Chemiosmotic coupling in Methanobacterium thermoautotrophicum: hydrogen-dependent adenosine 5'-triphosphate synthesis by subcellular particles

New biological paramagnetic center: octahedrally coordinated nickel(III) in the methanogenic bacteria

Methanobacterium bryantii was grown in medium supplemented with nickel-61, an isotope with a nuclear spin of 3/2. The appearance of nuclear hyperfine structure in the electron paramagnetic resonance spectrum of a cell extract identified a previously observed signal as nickel(III) in an environment of octahedral coordination with rhombic distortions.

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.

Evidence for adenylate nucleotide transport (ATP-ADP translocation) in vesicles of Frankia sp. strain EAN1pec

Atractyloside and carboxyatractyloside partially inhibited nitrogenase activity (acetylene reduction) by isolated vesicles of Frankia strain EAN1pec. Extracts of disrupted vesicles showed nitrogenase activity that was not affected by the inhibitors. The vesicles accumulated ATP by an atractyloside-sensitive mechanism. This inhibition of ATP uptake was reversed when vesicles were permeabilized by detergent. Uptake of ATP was inhibited by excess ATP and ADP, but not AMP or adenosine, and by a calcium-dependent ATPase inhibitor. Uptake was stimulated by calcium ions. Accumulation of ATP was accompanied by release of ADP and AMP from the vesicles. The ATP taken up by vesicles and cells grown with N2 as the nitrogen source was found in the corresponding cell pools only as ATP. The data indicate activity of an ATP-ADP translocase system in vesicles of this organism. The role of ATP translocation in the symbiosis between Frankia strain EAN1pec and plant root nodules is discussed.

Methanogenesis and ATP synthesis in methanogenic bacteria at low electrochemical proton potentials. An explanation for the apparent uncoupler insensitivity of ATP synthesis

The rate of methane formation from H2 and CO2, the intracellular ATP content and the electrochemical proton potential (delta mu H+) were determined in cell suspensions of Methanobacterium thermoautotrophicum, which were permeabilized for K+ with valinomycin (1.2 mumol/mg protein). In the absence of extracellular K+ the cells formed methane at a rate of 4 mumol min-1 (mg protein)-1, the intracellular ATP content was 20 nmol/mg protein and the delta mu H+ was 200 mV (inside negative). When K+ was added to the suspensions the measured delta mu H+ decreased to the value calculated from the [K+]in/[K+]out ratio. Using this method of delta mu H+ adjustment, it was found that lowering delta mu H+ from 200 mV ([K+]in/[K+]out = 1000) to 100 mV ([K+]in/[K+]out = 40) had no effect on the rate of methane formation and on the intracellular ATP content. At delta mu H+ values below 100 mV ([K+]in/[K+]out less than 40) both the rate of methanogenesis and the ATP content decreased. Methanogenesis completely ceased and the ATP content was 2 nmol/mg when delta mu H+ was adjusted to values lower 50 mV ([K+]in/[K+]out less than 7). The data show that methanogenesis from H2 and CO2 and ATP synthesis in M. thermoautotrophicum are possible at relatively low electrochemical proton potentials. Similar results were obtained with Methanosarcina barkeri. Protonophoric uncouplers like 3,5,3',4'-tetrachlorosalicylanilide (TCS) or 3,5-di-tert-butyl-4-hydroxy-benzylidenemalononitrile (SF 6847) were found not to dissipate delta mu H+ below 100 mV in M. thermoautotrophicum even when used at high concentrations (400 nmol/mg protein). This finding explains the observed uncoupler insensitivity of methanogenesis and ATP synthesis in this organism.

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.

Purification and some properties of the corrinoid-containing membrane protein from Methanobacterium thermoautotrophicum

The cytoplasmic membrane of the methanogenic archaebacterium Methanobacterium thermoautotrophicum does not contain cytochromes, but did contain a corrinoid protein of molecular mass about 33 kDa which, after treatment with 10 mg Triton X-100/mg protein, was contained in a protein complex of about 500 kDa. Washed membranes from 1 g dry cells contained about 70 nmol of the cobamide factor III (5-hydroxybenzimidazolyl cobamide) as the sole corrinoid. The corrinoid-containing protein complex was purified and some of its properties were studied. According to several criteria it is an integral membrane protein complex. The corrinoid-protein complex, after about 100-fold purification, gave a single band on native PAGE and still had molecular mass of about 500 kDa. In SDS-PAGE several subunits were observed: in addition to the corrinoid-carrying subunit of about 33 kDa, other polypeptides of approximately 28 kDa, 26 kDa, and possibly 23 kDa were present. One mole of the purified 500-kDa protein complex contained greater than or equal to eight moles of the cobamide factor III. It was estimated that the corrinoid-protein complex accounts for 8% of the membrane protein of M. thermoautotrophicum. The visible spectrum of the oxidized protein exhibited absorbance maxima at 547 nm, 511 nm, and a shoulder at 468 nm, which disappeared upon reduction with dithionite. The midpoint potential of this transition was around -145 mV (pH 7). With EPR a Co2+ signal was observed within -50 mV and -350 mV with a maximum around -200 mV. Possible reasons for the disappearance of the Co2+ signal at low redox potentials are discussed. The line shape of the Co2+ signal was similar to that of Co2+ in free corrinoids. The signal of Co2+ could also be evoked by reduction with 5 mM dithiothreitol. From the redox properties of the corrinoid membrane protein it may be expected that in vivo the cobalt may become reduced and reoxidized. Its possible function as an electron-mediating membrane protein in the metabolism of methanogenic bacteria is discussed.

