ATP hydrolysis and synthesis by the membrane-bound ATP synthetase complex of Methanobacterium thermoautotrophicum

Acetate as sole carbon and energy source for growth of methanosarcina strain 227

Methanosarcina strain 227 grew rapidly and produced methane on a mineral medium containing acetate as the sole added organic substrate. Cell yields but not doubling times were affected by the presence or absence of yeast extract. Greater cell yields occurred in yeast extract medium than in mineral medium. Radioactive labeling studies showed that acetate was decarboxylated in mineral medium, as was shown previously in complex medium. The specific radioactivity of methane produced per specific acitvity of acetate added was not significantly different in yeast extract medium compared with mineral medium. Unequivocal evidence indicates that the cleavage of acetate to methane and carbon dioxide provided the energy for growth in the presence or absence of other organic compounds; these latter compounds do not serve as energy sources, electron donors, or significant sources of methane during this aceticlastic reaction.

Isolation and characterization of a N,N'-dicyclohexylcarbodiimide-resistant mutant of Methanothermobacter thermautotrophicus with alterations to the ATP synthesis machinery

A spontaneous mutant of Methanothermobacter thermautotrophicus resistant toward the ATP-synthase inhibitor N,N'-dicyclohexylcarbodiimide (DCCD) was isolated. DCCD normally inhibits methanogenic electron-transport-driven ATP synthesis, however, the DCCD-resistant strain exhibited methanogenesis in the presence of 300 micromol/L DCCD. Total ATP synthesis was shown to be higher in the mutant strain, both in the presence and absence of DCCD. These results suggested a modification in the ATP-synthesizing system of the mutant strain. Using Blue Native PAGE combined with MALDI TOF/TOF mass spectrometry, increased concentrations of both the A(1) and A(o) subcomplexes of the A(1)A(o)-type synthase were identified in the mutant strain. However, no alterations were found in the structural genes (atp) for the A(1)A(o) ATP synthase. The results imply that DCCD resistance is a consequence of increased A(1)A(o) ATP synthase expression, and suggest that genes involved in regulating synthase expression are responsible for DCCD resistance.

ATP-dependent H+ -pump activity in inverted vesicles of Methanosarcina mazei Gö1 and characterization of membrane ATPase

ATP-dependent H+ -pump activity was found in inverted vesicles of Methanosarcina mazei Gö1 by using acridine orange as a fluorescent probe. The H+ -pump activity specifically required both Mg and sulfite ions, but azide, an inhibitor of F0F1-ATPase, did not inhibit the activity. The membranes prepared from M. mazei also had an Mg-ATPase activity, and at least the presence of vacuolar-type ATPase was detected.

Acid phosphatase synthesis in Aspergillus flavus

Proteins with phosphatase activity were produced during the growth of Aspergillus flavus in a phosphate-supplemented liquid synthetic medium. The best carbon and nitrogen sources for the synthesis of phosphatase were glucose and ammonium sulfate, respectively. The proteins were separated by molecular exclusion and ion exclusion chromatography (IEC) into three components one of which showed phosphatase activity. The molar mass of the enzyme was approximately 62 kDa. The purified enzyme exhibited an optimum activity at pH 4.0 and at 45 degrees C. The activity of the enzyme was stimulated by Ca2+ and Mg2+ but inhibited by fluoride, iodoacetic acid, ethylenediaminetetraacetic acid and 2,4-dinitrophenol, and exhibited an apparent KM of approximately 420 mumol/L.

Metabolism of methanogens

Methanogenic archaea convert a few simple compounds such as H2 + CO2, formate, methanol, methylamines, and acetate to methane. Methanogenesis from all these substrates requires a number of unique coenzymes, some of which are exclusively found in methanogens. H2-dependent CO2 reduction proceeds via carrier-bound C1 intermediates which become stepwise reduced to methane. Methane formation from methanol and methylamines involves the disproportionation of the methyl groups. Part of the methyl groups are oxidized to CO2, and the reducing equivalents thereby gained are subsequently used to reduce other methyl groups to methane. This process involves the same C1 intermediates that are formed during methanogenesis from CO2. Conversion of acetate to methane and carbon dioxide is preceded by its activation to acetyl-CoA. Cleavage of the latter compound yields a coenzyme-bound methyl moiety and an enzyme-bound carbonyl group. The reducing equivalents gained by oxidation of the carbonyl group to carbon dioxide are subsequently used to reduce the methyl moiety to methane. All these processes lead to the generation of transmembrane ion gradients which fuel ATP synthesis via one or two types of ATP synthases. The synthesis of cellular building blocks starts with the central anabolic intermediate acetyl-CoA which, in autotrophic methanogens, is synthesized from two molecules of CO2 in a linear pathway.

