Methanol conversion in Eubacterium limosum

A new metabolic trait in an acetogen: Mixed acid fermentation of fructose in a methylene-tetrahydrofolate reductase mutant of Acetobacterium woodii

To inactivate the Wood-Ljungdahl pathway in the acetogenic model bacterium Acetobacterium woodii, the genes metVF encoding two of the subunits of the methylene-tetrahydrofolate reductase were deleted. As expected, the mutant did not grow on C1 compounds and also not on lactate, ethanol or butanediol. In contrast to a mutant in which the first enzyme of the pathway (hydrogen-dependent CO2 reductase) had been genetically deleted, cells were able to grow on fructose, albeit with lower rates and yields than the wild-type. Growth was restored by addition of an external electron sink, glycine betaine + CO2 or caffeate. Resting cells pre-grown on fructose plus an external electron acceptor fermented fructose to two acetate and four hydrogen, that is, performed hydrogenogenesis. Cells pre-grown under fermentative conditions on fructose alone redirected carbon and electrons to form lactate, formate, ethanol as well as hydrogen. Apparently, growth on fructose alone induced enzymes for mixed acid fermentation (MAF). Transcriptome analyses revealed enzymes potentially involved in MAF and a quantitative model for MAF from fructose in A. woodii is presented.

Lactate formation from fructose or C1 compounds in the acetogen Acetobacterium woodii by metabolic engineering

Anaerobic, acetogenic bacteria are promising biocatalysts for a sustainable bioeconomy since they capture and convert carbon dioxide to acetic acid. Hydrogen is an intermediate in acetate formation from organic as well as C1 substrates. Here, we analyzed mutants of the model acetogen Acetobacterium woodii in which either one of the two hydrogenases or both together were genetically deleted. In resting cells of the double mutant, hydrogen formation from fructose was completely abolished and carbon was redirected largely to lactate. The lactate/fructose and lactate/acetate ratios were 1.24 and 2.76, respectively. We then tested for lactate formation from methyl groups (derived from glycine betaine) and carbon monoxide. Indeed, also under these conditions lactate and acetate were formed in equimolar amounts with a lactate/acetate ratio of 1.13. When the electron-bifurcating lactate dehydrogenase/ETF complex was genetically deleted, lactate formation was completely abolished. These experiments demonstrate the capability of A. woodii to produce lactate from fructose but also from promising C1 substrates, methyl groups and carbon monoxide. This adds an important milestone towards generation of a value chain leading from CO₂ to value-added compounds. KEY POINTS: • Resting cells of the ΔhydBA/hdcr mutant of Acetobacterium woodii produced lactate from fructose or methyl groups + CO • Lactate formation from methyl groups + CO was completely abolished after deletion of lctBCD • Metabolic engineering of a homoacetogen to lactate formation gives a potential for industrial applications.

Leveraging substrate flexibility and product selectivity of acetogens in two-stage systems for chemical production

Carbon dioxide (CO2 ) stands out as sustainable feedstock for developing a circular carbon economy whose energy supply could be obtained by boosting the production of clean hydrogen from renewable electricity. H2 -dependent CO2 gas fermentation using acetogenic microorganisms offers a viable solution of increasingly demonstrated value. While gas fermentation advances to achieve commercial process scalability, which is currently limited to a few products such as acetate and ethanol, it is worth taking the best of the current state-of-the-art technology by its integration within innovative bioconversion schemes. This review presents multiple scenarios where gas fermentation by acetogens integrate into double-stage biotechnological production processes that use CO2 as sole carbon feedstock and H2 as energy carrier for products' synthesis. In the integration schemes here reviewed, the first stage can be biotic or abiotic while the second stage is biotic. When the first stage is biotic, acetogens act as a biological platform to generate chemical intermediates such as acetate, formate and ethanol that become substrates for a second fermentation stage. This approach holds the potential to enhance process titre/rate/yield metrics and products' spectrum. Alternatively, when the first stage is abiotic, the integrated two-stage scheme foresees, in the first stage, the catalytic transformation of CO2 into C1 products that, in the second stage, can be metabolized by acetogens. This latter scheme leverages the metabolic flexibility of acetogens in efficient utilization of the products of CO2 abiotic hydrogenation, namely formate and methanol, to synthesize multicarbon compounds but also to act as flexible catalysts for hydrogen storage or production.