Locations of the hydrogenases of Methanobacterium formicicum after subcellular fractionation of cell extract

The F420 hydrogenase of Methanobacterium formicicum was associated with membranes isolated by sucrose density gradient ultracentrifugation of cell extract. The methyl viologen hydrogenase was present in the soluble fractions. Column chromatography with phenyl-Sepharose CL-4B revealed that the F420 hydrogenase was strongly hydrophobic, suggesting that it associates with isolated membranes through hydrophobic interactions.

Electron microscopy of nickel-containing methanogenic enzymes: methyl reductase and F420-reducing hydrogenase

Methanogens catalyze the hydrogen-dependent eight-electron reduction of carbon dioxide to methane. Two of the key catalysts in the eight-electron reduction pathway are the nickel-containing enzymes F420-reducing hydrogenase and methyl reductase. In the present study, the structures of these archaebacterial enzymes from Methanobacterium thermoautotrophicum delta H have been determined by electron microscopy. By negative stain techniques, F420 hydrogenase was found to be a ring structure with a diameter of 15.7 nm and an inner channel 4 nm in diameter. Shadow-casting experiments demonstrated that the rings were 8.5 nm deep, indicating a holoenzyme molecular weight of 8.0 X 10(5). Methyl reductase appeared to be an oligomeric complex of dimensions 8.5 by 9 by 11 nm, with a central stain-penetrating region. The morphology and known subunit composition suggest a model in which the subunits are arranged as an eclipsed pair of open trimers. Methyl reductase was also found in the form of larger aggregates and in paracrystalline arrays derived from highly concentrated solutions. The extremely large size of F420 hydrogenase and the methyl reductase supramolecular assemblies may have relevance in vivo in the construction of multiprotein arrays that function in methane biogenesis.

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.

Dose dependence of breath hydrogen and methane in healthy volunteers after ingestion of a commercial disaccharide mixture, Palatinit

Breath hydrogen and methane were determined by gas chromatography in eleven normal individuals given a low-fibre, mixed diet (control) and after ingestion of 20-50 g Palatinit/d, an equimolar mixture of D-glucosyl-alpha(1----1)-D-mannitol and D-glucosyl-alpha(1----6)-D-glucitol (Isomalt). A linear relation was found (r 0.85; P less than 0.001) between the amount of Palatinit ingested and breath H2 per 10 h in subjects who did not exhale methane. If methane was formed in addition to H2, the sum of both gases followed a linear dose-effect relation. The mouth-to-caecum time, indicated by the first increase in breath H2 after ingestion, was shortened by about half, yet no sign of diarrhoea was observed. Stool weight and stool frequency did not change significantly. The linear relation between a dose of 20-50 g Palatinit and exhalation of H2 (eventually plus methane) indicated that a relatively constant fraction of the dose given underwent cleavage and absorption in the small intestine, the remainder being transported into the large bowel. Microbial gas formation in the colon as well as the fractional transfer of these gases into the expiratory air occurred at fixed proportions, thus allowing an insight into colonic microbial contributions to carbohydrate utilization in the human large bowel.

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.

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.

Methanogenic cleavage of acetate by lysates of Methanosarcina barkeri

Cell lysates of acetate-grown Methanosarcina barkeri 227 were found to cleave acetate to CH4 and CO2. The aceticlastic reaction was identified by using radioactive methyl-labeled acetate. Cell lysates decarboxylated acetate in a nitrogen atmosphere, conserving the methyl group in methane. The rate of methanogenesis from acetate in the cell lysates was comparable to that observed with whole cells. Aceticlastic activity was found in the particulate fraction seperate from methylcoenzyme M methylreductase activity, which occurs in the soluble fraction. Pronase treatment eliminated methylcoenzyme M methylreductase activity in lysates and stimulated aceticlastic activity, indicating the aceticlastic activity was not derived from unbroken cells, which are unaffected by proteolytic treatment.

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.

Genome complexity of methanogenic bacteria

The genome complexities of different methanogenic bacteria were investigated by using an optical method to study renaturation kinetics of single-stranded DNA. The observed genome sizes ranged from 1.0 X 10(9) to 1.8 X 10(9) daltons, which is a typical range for procaryotic cells. Melting profiles of the DNA of three methanogenic species from different families show fractions which have a higher A . T content than the average DNA of that species.

The effects of ionophores and metabolic inhibitors on methanogenesis and energy-related properties of Methanobacterium bryantii

The effects of numerous ionophores and inhibitors were tested on methane synthesis, intracellular ATP and potassium concentrations, and the proton motive force of the methanogenic archaebacterium Methanobacterium bryantii. M. bryantii had an internal pH near 6.8 (and hence little delta pH during growth) with an electrical potential of --127 mV in growth medium and --105 mV in a pH 6.5 buffer. The study has identified agents which, in M. bryantii, can effectively cause a decline of intracellular ATP (gramicidin, acetylene) and potassium concentrations (gramicidin, nigericin), inhibit methane synthesis (acetylene, gramicidin, nigericin, triphenylmethylphosphonium bromide), eliminate the electrical potential (high extracellular potassium ion concentrations), and dissipate artificially imposed, inside alkaline, pH gradients (monensin, nigericin, carbonyl cyanide m-chlorophenylhydrazone). Carbonyl cyanide m-chlorophenylhydrazone was generally ineffective in media or buffers reduced with cysteine-sulfide but could be effective in cysteine-free solutions reduced with hydrogen sulfide.

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.

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.