Membrane ATPase from the aceticlastic methanogen Methanothrix thermophila

A new isolate of the aceticlastic methanogen Methanothrix thermophila utilizes only acetate as the sole carbon and energy source for methanogenesis (Y. Kamagata and E. Mikami, Int. J. Syst. Bacteriol. 41:191-196, 1991). ATPase activity in its membrane was found, and ATP hydrolysis activity in the pH range of 5.5 to 8.0 in the presence of Mg2+ was observed. It had maximum activity at around 70 degrees C and was specifically stimulated up to sixfold by 50 mM NaHSO3. The proton ATPase inhibitor N,N'-dicyclohexylcarbodiimide inhibited the membrane ATPase activity, but azide, a potent inhibitor of F0F1 ATPase (H(+)-translocating ATPase of oxidative phosphorylation), did not. Since the enzyme was tightly bound to the membranes and could not be solubilized with dilute buffer containing EDTA, the nonionic detergent nonanoyl-N-methylglucamide (0.5%) was used to solubilize it from the membranes. The purified ATPase complex in the presence of the detergent was also sensitive to N,N'-dicyclohexylcarbodiimide, and other properties were almost the same as those in the membrane-associated form. The purified enzyme revealed at least five kinds of subunits on a sodium dodecyl sulfate-polyacrylamide gel, and their molecular masses were estimated to be 67, 52, 37, 28, and 22 kDa, respectively. The N-terminal amino acid sequences of the 67- and 52-kDa subunits had much higher similarity with those of the 64 (alpha)- and 50 (beta)-kDa subunits of the Methanosarcina barkeri ATPase and were also similar to those of the corresponding subunits of other archaeal ATPases. The alpha beta complex of the M. barkeri ATPase has ATP-hydrolyzing activity, suggesting that a catalytic part of the Methanothrix ATPase contains at least the 67- and 52-kDa subunits.

ATP synthesis in Halobacterium saccharovorum: evidence that synthesis may be catalysed by an F0F1-ATP synthase

Halobacterium saccharovorum synthesized ATP in response to a pH shift from 8 to 6.2. Synthesis was inhibited by carbonyl cyanide m-chloro-phenylhydrazone, dicyclohexylcarbodiimide, and azide. Nitrate, an inhibitor of the membrane-bound ATPase previously isolated from this organism, did not inhibit ATP synthesis. N-Ethymaleimide, which also inhibited this ATPase, stimulated the production of ATP. These observations suggested that H. saccharovorum synthesized and hydrolysed ATP using different enzymes and that the vacuolar-like ATPase activity previously described in H. saccharovorum was an ATPase whose function is yet to be identified.

Chemiosmotic energy conversion and the membrane ATPase of Methanolobus tindarius

Electron transport phosphorylation has been demonstrated to drive ATP synthesis for the methanogenic archaebacterium Methanolobus tindarius: Protonophores evoked uncoupler effects and lowered the membrane potential delta psi. Under the influence of N,N'-dicyclohexylcarbodiimide [(cHxN)2C] the membrane potential increased while methanol turnover was inhibited. 2-Bromoethanesulfonate, an inhibitor of methanogenesis, had no effect on the membrane potential but, like (cHxN)2C and protonophores, decreased the intracellular ATP concentration. Labeling experiments with (cHxN)2(14)C showed membranes to contain a proteolipid, with a molecular mass of 5.5 kDa, that resembles known (cHxN)2C-binding proteins of F0-F1 ATPases. The (cHxN)2-sensitive membrane ATPase hydrolysed Mg.ATP at a pH optimum of 5.0 with a Km (ATP) of 2.5 mM (V = 77 mU/mg). It was inhibited competitively by ADP; Ki (ADP) = 0.65 mM. Azide or vanadate caused no significant loss in ATPase activity, but millimolar concentrations of nitrate showed an inhibitory effect, suggesting a relationship to ATPases from vacuolar membranes. In contrast, no inhibition occurred in the presence of bafilomycin A1. The ATPase was extractable with EDTA at low salt concentrations. The purified enzyme consists of four different subunits, alpha (67 kDa), beta (52 kDa), gamma (20 kDa) and beta (less than 10 kDa), as determined from SDS gel electrophoresis.

