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

Electron transfer-driven ATP synthesis in Methanococcus voltae is not dependent on a proton electrochemical gradient

Intracellular ATP levels in whole cells of Methanococcus voltae respond to electron transfer coupled to methanogenesis. ATP synthesis can also be induced by an artificially imposed transmembrane electrical potential [formed by electrogenic movement outwards of potassium (induced by valinomycin) or of protons (induced by an uncoupler], or by a pH gradient (acid outside). These results implicate the existence of a reversible ATPase coupled to electrogenic movement of an ion(s) other than potassium or proton, and that ionophores are competent to catalyze ion movement across the cytoplasmic membrane of this organism (which is the sole membrane structure in this species). ATP synthesis driven by electron transfer is, however, insensitive to the addition of such ionophores. These results indicate that although cells possess an ion-translocating ATPase (possibly involved in the maintenance of internal ionic composition homeostasis), methanogenesis-driven ATP synthesis does not involve the intermediacy of a transmembrane ion gradient. Primarily because methane formation has been previously demonstrated to involve true electron transfer, substrate-level phosphorylation (at least in analogy to other systems) has been generally ruled out. The results presented here suggest that at least one methanogenic bacterium may use a direct linkage of ATP synthesis to electron transfer.

Acylation of proteins of the archaebacteria Halobacterium cutirubrum and Methanobacterium thermoautotrophicum

Although the membrane lipids of extremely halophilic archaebacteria are exclusively derived from diphytanylglycerol diether, which is non-acylated, small amounts of fatty acids have been detected in these organisms. These fatty acids are formed by the action of a fatty acid synthase (FAS), shown to be present in the extreme halophile Halobacterium cutirubrum, despite the fact that only a fraction of the activity of FAS remains at the high salt concentration (> 4 M) present in the cytoplasm. It has now been demonstrated that fatty acids do not occur in lipid-bound form but largely in the form of acylated proteins in the red membrane of H. cutirubrum. In contrast, the bacteriorhodopsin of the purple membrane of this extreme halophile does not appear to be acylated. The thermophilic methanogen, Methanobacterium thermoautotrophicum had a much higher fatty acid synthase activity than the extreme halophile, and the synthase activity of the methanogen was optimal under its normal (anaerobic) growth conditions. The methanogen also utilized the resulting fatty acids to acylate its membrane proteins. The major fatty acids in both organisms were palmitic and stearic acids with small amounts of myristic and 18:1 acids, and these were bound to protein through both ester and amide linkages.

Na(+)-driven ATP synthesis in Methanobacterium thermoautotrophicum and its differentiation from H(+)-driven ATP synthesis by rhodamine 6G

Rhodamine 6G (3 microM) effectively inhibited delta pH-driven ATP synthesis in Methanobacterium thermoautotrophicum while delta pNA-driven ATP synthesis was not affected by it. Rhodamine 6G inhibited Mg(2+)-stimulated ATPase activity of membrane vesicles prepared from these cells but the ATPase catalytic sector detached from the membrane was insensitive to this inhibitor. Methanogenesis-driven ATP synthesis at pH 6.8 of the cells grown in the presence of 50 mM NaCl was inhibited by rhodamine 6G both in the presence of 5 mM and 50 mM NaCl. On the other hand, the methanogenesis-driven ATP synthesis at pH 8.0 of cells grown in the presence of 50 mM NaCl was slightly inhibited by rhodamine 6G in the presence of 5 mM NaCl and was not inhibited at all in the presence of 50 mM NaCl. The growth experiments have shown that cells of Methanobacterium thermoautotrophicum can grow under alkaline conditions even in the presence of rhodamine 6G and of high NaCl concentration when the growth media were inoculated with the cells which had been grown in the presence of 50 mM NaCl. These results indicate that sodium-motive force-driven ATP synthase in Methanobacterium thermoautotrophicum operates effectively at alkaline conditions and it might be the sole ATP synthesizing system when the proton motive force-supported ATP synthesis is inhibited by rhodamine 6G.