Metaproteomics reveals methyltransferases implicated in dichloromethane and glycine betaine fermentation by 'Candidatus Formimonas warabiya' strain DCMF

Dichloromethane (DCM; CH2Cl2) is a widespread pollutant with anthropogenic and natural sources. Anaerobic DCM-dechlorinating bacteria use the Wood-Ljungdahl pathway, yet dechlorination reaction mechanisms remain unclear and the enzyme(s) responsible for carbon-chlorine bond cleavage have not been definitively identified. Of the three bacterial taxa known to carry out anaerobic dechlorination of DCM, 'Candidatus Formimonas warabiya' strain DCMF is the only organism that can also ferment non-chlorinated substrates, including quaternary amines (i.e., choline and glycine betaine) and methanol. Strain DCMF is present within enrichment culture DFE, which was derived from an organochlorine-contaminated aquifer. We utilized the metabolic versatility of strain DCMF to carry out comparative metaproteomics of cultures grown with DCM or glycine betaine. This revealed differential abundance of numerous proteins, including a methyltransferase gene cluster (the mec cassette) that was significantly more abundant during DCM degradation, as well as highly conserved amongst anaerobic DCM-degrading bacteria. This lends strong support to its involvement in DCM dechlorination. A putative glycine betaine methyltransferase was also discovered, adding to the limited knowledge about the fate of this widespread osmolyte in anoxic subsurface environments. Furthermore, the metagenome of enrichment culture DFE was assembled, resulting in five high quality and two low quality draft metagenome-assembled genomes. Metaproteogenomic analysis did not reveal any genes or proteins for utilization of DCM or glycine betaine in the cohabiting bacteria, supporting the previously held idea that they persist via necromass utilization.

Open microbiome dominated by Clostridium and Eubacterium converts methanol into i-butyrate and n-butyrate

Isobutyrate (i-butyrate) is a versatile platform chemical, whose acid form is used as a precursor of plastic and emulsifier. It can be produced microbially either using genetically engineered organisms or via microbiomes, in the latter case starting from methanol and short-chain carboxylates. This opens the opportunity to produce i-butyrate from non-sterile feedstocks. Little is known on the ecology and process conditions leading to i-butyrate production. In this study, we steered i-butyrate production in a bioreactor fed with methanol and acetate under various conditions, achieving maximum i-butyrate productivity of 5.0 mM day^-1, with a concurrent production of n-butyrate of 7.9 mM day^-1. The production of i-butyrate was reversibly inhibited by methanogenic inhibitor 2-bromoethanesulfonate. The microbial community data revealed the co-dominance of two major OTUs during co-production of i-butyrate and n-butyrate in two distinctive phases throughout a period of 54 days and 28 days, respectively. The cross-comparison of product profile with microbial community composition suggests that the relative abundance of Clostridium sp. over Eubacterium sp. is correlated with i-butyrate productivity over n-butyrate productivity.

Methanol metabolism in the acetogenic bacterium Acetobacterium woodii

Methanol derived from plant tissue is ubiquitous in anaerobic sediments and a good substrate for anaerobes growing on C₁ compounds such as methanogens and acetogens. In contrast to methanogens little is known about the physiology, biochemistry and bioenergetics of methanol utilization in acetogenic bacteria. To fill this gap, we have used the model acetogen Acetobacterium woodii to study methanol metabolism using physiological and biochemical experiments paired with molecular studies and transcriptome analysis. These studies identified the genes and enzymes involved in acetogenesis from methanol and the redox carriers involved. We will present the first comprehensive model for carbon and electron flow from methanol in an acetogen and the bioenergetics of acetogenesis from methanol.