Sodium, protons, and energy coupling in the methanogenic bacteria

In this review, I focus on the bioenergetics of the methanogenic bacteria, with particular attention directed to the roles of transmembrane electrochemical gradients of sodium and proton. In addition, the mechanism of coupling ATP synthesis to methanogenic electron transfer is addressed. Evidence is reviewed which suggests that the methanogens possess great diversity in their bioenergetic machinery. In particular, in some methanogens the primary ion which is translocated coupled to metabolic energy is the proton, while others appear to utilize sodium. In addition, ATP synthesis driven by methanogenic electron transfer is accomplished in some organisms by a chemiosmotic mechanism and is coupled by a more direct mechanism in others. A possible explanation for this diversity (which is consistent with the relatedness of these organisms to each other and to other members of the Archaebacteria as determined by molecular biological techniques) is discussed.

Isolation of subunits from Methanosarcina barkeri ATPase: nucleotide-binding site in the alpha subunit

The alpha (62,000-dalton) and beta (49,000-dalton) subunits of Methanosarcina barkeri ATPase were purified to homogeneity. The subunits and ATPase complex were trypsinized in the presence of various nucleotides. ATP and ADP changed the trypsin sensitivity of the alpha subunit in the complex and isolated forms, suggesting the presence of a nucleotide-binding site in the alpha subunit.

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

Cell-free extracts of Methanobacterium thermoautotrophicum (strain delta H) were found to contain high concentrations of inorganic pyrophosphate (up to 40 mM). The compound was accumulated by the organism despite high activity of inorganic pyrophosphatase which was found to be present in the cell extracts (1-2 mumol min-1 mg protein-1). This activity was strongly inhibited at [PPi] greater than 1.0 mM. It was demonstrated that PPi synthesis occurred during methylcoenzyme M reduction under hydrogen atmosphere: in the first stage of the reaction for each mole of methane formed one mole of PPi was produced. Inhibition of the methylcoenzyme M reduction by 2-bromoethanesulfonic acid or by high concentrations (greater than 3 microM) of tetrachlorosalicylanilide also inhibited PPi synthesis. In contrast, low concentrations (1.3 microM) of tetrachlorosalicylanilide only inhibited PPi synthesis to the same extent as the methylcoenzyme M reduction was affected. In a later stage of the methylcoenzyme M reduction, PPi synthesis dropped and a second, as yet unidentified, unstable compound was formed. Synthesis of this compound also paralleled methane formation in a stoichiometric way and was affected by the inhibiting substances in a similar way as PPi synthesis.

Identification of a vanadate-sensitive, membrane-bound ATPase in the archaebacterium Methanococcus voltae

Membrane-bound ATPase activity was detected in the methanogen Methanococcus voltae. The ATPase was inhibited by vanadate, a characteristic inhibitor of E1E2 ATPases. The enzyme activity was also inhibited by diethylstilbestrol. However, it was insensitive to N,N'-dicyclohexylcarbodiimide, ouabain, and oligomycin. The enzyme displayed a high preference for ATP as substrate, was dependent on Mg2+, and had a pH optimum of approximately 7.5. The enzyme was completely solubilized with 2% Triton X-100. The enzyme was insensitive to oxygen and was stabilized by ATP. There was no homology with the Escherichia coli F0F1 ATPase at the level of DNA and protein. The membrane-bound M. voltae ATPase showed properties similar to those of E1E2 ATPases.