Na(+)-driven ATP synthesis in Methanobacterium thermoautotrophicum and its differentiation from H(+)-driven ATP synthesis by rhodamine 6G

Rhodamine 6G (3 microM) effectively inhibited delta pH-driven ATP synthesis in Methanobacterium thermoautotrophicum while delta pNa-driven ATP synthesis was not affected by it. Rhodamine 6G inhibited Mg(2+)-stimulated ATPase activity of membrane vesicles prepared from these cells but the ATPase catalytic sector detached from the membrane was insensitive to this inhibitor. Methanogenesis-driven ATP synthesis at pH 6.8 of cells grown in the presence of 50 mM NaCl was inhibited by rhodamine 6G both in the presence of 5 mM and 50 mM NaCl. On the other hand, the methanogenesis-driven ATP synthesis at pH 8.0 of cells grown in the presence of 50 mM NaCl was slightly inhibited by rhodamine 6G in the presence of 5 mM NaCl and was not inhibited at all in the presence of 50 mM NaCl. The growth experiments have shown that cells of Methanobacterium thermoautotrophicum can grow under alkaline conditions even in the presence of rhodamine 6G and of high NaCl concentration when the growth media were inoculated with the cells which had been grown in the presence of 50 mM NaCl. These results indicate that sodium-motive force-driven ATP synthase in Methanobacterium thermoautotrophicum operates effectively in alkaline conditions and it might be the sole ATP synthesizing system when the proton-motive force-supported ATP synthesis is inhibited by rhodamine 6G.

Mode of sodium ion action on methanogenesis and ATPase of the moderate halophilic methanogenis bacterium Methanohalophilus halophilus

Cells of Methanohalophilus halophilus swelled when exposed to hypotonic solutions of NaCl at pH 7.0. The swelling of the cells ceased in the presence of Mg2+. Methane formation by non-growing cells was strongly dependent on the NaCl concentration. Among other monovalent and divalent cations only Li+ and Mg2+ could partly substitute for a specific function of sodium ions. The artificial Na+/H+ antiporter, monensin, exerted a strong inhibitory effect on methane formation from methylamine. The membrane-bound Mg(2+)-stimulated ATPase of these cells was enhanced at low (40 mM) NaCl concentration while higher concentrations of this solute were inhibitory. The results obtained show that sodium ions are a prerequisite for optimal methane formation and ATPase activity in these cells. However, both of these processes required different sodium ion concentrations.

Inhibition of methylcoenzyme M methylreductase by a uridine 5'-diphospho-N-acetylglucosamine derivative

Uridine-5'-diphospho-N-acetylglucosamine, when oxidized with periodate to the corresponding aldehyde (o-UDP-GlcNAc), was a potent inhibitor of the methylcoenzyme M methylreductase reaction which catalyzes the reductive demethylation of methylcoenzyme M to methane. The oxidation product, o-UDP-GlcNAc, appears to bind to the UDP-GlcNAc site of the enzyme and inhibits the reduction of methylcoenzyme M both by MRF or its active hydrolytic fragment HS-HTP. The kinetic patterns indicate that o-UDP-GlcNAc inhibition is noncompetitive with HS-HTP or MRF, and the Hill coefficient indicated that there was cooperativity between the UDP and HS-HTP binding sites. The methylreductase enzyme was protected from o-UDP-GlcNAc inhibition by prior exposure to low concentrations of MRF. HS-HTP, at the same concentration as MRF, was not effective in protecting the enzyme from inhibition by o-UDP-GlcNAc.

Structure of a novel cofactor containing N-(7-mercaptoheptanoyl)-O-3-phosphothreonine