Carbon Isotope Fractionation during Catabolism and Anabolism in Acetogenic Bacteria Growing on Different Substrates

Homoacetogenic bacteria are versatile microbes that use the acetyl coenzyme A (acetyl-CoA) pathway to synthesize acetate from CO2 and hydrogen. Likewise, the acetyl-CoA pathway may be used to incorporate other 1-carbon substrates (e.g., methanol or formate) into acetate or to homoferment monosaccharides completely to acetate. In this study, we analyzed the fractionation of pure acetogenic cultures grown on different carbon substrates. While the fractionation of Sporomusa sphaeroides grown on C1 compounds was strong (εC1, -49‰ to -64‰), the fractionation of Moorella thermoacetica and Thermoanaerobacter kivui using glucose (εGlu= -14.1‰) was roughly one-third as strong, suggesting a contribution of less-depleted acetate from fermentative processes. ForM. thermoacetica, this could indeed be validated by the addition of nitrate, which inhibited the acetyl-CoA pathway, resulting in fractionation during fermentation (εferm= -0.4‰). In addition, we determined the fractionation into microbial biomass of T. kivui grown on H2/CO2(εanabol.= -28.6‰) as well as on glucose (εanabol.= +2.9‰).

Growth inhibition of Sporomusa ovata by incorporation of benzimidazole bases into cobamides

Phenolyl cobamides are unique members of a class of cobalt-containing cofactors that includes vitamin B12 (cobalamin). Cobamide cofactors facilitate diverse reactions in prokaryotes and eukaryotes. Phenolyl cobamides are structurally and chemically distinct from the more commonly used benzimidazolyl cobamides such as cobalamin, as the lower axial ligand is a phenolic group rather than a benzimidazole. The functional significance of this difference is not well understood. Here we show that in the bacterium Sporomusa ovata, the only organism known to synthesize phenolyl cobamides, several cobamide-dependent acetogenic metabolisms have a requirement or preference for phenolyl cobamides. The addition of benzimidazoles to S. ovata cultures results in a decrease in growth rate when grown on methanol, 3,4-dimethoxybenzoate, H2 plus CO2, or betaine. Suppression of native p-cresolyl cobamide synthesis and production of benzimidazolyl cobamides occur upon the addition of benzimidazoles, indicating that benzimidazolyl cobamides are not functionally equivalent to the phenolyl cobamide cofactors produced by S. ovata. We further show that S. ovata is capable of incorporating other phenolic compounds into cobamides that function in methanol metabolism. These results demonstrate that S. ovata can incorporate a wide range of compounds as cobamide lower ligands, despite its preference for phenolyl cobamides in the metabolism of certain energy substrates. To our knowledge, S. ovata is unique among cobamide-dependent organisms in its preferential utilization of phenolyl cobamides.

Genome-guided analysis of physiological and morphological traits of the fermentative acetate oxidizer Thermacetogenium phaeum

BACKGROUND

Thermacetogenium phaeum is a thermophilic strictly anaerobic bacterium oxidizing acetate to CO(2) in syntrophic association with a methanogenic partner. It can also grow in pure culture, e.g., by fermentation of methanol to acetate. The key enzymes of homoacetate fermentation (Wood-Ljungdahl pathway) are used both in acetate oxidation and acetate formation. The obvious reversibility of this pathway in this organism is of specific interest since syntrophic acetate oxidation operates close to the energetic limitations of microbial life.

RESULTS

The genome of Th. phaeum is organized on a single circular chromosome and has a total size of 2,939,057 bp. It comprises 3.215 open reading frames of which 75% could be assigned to a gene function. The G+C content is 53.88 mol%. Many CRISPR sequences were found, indicating heavy phage attack in the past. A complete gene set for a phage was found in the genome, and indications of phage action could also be observed in culture. The genome contained all genes required for CO(2) reduction through the Wood-Ljungdahl pathway, including two formyl tetrahydrofolate ligases, three carbon monoxide dehydrogenases, one formate hydrogenlyase complex, three further formate dehydrogenases, and three further hydrogenases. The bacterium contains a menaquinone MQ-7. No indications of cytochromes or Rnf complexes could be found in the genome.

