Inorganic pyrophosphate synthesis during methanogenesis from methylcoenzyme M by cell-free extracts of Methanobacterium thermoautotrophicum (strain delta H)

Cloning, purification, and characterization of inorganic pyrophosphatase from the hyperthermophilic archaea Pyrococcus horikoshii

The gene encoding inorganic pyrophosphatase (PPiase) from the hyperthermophilic archaea Pyrococcus horikoshii (Pho PPiase) was cloned in the Escherichia coli strain BL21/pET15b, and the recombinant PPiase was purified by Ni-chelating chromatography in only an one-step procedure. The PPiase showed optimal activity at 88°C and pH of 10.3. Kinetic analysis revealed Km, kcat, Vm of 14.27μM, 3436s(-1), and 34.35μmol/min/mg protein, respectively. Pho PPiase was stable against denaturant chemicals as well as heat. It retained 19.61% of the original activity after incubation at 100°C for 12h and 25.96% of the original activity in the presence of 8M urea after incubation at 50°C for 120h. Pho PPiase showed high specificity for inorganic pyrophosphate but low reactivity to sodium tripolyphosphate and sodium tetrapolyphosphate. ADP and ATP could not serve as substrates.

Identification and analysis of proton-translocating pyrophosphatases in the methanogenic archaeon Methansarcina mazei

Analysis of genome sequence data from the methanogenic archaeon Methanosarcina mazei Gö1 revealed the existence of two open reading frames encoding proton-translocating pyrophosphatases (PPases). These open reading frames are linked by a 750-bp intergenic region containing TC-rich stretches and are transcribed in opposite directions. The corresponding polypeptides are referred to as Mvp1 and Mvp2 and consist of 671 and 676 amino acids, respectively. Both enzymes represent extremely hydrophobic, integral membrane proteins with 15 predicted transmembrane segments and an overall amino acid sequence similarity of 50.1%. Multiple sequence alignments revealed that Mvp1 is closely related to eukaryotic PPases, whereas Mvp2 shows highest homologies to bacterial PPases. Northern blot experiments with RNA from methanol-grown cells harvested in the mid-log growth phase indicated that only Mvp2 was produced under these conditions. Analysis of washed membranes showed that Mvp2 had a specific activity of 0.34 U mg (protein)(-1). Proton translocation experiments with inverted membrane vesicles prepared from methanol-grown cells showed that hydrolysis of 1 mol of pyrophosphate was coupled to the translocation of about 1 mol of protons across the cytoplasmic membrane. Appropriate conditions for mvp1 expression could not be determined yet. The pyrophosphatases of M. mazei Gö1 represent the first examples of this enzyme class in methanogenic archaea and may be part of their energy-conserving system.

Methyl-coenzyme M reductase of Methanobacterium thermoautotrophicum delta H catalyzes the reductive dechlorination of 1,2-dichloroethane to ethylene and chloroethane

Reductive dechlorination of 1,2-dichloroethane (1,2-DCA) to ethylene and chloroethane (CA) by crude cell extracts of Methanobacterium thermoautotrophicum delta H with H2 as the electron donor was stimulated by Mg-ATP. The heterodisulfide of coenzyme M (CoM) and 7-mercaptoheptanoylthreonine phosphate together with Mg-ATP partially inhibited ethylene production but stimulated CA production compared Mg-ATP alone. The pH optimum for the dechlorination was 6.8 (at 60 degrees C). Michaelis-Menten kinetics for initial product formation rates with different 1,2-DCA concentrations indicated the enzymatic character of the dechlorination. Apparent Kms for 1,2-DCA of 89 and 119 microM and Vmaxs of 34 and 20 pmol/min/mg of protein were estimated for ethylene and CA production, respectively. 3-Bromopropanesulfonate, a specific inhibitor for methyl-CoM reductase, completely inhibited dechlorination of 1,2-DCA. Purified methyl-CoM reductase, together with flavin adenine dinucleotide and a crude component A fraction which reduced the nickel of factor F430 in methyl-CoM reductase, converted 1,2-DCA to ethylene and CA with H2 as the electron donor. In this system, methyl-CoM reductase was also able to transform its own inhibitor 2-bromoethanesulfonate to ethylene.