A plasma-membrane associated ATPase from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius

Thermoacidophilic archaebacteria have gained much interest because of their phylogenetic distance to eubacteria and eukaryotes and also because of their unique living conditions. Investigation of the energy-converting system therefore offers a key for understanding the evolutionary position and environmental adaptation of these unusual bacteria. A plasma-membrane-associated adenosine triphosphatase with specific activities of 0.3-0.6 mumol min-1 (mg protein)-1 has been detected in the thermoacidophilic archaebacterium Sulfolobus acidocaldarius (DSM 639). The enzyme exhibits two optima at pH 5.5 and 8.0, sulfite activation leads to only one optimum at pH 6.25. In the presence of the divalent cations Mg2+ or Mn2+ it hydrolyzes ATP with highest reactivity and also other purine and pyrimidine nucleotides, but not ADP and pyrophosphate. A specific stimulation by monovalent cations is not observed. The ATPase activity is not inhibited by N,N'-dicyclohexylcarbodiimide, azide or vanadate, but it is by the vascular ATPase inhibitor nitrate with an [I]50 of 8 mM. Linear Arrhenius plots up to 75 degrees C reflect pronounced adaptation to the hot environment of the archaebacterium. The solubilized ATPase as localized by activity staining in non-denaturating gels and further analyzed by sodium dodecyl sulfate electrophoresis is composed of two major polypeptides of 65 and 51 kDa reminiscent of the alpha and beta subunits of eubacterial and eukaryotic F0F1-ATPases. The ATPase is suggested as a probable candidate for a reversibly acting ATP synthase responsible for oxidative phosphorylation found in Sulfolobus acidocaldarius.

Immunoelectron microscopic demonstration of ATPase on the cytoplasmic membrane of the methanogenic bacterium strain Göl

ATPase was shown to be present on the cytoplasmic membrane of the methanogenic bacterium strain Göl. The enzyme was identified by an immunoelectron microscopic technique by using polyclonal antiserum directed against the beta subunit of Escherichia coli F0F1-ATPase. Negatively stained membrane vesicles exhibited a dense population of stalked particles similar in dimensions and fine structure to typical F0F1-ATPase particles.

Methanogenesis and ATP synthesis in a protoplast system of Methanobacterium thermoautotrophicum

When Methanobacterium thermoautotrophicum cells were incubated in 50 mM potassium phosphate buffer (pH 7.0) containing 1 M sucrose and autolysate from Methanobacterium wolfei, they were transformed into protoplasts. The protoplasts, which possessed no cell wall, lysed in buffer without sucrose. Unlike whole cells, the protoplasts did not show convoluted internal membrane structures. The protoplasts produced methane from H2-CO2 (approximately 1 mumol min-1 mg of protein-1) at about 50% the rate obtained for whole cells, and methanogenesis was coupled with ATP synthesis. Addition of the protonophore 3,5-di-tert-butyl-4-hydroxybenzylidenemalononitrile (SF-6847) to protoplast suspensions resulted in a dissipation of the membrane potential (delta psi), and this was accompanied by a parallel decrease in the rates of ATP synthesis and methanogenesis. In this respect protoplasts differed from whole cells in which ATP synthesis and methanogenesis were virtually unaffected by the addition of the protonophore. It is concluded that the insensitivity of whole cells to protonophores could be due to internal membrane structures. Membrane preparations produced from lysis of protoplasts or by sonication of whole cells gave comparatively low rates of methanogenesis (methylcoenzyme M methylreductase activity, less than or equal to 100 nmol of CH4 min-1 mg of protein-1), and no coupling with ATP synthesis could be demonstrated.

Characterization and purification of the membrane-bound ATPase of the archaebacterium Methanosarcina barkeri

Membrane-bound ATPase was found in membranes of the archaebacterium Methanosarcina barkeri. The ATPase activity required divalent cations, Mg2+ or Mn2+, and maximum activity was obtained at pH 5.2. The activity was specifically stimulated by HSO3- with a shift of optimal pH to 5.8, and N,N'-dicyclohexylcarbodiimide inhibited ATP hydrolysis. The enzyme could be solubilized from membranes by incubation in 1 mM Tris-maleate buffer (pH 6.9) containing 0.5 mM EDTA. The solubilized ATPase was purified by DEAE-Sepharose and Sephacryl S-300 chromatography. The molecular weight of the purified enzyme was estimated to be 420,000 by gel filtration through Sephacryl S-300. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate revealed two classes of subunit, Mr 62,000 (alpha) and 49,000 (beta) associated in the molar ratio 1:1. These results suggest that the ATPase of M. barkeri is similar to the F0F1 type ATPase found in many eubacteria.

