C1 metabolism inParacoccus denitrificans: Genetics ofParacoccus denitrificans
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Keywords:
Methanol dehydrogenase
Paracoccus denitrificans
Formate dehydrogenase
Formaldehyde dehydrogenase
Methanol dehydrogenase
Paracoccus denitrificans
Formate dehydrogenase
Formaldehyde dehydrogenase
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Pseudomonas sp. MS is capable of growth on a number of compounds containing only C(1) groups. They include trimethylsulphonium salts, methylamine, dimethylamine and trimethylamine. Although formaldehyde and formate will not support growth they are rapidly oxidized by intact cells. Methanol neither supports growth nor is oxidized. A particulate fraction of the cell oxidizes methylamine to carbon dioxide in the absence of any external electron acceptor. Formaldehyde and formate are more slowly oxidized to carbon dioxide by the particulate fraction, although they do not appear to be free intermediates in the oxidation of methylamine. Soluble NAD-linked formaldehyde dehydrogenase and formate dehydrogenase are also present. The particulate methylamine oxidase is induced by growth on methylamine, dimethylamine and trimethylamine, whereas the soluble formaldehyde dehydrogenase and formate dehydrogenase are induced by trimethylsulphonium nitrate as well as the aforementioned amines.
Dimethylamine
Formaldehyde dehydrogenase
Formate dehydrogenase
Methanol dehydrogenase
Electron acceptor
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ABSTRACT The bacterial nitric oxide reductase (NOR) is a divergent member of the family of respiratory heme-copper oxidases. It differs from other family members in that it contains an Fe B –heme-Fe dinuclear catalytic center rather than a Cu B –heme-Fe center and in that it does not pump protons. Several glutamate residues are conserved in NORs but are absent in other heme-copper oxidases. To facilitate mutagenesis-based studies of these residues in Paracoccus denitrificans NOR, we developed two expression systems that enable inactive or poorly active NOR to be expressed, characterized in vivo, and purified. These are (i) a homologous system utilizing the cycA promoter to drive aerobic expression of NOR in P. denitrificans and (ii) a heterologous system which provides the first example of the expression of an integral-membrane cytochrome bc complex in Escherichia coli . Alanine substitutions for three of the conserved glutamate residues (E125, E198, and E202) were introduced into NOR, and the proteins were expressed in P. denitrificans and E. coli . Characterization in intact cells and membranes has demonstrated that two of the glutamates are essential for normal levels of NOR activity: E125, which is predicted to be on the periplasmic surface close to helix IV, and E198, which is predicted to lie in the middle of transmembrane helix VI. The subsequent purification and spectroscopic characterization of these enzymes established that they are stable and have a wild-type cofactor composition. Possible roles for these glutamates in proton uptake and the chemistry of NO reduction at the active site are discussed.
Paracoccus denitrificans
Heterologous expression
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Methanol dehydrogenase
Formaldehyde dehydrogenase
Formate dehydrogenase
Chemostat
Derepression
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Methanol dehydrogenase-like protein XoxF of Methylobacterium extorquens AM1 exhibits a sequence identity of 50 % to the catalytic subunit MxaF of periplasmic methanol dehydrogenase in the same organism. The latter has been characterized in detail, identified as a pyrroloquinoline quinone (PQQ)-dependent protein, and shown to be essential for growth in the presence of methanol in this methylotrophic model bacterium. In contrast, the function of XoxF in M. extorquens AM1 has not yet been elucidated, and a phenotype remained to be described for a xoxF mutant. Here, we found that a xoxF mutant is less competitive than the wild-type during colonization of the phyllosphere of Arabidopsis thaliana, indicating a function for XoxF during plant colonization. A comparison of the growth parameters of the M. extorquens AM1 xoxF mutant with those of the wild-type during exponential growth revealed a reduced methanol uptake rate and a reduced growth rate for the xoxF mutant of about 30 %. Experiments with cells starved for carbon revealed that methanol oxidation in the xoxF mutant occurs less rapidly compared with the wild-type, especially in the first minutes after methanol addition. A distinct phenotype for the xoxF mutant was also observed when formate and CO(2) production were measured after the addition of methanol or formaldehyde to starved cells. The wild-type, but not the xoxF mutant, accumulated formate upon substrate addition and had a 1 h lag in CO(2) production under the experimental conditions. Determination of the kinetic properties of the purified enzyme showed a conversion capacity for both formaldehyde and methanol. The results suggest that XoxF is involved in one-carbon metabolism in M. extorquens AM1.