An extremely stable inorganic pyrophosphatase purified from the cytosol of a thermoacidophilic archaebacterium, Sulfolobus acidocaldarius strain 7

A highly active inorganic pyrophosphatase was purified to electrophoretical homogeneity from the cytosol of Sulfolobus acidocaldarius strain 7, an extremely thermoacidophilic archaebacterium. The enzyme has an apparent molecular mass of 80 kDa as estimated by gel permeation chromatography, and showed a 21-kDa polypeptide on SDS-PAGE, suggesting that the archaebacterial enzyme is similar to most of the eubacterial pyrophosphatases rather than eukaryotic ones. The pI = 5.1. The enzyme showed relatively high content of Pro and low content of Ser plus Thr. The optimal pH was 6.5 (at 56 degrees C). From the Arrhenius plot an activation energy of 11.2 kcal/mol was obtained between 37-95 degrees C. The specific activity was 617 mumol Pi release min-1 mg-1 at 56 degrees C. The S. acidocaldarius pyrophosphatase was extremely stable. Complete activity remained after incubation at 100 degrees C for 10 min. No dissociation into subunit or unfolding of polypeptide chain occurred in the presence of 8 M urea. Experiments using guanidine-HCl suggested that the transition between a native tetrameric state and an unfolded state is completely reversible, and essentially independent of any additional factors such as divalent metal cation or dithiothreitol.

Characterization of high affinity GTPase activity correlated to beta-adrenergic receptor stimulation of adenylyl cyclase in rat parotid membranes

beta-Adrenergic receptor stimulation of adenylyl cyclase involves the activation of a GTP-binding regulatory protein (G-protein, termed here Gs). Inactivation of this G-protein is associated with the hydrolysis of bound GTP by an intrinsic high affinity GTPase activity. In the present study, we have characterized the GTPase activity in a Gs-enriched rat parotid gland membrane fraction. Two GTPase activities were resolved; a high affinity GTPase activity displaying Michaelis-Menten kinetics with increasing concentrations of GTP, and a low affinity GTPase activity which increased linearly with GTP concentrations up to 10 mM. The beta-adrenergic agonist isoproterenol (10 microM) increased the Vmax of the high affinity GTPase component approx. 50% from 90 to 140 pmol/mg protein per min, but did not change its Km value (approximately 450 nM). Isoproterenol also stimulated adenylyl cyclase activity in parotid membranes both in the absence or presence of GTP. In the presence of a non-hydrolyzable GTP analogue, guanosine 5'-(3-O-thio)triphosphate (GTP gamma S), isoproterenol increased cAMP formation to the same extent as that observed with AlF-4. Cholera toxin treatment of parotid membranes led to the ADP-ribosylation of two proteins (approximately 45 and 51 kDa). Cholera toxin also specifically decreased the high affinity GTPase activity in membranes and increased cAMP formation induced by GTP in the absence or the presence of isoproterenol. These data demonstrate that the high affinity GTPase characterized here is the 'turn-off' step for the adenylyl cyclase activation seen following beta-adrenergic stimulation of rat parotid glands.

Ammonia assimilation and glutamate incorporation in coenzyme F420 derivatives of Methanosarcina barkeri

Methanosarcina barkeri was able to grow on L-alanine and L-glutamate as sole nitrogen sources. Cell yields were 0.5 g/l and 0.7 g/l (wet wt), respectively. The mechanism of ammonia assimilation in Methanosarcina barkeri strain MS was studied by analysis of enzyme activities. Activity levels of nitrogen-assimilating enzymes in extracts of cells grown on different nitrogen sources (ammonia, 0.05-100 mM; L-alanine, 10 mM; L-glutamate, 10 mM) were compared. Activities of glutamate dehydrogenase, glutamate synthase, glutamine synthetase, glutamate oxaloacetate transaminase and glutamate pyruvate transaminase could be measured in cells grown on these three nitrogen sources. Alanine dehydrogenase was not detected under the growth conditions used. None of the measured enzyme activities varied significantly in response to the NH4+ concentration. The length of the poly-gamma-glutamyl side chain of F420 derivatives turned out to be independent of the concentration of ammonia in the culture medium.

Conservation of functional residues between yeast and E. coli inorganic pyrophosphatases

The alignments of the amino acid sequences of inorganic pyrophosphatase (PPase) from Saccharomyces cerevisiae (Y1-PPase, 286 amino acids) and Escherichia coli (E-PPase, 175 amino acids) are examined in the light of crystallographic and chemical modification results placing specific amino acid residues at the active site of the yeast enzyme. The major results are: (1) the full E-PPase sequence aligns within residues 28-225 of Y1-PPase, raising the possibility that the N-terminal and C-terminal portions of Y1-PPase may not be essential for activity, and (2) that whereas the overall identity between the two sequences is only modest (22-27% depending on the choice of alignment parameters), of some 17 putative active site residues, 14-16 are identical between Y-PPase and E-PPase. PPase thus appears to be an example of enzymes from widely divergent species that conserve common functional elements within the context of substantial overall sequence variation.

Methanogenesis involving a novel carrier of C1 compounds in Methanogenium tationis

The pathway of CO2 reduction to methane in Methanogenium tationis and Methanogenium thermophilicum is similar to that observed in other methanogens. In M. tationis a novel pterin, tatiopterin, is present. This pterin appears to be a structural and functional analog of methanopterin and sarcinapterin. Folate could not substitute for tatiopterin.