The cofactor required in the methylcoenzyme M methylreductase reaction was shown to be a large molecule with an Mr of 1149.21 in the free acid form. The cofactor, named MRF for methyl reducing factor, was identified from analyses by fast atom bombardment mass spectrometry and 1H, 13C, and 31P NMR spectroscopy as uridine 5'-[N-(7-mercaptoheptanoyl)-O-3-phosphothreonine-P-yl(2-acetamido- 2-deoxy- beta-mannopyranuronosyl)(acid anhydride)]-(1----4)-O-2-acetamido-2-deoxy- alpha-glucopyranosyl diphosphate. MRF contains N-(7-mercaptoheptanoyl)threonine O-3-phosphate (HS-HTP) [No11, K. M., Rinehart, K. L., Tanner, R. S., & Wolfe, R. S. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 4238-4242] and is linked to C-6 of 2-acetamido-2-deoxymannopyranuronic acid of the UDP-disaccharide through a carboxylic-phosphoric anhydride linkage. It is postulated that this bond is responsible for the instability of the molecule and its hydrolysis during isolation. Analyses of Eadie and Hofstee plots of the methylcoenzyme M methylreductase reaction indicate that MRF has a 6-fold lower Km(app) than HS-HTP and a 50% greater Vmax. This suggests that the UDP-disaccharide moiety may be of importance in the binding of MRF to the enzyme active site.

ATP synthesis coupled to methane formation from methyl-CoM and H2 catalyzed by vesicles of the methanogenic bacterial strain Gö1

Methanogenesis from methyl-CoM and H2, as catalyzed by inside-out vesicle preparations of the methanogenenic bacterium strain Gö1, was associated with ATP synthesis. That this ATP synthesis proceeded via an uncoupler-sensitive transmembrane proton gradient was concluded from the following results: 1. Various inhibitors that affected methane formation (e.g. 2-bromomethanesulfonate) also prevented ATP synthesis. 2. The protonophore 3,5-di-tert-butyl-4-hydroxybenzylidenemalononitrile, in combination with the K+ ionophore valinomycin, inhibited ATP synthesis completely without affecting methanogenesis. 3. The ATP synthase inhibitor diethylstilbestrol inhibited ATP synthesis. 4. Addition of the detergent sulfobetaine inhibited both methane formation and ATP synthesis; the former but not the latter could be restored by adding titanium(III) citrate as electron donor. In addition it was shown that ATP synthesis could also be driven by transmembrane proton gradients artificially imposed on the vesicles. Furthermore net methanogenesis-dependent ATP formation was shown by measuring [32P]phosphate incorporation.

Structure determination of the UDP-disaccharide fragment of cytoplasmic cofactor isolated from Methanobacterium thermoautotrophicum

The methylcoenzyme M methylreductase reaction has an absolute requirement for 7-mercaptoheptanoylthreonine phosphate or component B, which is the active component of the intact molecule previously referred to as cytoplasmic cofactor. A hydrolytic fragment of cytoplasmic cofactor has been purified and identified as uridine 5'-(O-2-acetamido-2-deoxy-beta-manno-pyranuronosyl acid (1----4)-2-acetamido-2-deoxy-alpha-glucopyranosyl diphosphate) by high resolution NMR and fast atom bombardment mass spectro-metry. It is postulated that UDP-disaccharide may function to anchor 7-mercaptoheptanoyl threonine phosphate at the active site of the methyl-reductase enzyme complex.

Structure of purified cytoplasmic cofactor from Methanobacterium thermoautotrophicum

The reduction of methylcoenzyme M to methane is known to require a heat stable and oxygen sensitive cofactor. Recently it has been shown that the active site of this cofactor is 7-mercaptoheptanoylthreonine phosphate. The present study shows that in the complete structure of this cofactor 7-mercaptoheptanoylthreonine phosphate is linked by pyrophosphate to two N-acetyl-glucosamine residues and an unidentified terminal group R with m/z 214. By fast-atom-bombardment mass spectrometry the intact cofactor, isolated as the mixed disulfide with 2-mercaptoethanol, was shown to have a molecular weight of 1084.5. The pyrophosphate bond is quite labile and undergoes hydrolysis or prolonged storage. This lability of the pyrophosphate bond may explain why the intact cofactor has not been isolated until now.