31P-NMR spectra of methanogens: 2,3-cyclopyrophosphoglycerate is detectable only in methanobacteria strains

The unique compound 2,3-cyclopyrophosphoglycerate occurs at a detectable concentration in the genera Methanobacterium and Methanobrevibacter but not in Methanococcus, Methanospirillum and Methanosarcina, as shown by a 31P-NMR survey of several different methanogens. Metabolic poisons (carbonyl cyanide m-chlorophenylhydrazone and valinomycin) do not decrease the level of the cyclic pyrophosphate in Methanobacterium thermoautotrophicum; therefore, it cannot be a phosphagen, i.e., an energy storage material. 13CO2 is rapidly incorporated into this cyclic compound which represents the major soluble carbon as well as the phosphorus component of this methanobacteria. 13C-NMR analysis demonstrates that the pKa of the 2,3-cyclopyrophosphoglycerate carboxyl group is 2.55. The unusual pseudomurein cell wall structure of methano- and methanobrevibacteria necessitates a high demand on carbohydrate metabolism. For this reason, and the fact that when its concentration is decreased no new phosphorus resonances appear in the high resolution spectra, it is suggested that 2,3-cyclopyrophosphoglycerate has a function in carbohydrate metabolism.

Dicyclohexylcarbodiimide-sensitive ATPase in Halobacterium saccharovorum

Membranes from Halobacterium saccharovorum contained a cryptic ATPase which required Mg2+ or Mn2+ and was activated by Triton X-100. The optimal pH for ATP hydrolysis was 9-10. ATP or GTP were hydrolyzed at the same rate while ITP, CTP, and UTP were hydrolyzed at about half that rate. The products of ATP hydrolysis were ADP and phosphate. The ATPase required high concentrations (3.5 M) of NaCl for maximum activity. ADP was a competitive inhibitor of the activity, with an apparent Ki of 50 microM. Dicyclohexylcarbodiimide (DCCD) inhibited ATP hydrolysis. The inhibition was marginal at the optimum pH of the enzyme. When the ATPase was preincubated with DCCD at varying pH values, but assayed at the optimal pH for activity, DCCD inhibition was observed to increase with increasing acidity of the preincubation medium. DCCD inhibition was also dependent on time of preincubation, and protein and DCCD concentrations. When preincubated at pH 6.0 for 4 h at a protein:DCCD ratio of 40 (w/w), ATPase activity was inhibited 90%.

Membrane-bound ATPase of a thermoacidophilic archaebacterium, Sulfolobus acidocaldarius

The membranes of Sulfolobus, a thermoacidophilic archaebacterium showed two types of ATP hydrolyzing activity. One was that of a neutral ATPase at an optimum pH around 6.5. This enzyme was activated by 10 mM sulfate with a shift of optimum pH to 5. In these respects, the enzyme was similar to membrane-bound ATPase of Thermoplasma, another thermoacidophilic archaebacterium, reported by Searcy and Whatley [1982) Zbl. Bakt. Hyg., I. Abt. Orig. C3, 245-257). The enzyme hydrolyzed ATP and other NTPs, but not ADP or AMP. It was highly thermostable, but irreversibly inactivated in 0.1 M HCl. The other activity was that of an acidic apyrase at an optimum pH around 2.5. This enzyme was extremely stable toward high temperature and acid and inhibited by sulfate. Both of these ATP hydrolyzing enzymes were resistant to N,N'-dicyclohexylcarbodiimide (DCCD), azide, oligomycin, N'-ethylmaleimide, p-chloromercuribenzoate, orthovanadate, or ouabain. Sulfolobus ATPases differ from F1 and other transport ATPases so far described.