Methanol dehydrogenase
Formate dehydrogenase
Pyrroloquinoline Quinone
Formaldehyde dehydrogenase
Methylobacterium
Wild type
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Mutants deficient in the metabolism of one-carbon compounds have been obtained by treating Paracoccus denitrificans with the mutagen N-methyl-N'-nitro-N-nitrosoguanidine. Mutants were selected without enrichment procedures by newly developed plate screening tests. The obtained mutants were characterized by their growth responses, cytochrome composition, enzyme activities, and immunogenic reaction with antisera against methanol dehydrogenase. By these criteria five mutant classes could be distinguished. Class I mutants are involved in the expression of methanol dehydrogenase. Three mutants of this class have a defect in the structural gene. A double mutant was found with defects in the expression of both methanol dehydrogenase and hydrogenase. Class II mutants have a defect in a regulatory gene involved in the regulation of both methanol dehydrogenase and methylamine dehydrogenase. Class III mutants are deficient in formaldehyde metabolism. A defect may exist in the expression of a second non-NAD-linked formaldehyde dehydrogenase which was postulated to be involved in C1 metabolism. Class IV mutants are deficient in cytochrome c. Mutants of class V have a defect in synthesis of the molybdenum cofactor essential for the function of formate dehydrogenase.
Formate dehydrogenase
Paracoccus denitrificans
Formaldehyde dehydrogenase
Methanol dehydrogenase
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Summary: Methanol dissimilatory enzymes detected in the methanol autotroph Xanthobacter H4-14 were a typical phenazine methosulphate-linked methanol dehydrogenase, a NAD+-linked formate dehydrogenase, and a dye-linked formaldehyde dehydrogenase that could be assayed only by activity stains of polyacrylamide gels. This same methanol dehydrogenase activity was found in ethanol-grown cells and was apparently utilized for ethanol oxidation. Formaldehyde dehydrogenase activities were investigated in Paracoccus denitrificans, Xanthobacter H4-14, and Pseudomonas AM1. P. denitrificans contained a previously reported NAD+-linked, GSH-dependent activity, but both Xanthobacter H4-14 and Pseudomonas AM1 contained numerous activities detected by activity stains of polyacrylamide gels. Induction studies showed that in Xanthobacter H4-14, a 10 kDal polypeptide, probably a dehydrogenase-associated cytochrome c, was co-induced with methanol dehydrogenase, but the formaldehyde and formate dehydrogenases were not co-regulated. Analogous induction experiments revealed similar patterns in P. denitrificans, but no evidence for co-regulation of dissimilatory activities in Pseudomonas AM1.
Paracoccus denitrificans
Formate dehydrogenase
Methanol dehydrogenase
Formaldehyde dehydrogenase
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Assimilation of methylamine by Paracoccus denitrificans involves the following enzymes: a periplasmic methylamine dehydrogenase, a formaldehyde transport system, cytoplasmic formaldehyde and formate dehydrogenase. Formaldehyde transport follows saturation kinetics with a high substrate affinity (Km= 7 μM), and is severely inhibited by iodoacetate, cyanide and p-trifluoromethoxy carbonylcyanide phenylhydrazone. Expression of the formaldehyde carrier is regulated by the carbon source.
Paracoccus denitrificans
Formaldehyde dehydrogenase
Formate dehydrogenase
Enzyme Kinetics
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Formaldehyde is an intermediate formed during the metabolism of methanol or other methylated compounds. Many Gram-negative bacteria generate formaldehyde from methanol via a periplasmic pyrroloquinoline quinone (PQQ)-dependent dehydrogenase in which the α subunit of an α 2 β 2 tetramer has catalytic activity. The genome of the facultative formaldehyde-oxidizing bacterium Rhodobacter sphaeroides encodes XoxF, a homologue of the catalytic subunit of a proposed PQQ-containing dehydrogenase of Paracoccus denitrificans. R. sphaeroides xoxF is part of a gene cluster that encodes periplasmic c-type cytochromes, including CycI, isocytochrome c 2 and CycB (a cyt c 553i homologue), as well as adhI, a glutathione-dependent formaldehyde dehydrogenase (GSH-FDH), and gfa, a homologue of a glutathione–formaldehyde activating enzyme (Gfa). To test the roles of XoxF, CycB and Gfa in formaldehyde metabolism by R. sphaeroides, we monitored photosynthetic growth with methanol as a source of formaldehyde and whole-cell methanol-dependent oxygen uptake. Our data show that R. sphaeroides cells lacking XoxF or CycB do not exhibit methanol-dependent oxygen uptake and lack the capacity to utilize methanol as a sole photosynthetic carbon source. These results suggest that both proteins are required for formaldehyde metabolism. R. sphaeroides Gfa is not essential to activate formaldehyde, as cells lacking gfa are capable of both methanol-dependent oxygen uptake and growth with methanol as a photosynthetic carbon source.
Rhodobacter sphaeroides
Formaldehyde dehydrogenase
Methanol dehydrogenase
Paracoccus denitrificans
Pyrroloquinoline Quinone
Methylotroph
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