Structure of two new aminophospholipids from Methanobacterium thermoautotrophicum

The methanogenic bacterium Methanobacterium thermoautotrophicum (A.T.C.C. 29183) was shown to contain two new aminophospholipids. These are 2-aminoethyl phosphate ester of diphytanylglycerol diether and a sugar containing bisdiphytanyldiglycerol tetraether. The two aminophospholipids were stable to acid methanolysis except for the sugar on the bisdiphytanyldiglycerol tetraether. Strong acid (6 M-HCl) hydrolysed the alkyl ether and aminophosphate ester bonds. The structure of the phosphate linkage was demonstrated by 31P n.m.r., and the 2-ethanolamine structure was elucidated by 1H- and 13C-n.m.r. spectroscopy and by fast-atom-bombardment m.s.

Purification and use of Methanobacterium wolfei pseudomurein endopeptidase for lysis of Methanobacterium thermoautotrophicum

The pseudomurein-degrading enzyme from autolysates of Methanobacterium wolfei was purified approximately 500-fold to electrophoretic homogeneity by ion-exchange chromatography under anaerobic conditions. Analysis of the soluble cell wall fragments produced by the pure enzyme from a cell wall preparation of M. thermoautotrophicum indicated that it is a peptidase hydrolyzing the epsilon-Ala-Lys bond of pseudomurein. A partially purified preparation of pseudomurein endopeptidase was free of nuclease activity and thus proved useful for the preparation in high yields of undegraded chromosomal and plasmid DNA from M. thermoautotrophicum. The partially purified enzyme was also used for the preparation of protoplasts, which were stabilized by 0.8 M sucrose. Under growth conditions the protoplasts produced methane and increased up to 100-fold in size, but failed to regenerate a cell wall.

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.

Tetrahydromethanopterin methyltransferase, a component of the methane synthesizing complex of Methanobacterium thermoautotrophicum

A new enzyme, tetrahydromethanopterin methyltransferase, which catalyzes the transfer of methyl groups from methyl-tetrahydromethanopterin to 2-mercaptoethane-sulfonate, has been found in the methane synthesizing complex of Methanobacterium thermoautotrophicum. The enzyme is oxygen sensitive and has a well defined pH optimum at pH 6.7. There was no methyl group transfer when the enzyme was heated to 100 degrees for 5 min. The product of the forward reaction, methyl-CoM, was positively identified by TLC and high voltage paper electrophoresis. The demethylation of methyl-CoM, in the absence of methane synthesis, was dependent on the addition of H4MPT which suggests that the enzyme reaction is reversible.

The role of tetrahydromethanopterin and cytoplasmic cofactor in methane synthesis

A fraction previously isolated from acid-treated supernatant fraction of Methanobacterium thermoautotrophicum by DEAE-Sephadex chromatography [Sauer, Mahadevan & Erfle (1984) Biochem. J. 221, 61-97] which was absolutely required for methane synthesis, has been separated into two compounds, tetrahydromethanopterin (H4MPT) and an as-yet-unidentified cofactor we call 'cytoplasmic cofactor'. H4MPT was identified by its u.v. spectrum and by 13C- and 1H-n.m.r. spectroscopy. The reduction of 2-(methylthio)ethanesulphonic acid (CH3-S-CoM) to methane by the membrane fraction from M. thermoautotrophicum was completely dependent on the addition of cytoplasmic cofactor. Methane synthesis from CO2, however, was only partially dependent on cofactor addition, and 57% of the original activity was retained in its absence. The kinetics of 14C labelling were consistent with the scheme methyl-H4MPT----CH3-S-CoM----methane, as has been proposed. This is the first time that direct experimental evidence has been presented to show that the proposed methyl transfer from H4MPT to coenzyme M (HS-CoM) actually occurs.

Enzymatic lysis of the pseudomurein-containing methanogen Methanobacterium formicicum

A streptomycete isolated from cow manure produces an extracellular enzyme capable of lysing the pseudomurein-containing methanogen Methanobacterium formicicum. The lytic activity has been partially purified from culture fluid and appears to be a serine protease. Similar lytic activity has been fractionated from pronase. Optimal conditions have been developed for lysis of M. formicicum by commercial preparations of proteinase K. The three lytic enzymes have been partially characterized. The results with the three enzyme preparations tend to confirm that proteolytic enzymes are capable of lysing methanogen cells.

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.