ATP synthesis in Methanobacterium thermoautotrophicum coupled to CH4 formation from H2 and CO2 in the apparent absence of an electrochemical proton potential across the cytoplasmic membrane

Methanogenic bacteria are considered to couple methane formation with the synthesis of ATP by a chemiosmotic mechanism. This hypothesis was tested with Methanobacterium thermoautotrophicum. Methane formation from H2 and CO2 (2.5 - 3 mumol X min-1 X mg cells-1) by cell suspensions of this organism resulted in the formation of an electrochemical proton potential (delta mu H +) across the cytoplasmic membrane of 230 mV (inside negative) and in the synthesis of ATP up to an intracellular concentration of 5 - 7 nmol/mg. The addition of ionophores at concentrations which completely dissipated delta mu H + without inhibiting methane formation did not result in an inhibition of ATP synthesis. It thus appears that delta mu H + across the cytoplasmic membrane is not the driving force for the synthesis of ATP in M. thermoautotrophicum.

The bioenergetics of methanogenesis

The reduction of CO2 or any other methanogenic substrate to methane serves the same function as the reduction of oxygen, nitrate or sulfate to more reduced products. These exergonic reactions are coupled to the production of usable energy generated through a charge separation and a protonmotive-force-driven ATPase. For the understanding of how methanogens derive energy from C-1 unit reduction one must study the biochemistry of the chemical reactions involved and how these are coupled to the production of a charge separation and subsequent electron transport phosphorylation. Data on methanogenesis by a variety of organisms indicates ubiquitous use of CH3-S-CoM as the final electron acceptor in the production of methane through the methyl CoM reductase and of 5-deazaflavin as a primary source of reducing equivalents. Three known enzymes serve as catalysts in the production of reduced 5-deazaflavin: hydrogenase, formate dehydrogenase and CO dehydrogenase. All three are potential candidates for proton pumps. In the organisms that must oxidize some of their substrate to obtain electrons for the reduction of another portion of the substrate to methane (e.g., those using formate, methanol or acetate), the latter two enzymes may operate in the oxidizing direction. CO2 is the most frequent substrate for methanogenesis but is the only substrate that obligately requires the presence of H2 and hydrogenase. Growth on methanol requires a B12-containing methanol-CoM methyl transferase and does not necessarily need any other methanogenic enzymes besides the methyl-CoM reductase system when hydrogenase is present. When bacteria grow on methanol alone it is not yet clear if they get their reducing equivalents from a reversal of methanogenic enzymes, thus oxidizing methyl groups to CO2. An alternative (since these and acetate-catabolizing methanogens possess cytochrome b) is electron transport and possible proton pumping via a cytochrome-containing electron transport chain. Several of the actual components of the methanogenic pathway from CO2 have been characterized. Methanofuran is apparently the first carbon-carrying cofactor in the pathway, forming carboxy-methanofuran. Formyl-FAF or formyl-methanopterin (YFC, a very rapidly labelled compound during 14C pulse labeling) has been implicated as an obligate intermediate in methanogenesis, since methanopterin or FAF is an essential component of the carbon dioxide reducing factor in dialyzed extract methanogenesis. FAF also carries the carbon at the methylene and methyl oxidation levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Methane synthesis by membrane vesicles and a cytoplasmic cofactor isolated from Methanobacterium thermoautotrophicum

Methanobacterium thermoautotrophicum when grown on ordinary culture medium has a tough cell wall which is lysozyme-resistant and difficult to disrupt by physical means. The cell wall, however, can be weakened by the addition of D-sorbitol to the growth medium and the organisms form protoplasts after lysozyme addition. This technique allowed the isolation of two types of intracellular small vesicles: (a) isolated by disruption of the total cell population (lysozyme-sensitive and lysozyme-resistant cells) by ultrafrequency sound and (b) isolated by osmotic lysis of protoplasts. For the first time, a small vesicle fraction isolated as in (a) was capable of synthesizing methane from CO2 and H2 without cytoplasm. There was, however, an absolute requirement for a small, heat-stable, oxygen-sensitive cofactor which was isolated from the cytoplasm. Methane synthesis with this vesicle fraction was inhibited by the detergent deoxycholate, and by the protonophores 2,4-dinitrophenol and carbonyl cyanide m-chlorophenylhydrazone. Mg2+-ATPase appeared to be located on the outer or cytoplasmic surface of the small vesicle fraction isolated as in (b). The results were consistent with a previously made suggestion [Sauer, Erfle & Mahadevan (1981) J. Biol. Chem. 256, 9843-9848] that the interior of the small intracellular vesicles becomes acid during methane synthesis.

