfer partner or substrate. Experiments with xanthine dehydrogenase using 18O-labelled water have confirmed that the oxygen is incorporated into the product during substrate oxidation and this distinguishes the mononuclear molybdoenzymes from monoxygenases where molecular oxygen rather than water acts as an oxygen atom donor (5). The last decade has seen a resurgence of interest in Mo enzymes as a consequence of the new structural information and also because the remarkable b i o e n e r g e t i c d i v e r s i t y o f m i c r o o r g a n i s m s i s underpinned to a large degree by Mo enzymes (6). Molybdenum hydroxylases and examples of the sulfite dehydrogenase family can be found in all three domains of life (4). In contrast, the DMSO Reductase family appears to be restricted to bacteria and archaea (5).

Sunao Yamazaki - One of the best experts on this subject based on the ideXlab platform.

Joel H. Weiner - One of the best experts on this subject based on the ideXlab platform.

  • X-ray absorption spectroscopic characterization of the molybdenum site of Escherichia coli dimethyl sulfoxide Reductase.
    Inorganic chemistry, 2007
    Co-Authors: Graham N. George, Christian J. Doonan, Richard A. Rothery, Nasim Boroumand, Joel H. Weiner
    Abstract:

    Structural studies of dimethyl sulfoxide (DMSO) Reductases were hampered by modification of the active site during purification. We report an X-ray absorption spectroscopic analysis of the molybdenum active site of Escherichia coli DMSO Reductase contained within its native membranes. The enzyme in these preparations is expected to be very close to the form found in vivo. The oxidized active site was found to have four Mo-S ligands at 2.43 A, one Mo=O at 1.71 A, and a longer Mo-O at 1.90 A. We conclude that the oxidized enzyme is a monooxomolybdenum(VI) species coordinated by two molybdopterin dithiolenes and a serine. The bond lengths determined for E. coli DMSO Reductase are very similar to those determined for the well-characterized Rhodobacter sphaeroides DMSO Reductase, suggesting similar active site structures for the two enzymes. Furthermore, our results suggest that the form found in vivo is the monooxobis(molybdopterin) species.

  • Glutamate 87 is important for menaquinol binding in DmsC of the DMSO Reductase (DmsABC) from Escherichia coli.
    Biochimica et Biophysica Acta, 2003
    Co-Authors: Paulina Geijer, Joel H. Weiner
    Abstract:

    Escherichia coli dimethylsulfoxide (DMSO) Reductase is a trimeric enzyme with a catalytic dimer (DmsAB) and an integral membrane anchor (DmsC). Using site-directed mutagenesis, we examined six residues in the periplasmic loop between helices two and three, potentially involved in menaquinol binding in DmsC. Mutants were characterised for growth, enzyme expression and activity, and 2-n-heptyl-4-hydroxoquinoline N-oxide (HOQNO) inhibitor binding. Mutations of leucine 66, glycine 67, arginine 71, phenylalanine 73 and serine 75 had no effect on menaquinol binding. Only a glutamate residue (E87) located in helix three was important for menaquinol binding. E87 was replaced with lysine, glutamine and aspartate. All three mutants were assembled into the membrane. Neither the lysine nor the glutamine mutant enzymes were able to support anaerobic growth on glycerol/DMSO minimal media or oxidise lapachol. The glutamine mutant bound the inhibitor with lower affinity compared to wild-type, whereas in the lysine mutant, binding was almost abolished. The aspartate mutant behaved as a wild-type enzyme. The data shows that E87 is important for menaquinol binding and oxidation and is likely to act as a proton acceptor in the menaquinol binding site.