CONCLUSIONS

The information obtained from the genome sequence indicates that Th. phaeum differs basically from the three homoacetogenic bacteria sequenced so far, i.e., the sodium ion-dependent Acetobacterium woodii, the ethanol-producing Clostridium ljungdahlii, and the cytochrome-containing Moorella thermoacetica. The specific enzyme outfit of Th. phaeum obviously allows ATP formation both in acetate formation and acetate oxidation.

Acidification of methanol-fed anaerobic granular sludge bioreactors by cobalt deprivation: Induction and microbial community dynamics

The acidification of mesophilic (30 degrees C) methanol-fed upflow anaerobic sludge bed (UASB) reactors induced by cobalt deprivation from the influent was investigated by coupling the reactor performance (pH 7.0; organic loading rate 4.5 g COD . L(-1) . d(-1)) to the microbial ecology of the bioreactor sludge. The latter was investigated by specific methanogenic activity (SMA) measurements and fluorescence in situ hybridization (FISH) to quantify the abundance of key organisms over time. This study hypothesized that under cobalt limiting conditions, the SMA on methanol of the sludge gradually decreases, which ultimately results in methanol accumulation in the reactor effluent. Once the methanol accumulation surpasses a threshold value (about 8.5 mM for the sludge investigated), reactor acidification occurs because acetogens outcompete methylothrophic methanogens at these elevated methanol concentrations. Methanogens present in granular sludge at the time of the acidification do not use methanol as the direct substrate and are unable to degrade acetate. Methylotrophic/acetoclastic methanogenic activity was found to be lost within 10 days of reactor operation, coinciding with the disappearance of the Methanosarcina population. The loss of SMA on methanol can thus be used as an accurate parameter to predict reactor acidification of methanol-fed UASB reactors operating under cobalt limiting conditions.

Enhanced anaerobic biotransformation of carbon tetrachloride with precursors of vitamin B(12) biosynthesis

Relatively low concentrations of Vitamin B(12) are known to accelerate the anaerobic biotransformation of carbon tetrachloride (CT) and chloroform (CF). However, the addition of vitamin B(12) for field-scale bioremediation is expected to be costly. The present study considered a strategy to generate vitamin B(12) by addition of biosynthetic precursors. One of the precursors, porphobilinogen (PB) involved in the formation of the corrin ring, significantly increased the CT biotransformation rates by 2.7-, 8.8- and 10.9-fold when supplemented at 160, 500 and 900 microM, respectively. A positive control with 10 microM of vitamin B(12) resulted in a 5.9-fold increase in the CT-bioconversion rate. PB additions provided high molar yields of inorganic chloride (57% of CT organochlorine), comparable to that obtained with vitamin B(12) supplemented cultures. The primary substrate, methanol, known to induce vitamin B(12) production in methanogens and acetogens, was required for PB to have a significant impact on CT conversion. The observation suggests that PB's role was due to stimulating vitamin B(12) biosynthesis. The present study therefore provides insights on how to achieve vitamin B(12) enhanced rates of CT bioremediation through the use of less complex compounds that are precursors of vitamin B(12). Although PB is a costly chemical, its large impact points to corrin ring formation as the rate-limiting step.