Coupling of ATP synthesis and methane formation from methanol and molecular hydrogen in Methanosarcina barkeri

The addition of methanol to a cell suspension of Methanosarcina barkeri resulted in an increase of the intracellular ATP concentration from 1 nmol/mg to 10 nmol/mg protein and in the formation of a proton-motive force delta p of -130 mV. delta p consisted of more than 90% of the membrane potential delta psi. These values were similar under N2 and under H2. The addition of the uncoupler tetrachlorosalicylanilide to the above system under N2 led to a drastic decrease of both, the ATP concentration and the delta p and to a stop of methanogenesis. With methanol and H2, however, methane formation continued, although the effect of the uncoupler on the ATP pool and on delta p was a under N2. The proton-translocating ATPase inhibitor N,N'-dicyclohexylcarbodiimide caused a rapid exhaustion of the ATP pool and a discontinuation of methane synthesis, whereas delta p was unaffected. Inhibition of methane formation under these conditions could be relieved by the addition of the uncoupler tetrachlorosalicylanilide. These results demonstrate that methane formation according to the equation CH3OH + H2----H2----CH4 + H2O was coupled to ATP synthesis by a chemiosmotic mechanism and was under the control of delta psi: Methane formation only proceeded if the delta psi generated was used for ATP synthesis or if an uncoupler was present. Under N2, methane formation according to the equation 4CH3OH ----CO2 + 3CH4 + 2H2O was abolished by an uncoupler, because one step in the oxidation of methanol to 1 CO2 apparently depended on an energized state of the membrane.

Activation of the methylreductase system from Methanobacterium bryantii by ATP

The methylreductase of Methanobacterium bryantii required ATP for activity. There was sufficient ATP synthesis in extracts to account for the observed activity. Hexokinase inhibited the methylreductase by competing for endogenously synthesized ATP. The uncoupler, carbonyl cyanide p-trifluoromethyoxyphenyl hydrazone, inhibited only at concentrations greater than 0.5 mM, and detergents and non-halogenated membrane-permeable-ions did not inhibit. Thus, membrane proton gradients are not important in activation. In addition, maximal activation was obtained with less than 0.25 mM ATP, was inhibited by beta, gamma-imido ATP, and was strongly temperature dependent. The activated state was very unstable, having a half-life of 5 to 15 min. After gel filtration at 5 degrees C, the methylreductase retained partial activity for a short time in the absence of ATP. These observations indicate that activation involves the modification of a protein or protein-bound cofactor of the methylreductase system.

Chromophoric derivatives of coenzyme MF430, a proposed coenzyme of methanogenesis in Methanobacterium thermoautotrophicum

Factors F430 are nickel tetrapyrroles from methanogenic bacteria. Two methods are described to extract these compounds from cells of Methanobacterium thermoautotrophicum, namely, by boiling with 40% ethanol and by treatment of disrupted cells with HClO4 at pH 2 and 0 degrees C. The subsequent purification procedures involving column chromatography are outlined. Ethanol extraction yielded one yellow compound which will be denoted coenzyme MF430 (CoMF430). Extraction with HCIO4 yielded a yellow derivative, called Factor F430II, and a red component (F560). In addition, a number of derivatives were prepared by preparative thin-layer chromatography, acid hydrolysis, and methanolysis of acid hydrolyzates. On the basis of ultraviolet-visible light absorption and mass spectral data, it was concluded that the methylated chromophores obtained by treatment of acid hydrolyzates are derivatives of Ni(II)sirohydrochlorin and its pi-cation radical. Reduction studies and ultraviolet-visible light, 1H-NMR, and mass spectroscopy indicate that the chromophoric derivatives of CoMF430 differ from the native compound with respect to the reduction level of the tetrapyrrole and the structural elements that are attached to the chromophore.