  • Hydroxylated naphthoquinones as substrates for Escherichia coli anaerobic Reductases
    Biochemical Journal, 1998
    Co-Authors: Richard A. Rothery, Indraneal Chatterjee, Gregory K. Kiema, Mark T. Mcdermott, Joel H. Weiner
    Abstract:

    We have used two hydroxylated naphthoquinol menaquinol analogues, reduced plumbagin (PBH2, 5-hydroxy-2-methyl-1,4-naphthoquinol) and reduced lapachol [LPCH2, 2-hydroxy-3-(3-methyl-2-butenyl)-1, 4-naphthoquinol], as substrates for Escherichia coli anaerobic Reductases. These compounds have optical, solubility and redox properties that make them suitable for use in studies of the enzymology of menaquinol oxidation. Oxidized plumbagin and oxidized lapachol have well resolved absorbances at 419 nm (epsilon=3.95 mM-1. cm-1) and 481 nm (epsilon=2.66 mM-1.cm-1) respectively (in Mops/KOH buffer, pH 7.0). PBH2 is a good substrate for nitrate Reductase A (Km=282+/-28 microM, kcat=120+/-6 s-1) and fumarate Reductase (Km=155+/-24 microM, kcat=30+/-2 s-1), but not for DMSO Reductase. LPCH2 is a good substrate for nitrate Reductase A (Km=57+/-35 microM, kcat=68+/-13 s-1), fumarate Reductase (Km=85+/-27 microM, kcat=74+/-6 s-1) and DMSO Reductase (Km=238+/-30 microM, kcat=191+/-21 s-1). The sensitivity of enzymic LPCH2 and PBH2 oxidation to 2-n-heptyl-4-hydroxyquinoline N-oxide inhibition is consistent with their oxidation occurring at sites of physiological quinol binding.

  • Interaction of an engineered [3Fe-4S] cluster with a menaquinol binding site of Escherichia coli DMSO Reductase.
    Biochemistry, 1996
    Co-Authors: Richard A. Rothery, Joel H. Weiner
    Abstract:

    We have characterized by EPR the interaction of the Em,7 = -50 mV [4Fe-4S] cluster of Escherichia coli DMSO Reductase (DmsABC) with a menaquinol (MQH2) binding site. Potentiometric titrations indicate that in DmsAB(C102S)C, the Em,7 = -50 mV [4Fe-4S] cluster is replaced by an Em,7 = +260 mV [3Fe-4S] cluster. The Q-pool coupling assay in combination with the MQH2 analog HOQNO (2-n-heptyl-4-hydroxyquinoline-N-oxide) was used to examine the effect of the DmsB(Cl02S) mutation on physiological electron transfer through DmsABC. Forward electron transfer through the mutant (MQH2 to DmsA) is blocked in the Q-pool coupling assay, but reverse electron transfer (DmsA to MQ) is not. HOQNO elicits a significant change in the EPR line shape of the oxidized DmsAB(Cl02S)C [3Fe-4S] cluster but has no effect on the line shape of the reduced [4Fe-4S] clusters. We have identified a residue in DmsC involved in MQH2 oxidation. DmsC(H65), and in a double mutant, DmsAB(C102S)C(H65R), the DmsC mutation blocks the HOQNO effect on the [3Fe-4S] EPR line shape, suggesting, that the DmsC(H65R) mutation either blocks HOQNO binding or blocks a conformational link between a HOQNO binding site and the DmsB(C102S) [3Fe-4S] cluster. These results suggest that the MQH2 binding site of DmsC is conformationally and functionally linked to the Em,7 = -50 mV [4Fe-4S] cluster of DmsB.

  • Differentiation of the multiple S- and N-oxide-reducing activities ofEscherichia coli
    Current Microbiology, 1991
    Co-Authors: D. Sambasivarao, Joel H. Weiner
    Abstract:

    In Escherichia coli , several terminal Reductases catalyze the reduction of S- and N-oxide compounds. We have used mutants missing either the constitutive dimethylsulfoxide (DMSO) Reductase, dmsABC , and/or the inducible trimethylamine N-oxide (TMAO) Reductase, torA , to define the roles of each Reductase. These studies indicated that the constitutive DMSO Reductase can sustain growth on DMSO, TMAO, methionine sulfoxide (MetSO), and other N-oxide compounds. Only one inducible TMAO Reductase is expressed in E. coli , and this enzyme sustains growth on TMAO but not DMSO or MetSO. Characterization of a torA ^−, dms^−double mutant revealed that adenosine N-oxide (ANO) Reductase is specifically required for anaerobic respiration on ANO in this mutant.