Methanol utilization by a novel thermophilic homoacetogenic bacterium, Moorella mulderi sp. nov., isolated from a bioreactor

A thermophilic, anaerobic, spore-forming bacterium (strain TMS) was isolated from a thermophilic bioreactor operated at 65 degrees C with methanol as the energy source. Cells were gram-positive straight rods, 0.4-0.6 microm x 2-8 microm, growing as single cells or in pairs. The temperature range for growth was 40-70 degrees C with an optimum at 65 degrees C. Growth was observed from pH 5.5 to 8.5, and the optimum pH was around 7. The salinity range for growth was 0-45 g NaCl l(-1 )with an optimum at 10 g l(-1). The isolate was able to grow on methanol, H(2)-CO(2 )(80/20%, v/v), formate, lactate, pyruvate, glucose, fructose, cellobiose and pectin. The bacterium reduced thiosulfate to sulfide. The G+C content of the DNA was 53 mol%. Comparison of 16S rRNA genes revealed that strain TMS is related to Moorella glycerini (96%, sequence similarity), Moorella thermoacetica (92%) and Moorella thermoautotrophica (92%). On the basis of physiological and phylogenetic differences, strain TMS is proposed as a new species within the genus Moorella, Moorella mulderi sp. nov. (=DSM 14980, =ATCC BAA-608).

Growth kinetics of Acetobacterium sp. on methanol-formate in continuous culture

The fermentative metabolism of Acetobacterium sp. grown on methanol-formate in continuous culture is described. The reaction stoichiometry of methanol-formate, including cells, were as follows: CH3OH + 1.13HCOOH --> 0.87CH3COOH + 0.47 cell C. Formate enhanced growth yields by approximately 60% compared with methanol-CO2-grown cultures. Comparison of yields on methanol-formate allowed calculation of an energy yield of 1.3 mol ATP per mol acetate formed during homoacetate fermentation. The magnitudes of YEG,the theoretical maximum yield of YE, and m, the maintenance coefficient, were determined by growing the organism in methanol-formate and resulted in 16.5 g cell (mol methanol catabolized)-1 and 0.674 mmol methanol catabolized (g cell)-1 h-1, respectively. It is concluded that formate might replace CO2 as a source of carboxyl donor.

Metabolism of homocetogens

Homoacetogenic bacteria are strictly anaerobic microorganisms that catalyze the formation of acetate from C1 units in their energy metabolism. Most of these organisms are able to grow at the expense of hydrogen plus CO2 as the sole energy source. Hydrogen then serves as the electron donor for CO2 reduction to acetate. The methyl group of acetate is formed from CO2 via formate and reduced C1 intermediates bound to tetrahydrofolate. The carboxyl group is derived from carbon monoxide, which is synthesized from CO2 by carbon monoxide dehydrogenase. The latter enzyme also catalyzes the formation of acetyl-CoA from the methyl group plus CO. Acetyl-CoA is then converted either to acetate in the catabolism or to cell carbon in the anabolism of the bacteria. The homoacetogens are very versatile anaerobes, which convert a variety of different substrates to acetate as the major end product.

Purification and characterization of a methanol-induced cobamide-containing protein from Sporomusa ovata

The major cobamide-containing protein from methanol-utilizing Sporomusa ovata was 8-fold enriched to apparent homogeneity. The protein exhibited a molecular mass of 40 kDa and of 38 kDa determined by gel filtration and by SDS-polyacrylamide gel electrophoresis, respectively. This finding indicates a monomeric protein structure. Monospecific polyclonal antisera raised against the protein did not cross react with another cobamide-containing protein from Sporomusa cells. Only the 40 kDa cobamide-containing protein was induced by methanol, since proteins from cells grown on 3,4-dimethoxybenzoate, betaine H2/CO2, or fructose showed faint or no cross reaction. Hence, the 40 kDa cobamide-containing protein is presumably involved in the methyl-transfer reaction of the methanol metabolism. The purified enzyme revealed 1.1 mol of p-cresolyl cobamide per mol of protein, but it lacked of iron-sulfur centers. Remarkably, the cofactor was firmly bound to its protein.