Structural elements of methanopterin, a novel pterin present in Methanobacterium thermoautotrophicum

During short-time labeling experiments, cells of Methanobacterium thermoautotrophicum incorporate a substantial part of 14CO2 in a yellow fluorescent compound (called YFC) [Daniels, L. & Zeikus, J. G. (1978) J. Bacteriol. 136, 75-84]. As the compound was present only in small amounts, its more abundant, metabolic precursor was identified, extracted and purified by column chromatography. The chromophore of this compound is 2-amino-4-hydroxypteridine (pterin) as indicated by its ultraviolet-visible-light absorption and fluorescence properties. Decomposition studies revealed the presence of a number of structural elements, viz. glutamic acid, phosphate and a hexosamine. 1H-NMR and 13C-NMR spectra pointed to the presence of additional, as yet unidentified, elements. The compound is a complex, novel pterin derivative, which we have called methanopterin.

A novel one-carbon carrier (carboxy-5,6,7,8-tetrahydromethanopterin) isolated from Methanobacterium thermoautotrophicum and derived from methanopterin

During short-term labeling experiments, cells of Methanobacterium thermoautotrophicum incorporated a substantial part of 14CO2 in a compound with a bright yellow fluorescence on dry thin-layer chromatography plates and called yellow fluorescent compound (YFC) [Daniels, L. and Zeikus, J.G. (1978) J. Bacteriol. 136, 75-84]. This compound was extracted and purified by ion-exchange column chromatography with formic acid gradients up to 0.3 M. Out of 325 g wet cells of M. thermoautotrophicum about 4 mg of the compound were isolated. This material and some degradation products obtained from it were studied by means of chemical decomposition, ultraviolet-visible-light spectroscopy and preliminary 1H-NMR spectroscopy. It has structural elements in common with methanopterin (see preceding paper in this journal); these elements are a pterin group, glutamate, a hexosamine. The pterin in this compound is present in a reduced form, presumably as 5,6,7,8-tetrahydromethanopterin, and the additional one-carbon unit is probably present as a carboxy group. Probably the first step of methanogenesis implies a carboxylation of methanopterin and a concomitant reduction of the pterin. The trivial name carboxy-5,6,7,8-tetrahydromethanopterin is introduced for the compound.

Product isotope effects on in vivo methanogenesis by Methanobacterium thermoautotrophicum

The hydrogen in methane produced by cultures of Methanobacterium thermoautotrophicum originates from water. In H2O/D2O mixtures, a methane product isotope effect is observed that increases rapidly as the water deuterium enrichment approaches 100%. This effect is due to the intracellular production of protons from H2, catalyzed by hydrogenase, which occurs at 12% the rate of water diffusion through the cell membrane. We estimate that water diffusion through the thick cell membrane of M. thermoautotrophicum is retarded by a factor of 10(6) over the free diffusion rate. The intracellular production of H+ suggests that either (1) hydrogenase is not directly involved in the production of a chemiosmotic proton gradient or (2) if it is involved, the proton gradient exists between the cytosol and the interior of vesicles observed in this bacterium. The intrinsic deutrium product isotope effect in methanogenesis is 1.20 +/- 0.1, comparable to anabolic deuterium product isotope effects in other autotrophs. An algebraic model incorporating the intracellular H2 to H+ flux accurately predicts the distribution of deuterated methane species at all levels of water deuterium enrichment.

Methane production by the membranous fraction of Methanobacterium thermoautotrophicum

Intact membrane vesicles are required to synthesize methane from CO2 and H2 by disrupted preparations of Methanobacterium thermoautotrophicum cells. When membrane vesicles were removed by high-speed centrifugation at 226 600 g, the remaining supernatant fraction no longer synthesized methane. Alternatively, if vesicle structure was disrupted by passage through a Ribi cell fractionator at very high pressures (345 MPa), the bacterial cell extract, with all the particulate fraction in it, did not synthesize methane. Methyl-coenzyme M, a new coenzyme first described by McBride & Wolfe [(1971) Biochemistry 10, 2317--2324], was shown to stimulate methane production from CO2 and H2, as previously reported, but the methyl group of the coenzyme did not appear to be a precursor of methane in this reaction. No methyl-coenzyme M reductase activity was detected in the cytoplasmic fraction of M. thermoautotrophicum cells.