Isolation and characterization of acetyl-coenzyme A synthetase from Methanothrix soehngenii

In Methanothrix soehngenii, acetate is activated to acetyl-coenzyme A (acetyl-CoA) by an acetyl-CoA synthetase. Cell extracts contained high activities of adenylate kinase and pyrophosphatase, but no activities of a pyrophosphate:AMP and pyrophosphate:ADP phosphotransferase, indicating that the activation of 1 acetate in Methanothrix requires 2 ATP. Acetyl-CoA synthetase was purified 22-fold in four steps to apparent homogeneity. The native molecular mass of the enzyme from M. soehngenii estimated by gel filtration was 148 kilodaltons (kDa). The enzyme was composed of two subunits with a molecular mass of 73 kDa in an alpha 2 oligomeric structure. The acetyl-CoA synthetase constituted up to 4% of the soluble cell protein. At the optimum pH of 8.5, the Vmax was 55 mumol of acetyl-CoA formed per min per mg of protein. Analysis of enzyme kinetic properties revealed a Km of 0.86 mM for acetate and 48 microM for coenzyme A. With varying amounts of ATP, weak sigmoidal kinetic was observed. The Hill plot gave a slope of 1.58 +/- 0.12, suggesting two interacting substrate sites for the ATP. The kinetic properties of the acetyl-CoA synthetase can explain the high affinity for acetate of Methanothrix soehngenii.

Stimulation of the methylcoenzyme M reduction by uridine-5'-diphospho-sugars in cell-free extracts of Methanobacterium thermoautotrophicum (strain delta H)

The hydrogen-dependent reduction of methylcoenzyme M catalyzed by coenzyme-depleted cell-free extracts of Methanobacterium thermoautotrophicum was stimulated by micromolar concentrations of a UDP-disaccharide present in the organism. The compound was isolated and identified as UDP-1-O-alpha-D-2-acetamido-2-deoxyglucopyranose (UDPGlcpNAc) glycosidically linked to 2-acetamido-2-deoxymannopyranosyluronic acid. Maximal stimulation was observed when both the UDP-disaccharide and mercaptoheptanoylthreonine phosphate were present in the reaction mixtures. The UDP derivative isolated was not specific in its action: other UDP-sugars tested in micromolar concentrations stimulated the methylcoenzyme M reduction to the same extent. The activated sugars presumably substitute for ATP, which is usually required in much higher concentrations to activate the methylcoenzyme M reductase system.

Methyl-coenzyme-M reductase from Methanobacterium thermoautotrophicum (strain Marburg). Purity, activity and novel inhibitors

Methyl-coenzyme-M reductase from Methanobacterium thermoautotrophicum (strain Marburg) was purified to a stage where, besides the alpha, beta and gamma subunits, no additional polypeptides were detectable in the preparation. Under appropriate conditions the enzyme was found to catalyze the reduction of methyl-CoM with 7-mercaptoheptanoylthreonine phosphate (H-S-HTP) to CH4 at a specific rate of 2.5 mumol.min-1.mg protein-1. This finding contradicts a recent report that methyl-CoM reductase is only active when some contaminating proteins are present. The two polypeptides encoded by the open reading frames ORF1 and ORF2 of the methyl-CoM reductase transcription unit did not co-purify with the alpha, beta and gamma subunits. They were neither required nor did they stimulate the activity under the assay conditions. 3-Bromopropanesulfonate (apparent Ki = 0.05 microM) and 2-azidoethanesulfonate (apparent Ki = 1 microM) were found to be two new competitive inhibitors of methyl-CoM reductase. Both inhibitors were considerably more effective than the "classical" 2-bromoethanesulfonate (apparent Ki = 4 microM).

Energy-dependent, high-affinity transport of nickel by the acetogen Clostridium thermoaceticum

The nickel transport system of Clostridium thermoaceticum was investigated with 63NiCl2 and an anaerobic microfiltration transport assay. Transport was optimal at pH 7 to pH 7.5 and 65 degrees C and decreased in the presence of metabolic uncouplers and inhibitors. Exogenous nickel was concentrated 3,000-fold over the apparent nickel concentration gradient during typical transport assays. Stored cellular energy appeared to provide a short-term energy source to power nickel transport, and starvation experiments demonstrated external energy source stimulation of nickel translocation. The apparent Km and Vmax for nickel transport by carbon monoxide-dependent chemolithotrophic cells approximated 3.2 microM Ni and 400 pmol of Ni transported per min per mg of cells (dry weight), respectively. Magnesium, calcium, cobalt, iron, manganese, and zinc did not inhibit the transport of nickel.