N5-methyltetrahydromethanopterin: coenzyme M methyltransferase in methanogenic archaebacteria is a membrane protein

An assay is described that allows the direct measurement of the enzyme activity catalyzing the transfer of the methyl group from N5-methyltetrahydromethanopterin (CH3-H4MPT) to coenzyme M (H-S-CoM) in methanogenic archaebacteria. With this method the topology, the partial purification, and the catalytic properties of the methyltransferase in methanol- and acetate-grown Methanosarcina barkeri and in H2/CO(2)-grown Methanobacterium thermoautotrophicum were studied. The enzyme activity was found to be associated almost completely with the membrane fraction and to require detergents for solubilization. The transferase activity in methanol-grown M. barkeri was studied in detail. The membrane fraction exhibited a specific activity of CH3-S-CoM formation from CH3-H4MPT (apparent Km = 50 microM) and H-S-CoM (apparent Km = 250 microM) of approximately 0.6 mumol.min-1.mg protein-1. For activity the presence of Ti(III) citrate (apparent Km = 15 microM) and of ATP (apparent Km = 30 microM) were required in catalytic amounts. Ti(III) could be substituted by reduced ferredoxin. ATP could not be substituted by AMP, CTP, GTP, S-adenosylmethionine, or by ATP analogues. The membrane fraction was methylated by CH3-H4MPT in the absence of H-S-CoM. This methylation was dependent on Ti(III) and ATP. The methylated membrane fraction catalyzed the methyltransfer from CH3-H4MPT to H-S-CoM in the absence of ATP and Ti(III). Demethylation in the presence of H-S-CoM also did not require Ti(III) or ATP. Based on these findings a mechanism for the methyltransfer reaction and for the activation of the enzyme is proposed.

Isolation of a 5-hydroxybenzimidazolyl cobamide-containing enzyme involved in the methyltetrahydromethanopterin: coenzyme M methyltransferase reaction in Methanobacterium thermoautotrophicum

Formaldehyde conversion into methyl-coenzyme M involves (a) reaction of the substrate with 5,6,7,8-tetrahydromethanopterin (H4MPT) giving 5,10-methylene-H4MPT, followed by its reduction to 5-methyl-H4MPT and (b) transfer of the methyl group from the latter compound to coenzyme M. The reactions were studied in a resolved system from Methanobacterium thermoautotrophicum strain delta H. The first part (a) of the reactions was catalyzed by the 55% ammonium sulfate supernatant of cell-free extracts. The methyltransferase step (b) was dependent on an oxygen-sensitive enzyme, called methyltransferase a (MTa). Isolation of MTa was achieved by gel filtration on Sephacryl S-400. MTa was a high-molecular-weight complex of at least 2000 kDa and between 900 to 1500 kDa when purified in the absence and presence of the detergent CHAPS, respectively. The enzyme consisted of 100 kDa units composed of three subunits in an alpha beta gamma configuration with apparent molecular masses of 35, 33 and 31 kDa, respectively. The corrinoid, 5-hydroxybenzymidazolyl cobamide (B12HBI, Factor III) copurified with MTa and the latter contained 2 nmol B12HBI per mg protein. B12HBI present in MTa could be methylated under the appropriate conditions by 5-methyl-H4MPT. These findings suggest that the corrinoid is a prosthetic group of MTa. MTa may be homologous to the corrinoid membrane protein purified before from M. thermoautotrophicum strain Marburg (Schulz, H., Albracht, S.P.J., Coremans, J.M.C.C. and Fuchs, G. (1988) Eur. J. Biochem. 171, 589-597).

Sodium dependence of acetate formation by the acetogenic bacterium Acetobacterium woodii

Growth of Acetobacterium woodii on fructose was stimulated by Na+; this stimulation was paralleled by a shift of the acetate-fructose ratio from 2.1 to 2.7. Growth on H2-CO2 or on methanol plus CO2 was strictly dependent on the presence of sodium ions in the medium. Acetate formation from formaldehyde plus H2-CO by resting cells required Na+, but from methanol plus H2-CO did not. This is analogous to H2-CO2 reduction to methane by Methanosarcina barkeri, which involves a sodium pump (V. Müller, C. Winner, and G. Gottschalk, Eur. J. Biochem. 178:519-525, 1988). This suggests that the reduction of methylenetetrahydrofolate to methyltetrahydrofolate is the Na+-requiring reaction. A sodium gradient (Na+ out/Na+ in = 32, delta pNa = -91 mV) was built up when resting cells of A. woodii were incubated under H2-CO2. Acetogenesis was inhibited when the delta pNa was dissipated by monensin.

Metabolism of the 18O-methoxy substituent of 3-methoxybenzoic acid and other unlabeled methoxybenzoic acids by anaerobic bacteria

O-methyl substituents of aromatic compounds can provide C1 growth substrates for facultative and strict anaerobic bacteria isolated from diverse environments. The mechanism of the bioconversion of methoxylated benzoic acids to the hydroxylated derivatives was investigated with a model substrate and cultures of one anaerobic consortium, eight strict anaerobic bacteria, and one facultative anaerobic microorganism. Using high-pressure liquid chromatography and gas chromatography-mass spectral analysis, we found that a haloaromatic dehalogenating consortium, a dehalogenating isolate from that consortium, Eubacterium limosum, and a strain of Acetobacterium woodii metabolized 3-[methoxy-18O]methoxybenzoic acid (3-anisic acid) to 3-[hydroxy-18O]hydroxybenzoic acid stoichiometrically at rates of 1.5, 3.2, 52.4, and 36.7 nmol/min per mg of protein, respectively. A different strain of Acetobacterium and strains of Syntrophococcus, Clostridium, Desulfotomaculum, Enterobacter, and an anaerobic bacterium, strain TH-001, were unable to transform this compound. The O-demethylating ability of E. limosum was induced only with appropriate methoxylated benzoates but not with D-glucose, lactate, isoleucine, or methanol. Cross-acclimation and growth experiments with E. limosum showed a rate of metabolism that was an order of magnitude slower and showed no growth with either 4-methoxysalicylic acid (2-hydroxy-4-methoxybenzoic acid) or 4-anisic acid (4-methoxybenzoic acid) when adapted to 3-anisic acid. However, A. woodii NZva-16 showed slower rates and no growth with 3- or 4-methoxysalicylic acid when adapted to 3-anisic acid in similar experiments. The results clearly indicate a methyl rather than methoxy group removal mechanism for such reactions.

Interconversion of F430 derivatives of methanogenic bacteria

F430 is the prosthetic group of the methylcoenzyme M reductase of methanogenic bacteria. The compound isolated from Methanosarcina barkeri appears to be identical to the one obtained from the only distinctly related Methanobacterium thermoautotrophicum. F430 is thermolabile and in the presence of acetonitrile or C10-4 two epimerization products are obtained upon heating; in the absence of these compounds F430 is oxidized to 12,13-didehydro-F430. The latter is stereoselectively reduced under H2 atmosphere to F430 by cell-free extracts of M. barkeri or M. thermoautotrophicum. H2 may be replaced by the reduced methanogenic electron carrier coenzyme F420.

Purification and properties of methanol:5-hydroxybenzimidazolylcobamide methyltransferase from Methanosarcina barkeri

Methanol:5-hydroxybenzimidazolylcobamide methyltransferase from Methanosarcina barkeri has been purified to approximately 90% homogeneity by ion-exchange chromatography on DEAE-cellulose and QAE-A50 Sephadex columns. The molecular weight, estimated by gel electrophoresis, was found to be 122,000, and the enzyme contained two different subunits with molecular weights of 34,000 and 53,000, which indicates an alpha 2 beta structure. The enzyme contains three or four molecules of 5-hydroxybenzimidazolylcobamide, which could be removed by treatment of the enzyme with 2-mercaptoethanol or sodium dodecyl sulfate. In both cases the enzyme dissociated into its subunits. For stability, the enzyme required the presence of divalent cations such as Mg2+, Mn2+, Sr2+, Ca2+, or Ba2+. ATP, GTP, or CTP was needed in a reductive activation process of the enzyme. This activation was brought about by a mixture of H2, ferredoxin, and hydrogenase, but also by CO, which is thought to reduce the corrinoid chemically. The CO dehydrogenase-like activity of the methyltransferase is discussed.