Methanofuran (carbon dioxide reduction factor), a formyl carrier in methane production from carbon dioxide in Methanobacterium

Molecular basis of hyper-thermostability in the thermophilic archaeal aldolase MfnB

Methanogenic archaea are chemolithotrophic prokaryotes that can reduce carbon dioxide with hydrogen gas to form methane. These microorganisms make a significant contribution to the global carbon cycle, with methanogenic archaea from anoxic environments estimated to contribute > 500 million tons of global methane annually. Archaeal methanogenesis is dependent on the methanofurans; aminomethylfuran containing coenzymes that act as the primary C1 acceptor molecule during carbon dioxide fixation. Although the biosynthetic pathway to the methanofurans has been elucidated, structural adaptations which confer thermotolerance to Mfn enzymes from extremophilic archaea are yet to be investigated. Here we focus on the methanofuran biosynthetic enzyme MfnB, which catalyses the condensation of two molecules of glyceralde-3-phosphate to form 4‑(hydroxymethyl)-2-furancarboxaldehyde-phosphate. In this study, MfnB enzymes from the hyperthermophile Methanocaldococcus jannaschii and the mesophile Methanococcus maripaludis have been recombinantly overexpressed and purified to homogeneity. Thermal unfolding studies, together with steady-state kinetic assays, demonstrate thermoadaptation in the M. jannaschii enzyme. Molecular dynamics simulations have been used to provide a structural explanation for the observed properties. These reveal a greater number of side chain interactions in the M. jannaschii enzyme, which may confer protection from heating effects by enforcing spatial residue constraints.

Model Organisms To Study Methanogenesis, a Uniquely Archaeal Metabolism

Methanogenic archaea are the only organisms that produce CH4 as part of their energy-generating metabolism. They are ubiquitous in oxidant-depleted, anoxic environments such as aquatic sediments, anaerobic digesters, inundated agricultural fields, the rumen of cattle, and the hindgut of termites, where they catalyze the terminal reactions in the degradation of organic matter. Methanogenesis is the only metabolism that is restricted to members of the domain Archaea. Here, we discuss the importance of model organisms in the history of methanogen research, including their role in the discovery of the archaea and in the biochemical and genetic characterization of methanogenesis. We also discuss outstanding questions in the field and newly emerging model systems that will expand our understanding of this uniquely archaeal metabolism.

Glutamate: A multifunctional amino acid in plants

Glutamate (Glu) is a versatile metabolite and a signaling molecule in plants. Glu biosynthesis is associated with the primary nitrogen assimilation pathway. The conversion between Glu and 2-oxoglutarate connects Glu metabolism to the tricarboxylic acid cycle, carbon metabolism, and energy production. Glu is the predominant amino donor for transamination reactions in the cell. In addition to protein synthesis, Glu is a building block for tetrapyrroles, glutathione, and folate. Glu is the precursor of γ-aminobutyric acid that plays an important role in balancing carbon/nitrogen metabolism and various cellular processes. Glu can conjugate to the major auxin indole 3-acetic acid (IAA), and IAA-Glu is destined for oxidative degradation. Glu also conjugates with isochorismate for the production of salicylic acid. Accumulating evidence indicates that Glu functions as a signaling molecule to regulate plant growth, development, and defense responses. The ligand-gated Glu receptor-like proteins (GLRs) mediate some of these responses. However, many of the Glu signaling events are GLR-independent. The receptor perceiving extracellular Glu as a danger signal is still unknown. In addition to GLRs, Glu may act on receptor-like kinases or receptor-like proteins to trigger immune responses. Glu metabolism and Glu signaling may entwine to regulate growth, development, and defense responses in plants.

The Wolfe cycle of carbon dioxide reduction to methane revisited and the Ralph Stoner Wolfe legacy at 100 years

Methanogens are a component of anaerobic microbial consortia decomposing biomass to CO₂ and CH₄ that is an essential link in the global carbon cycle. One of two major pathways of methanogenesis involves reduction of the methyl group of acetate to CH₄ with electrons from oxidation of the carbonyl group while the other involves reduction of CO₂ to CH₄ with electrons from H₂ or formate. Pioneering investigations of the CO₂ reduction pathway by Ralph S. Wolfe in the 70s and 80s contributed findings impacting the broader fields of biochemistry and microbiology that directed discovery of the domain Archaea and expanded research on anaerobic microbes for decades that continues to the present. This review presents an historical overview of the CO₂ reduction pathway (Wolfe cycle) with recent developments, and an account of Wolfe's larger and enduring impact on the broad field of biology 100 years after his birth.

Energy conservation under extreme energy limitation: the role of cytochromes and quinones in acetogenic bacteria

Acetogenic bacteria are a polyphyletic group of organisms that fix carbon dioxide under anaerobic, non-phototrophic conditions by reduction of two mol of CO₂ to acetyl-CoA via the Wood–Ljungdahl pathway. This pathway also allows for lithotrophic growth with H₂ as electron donor and this pathway is considered to be one of the oldest, if not the oldest metabolic pathway on Earth for CO₂ reduction, since it is coupled to the synthesis of ATP. How ATP is synthesized has been an enigma for decades, but in the last decade two ferredoxin-dependent respiratory chains were discovered. Those respiratory chains comprise of a cytochrome-free, ferredoxin-dependent respiratory enzyme complex, which is either the Rnf or Ech complex. However, it was discovered already 50 years ago that some acetogens contain cytochromes and quinones, but their role had only a shadowy existence. Here, we review the literature on the characterization of cytochromes and quinones in acetogens and present a hypothesis that they may function in electron transport chains in addition to Rnf and Ech.

Comparison of the radiofrequency ablation versus laparoscopic adrenalectomy for aldosterone-producing adenoma: a meta-analysis of perioperative outcomes and safety

Radiofrequency ablation (RFA) has emerged as a new treatment for primary aldosteronism owing to aldosterone-producing adenoma (APA). We aimed to compare the perioperative outcomes and safety of RFA and laparoscopic adrenalectomy (LA) for patients with APA. We searched PubMed, EMBASE, and the Cochrane Library for all literatures published from January 2001 to September 2020 to compare RFA with LA for APA. After data extraction and quality assessments, we used Review Manager 5.4.1 and STATA 14.0 to pool the data. Four retrospective studies consisting of 170 patients were obtained. Patients who underwent RFA were associated with shorter operative time (standard mean difference (SMD): -1.98, 95% confidence interval (CI): -3.86 to 0.11, P = 0.04), less intraoperative blood loss (SMD: -0.61, 95% CI: -0.96 to -0.26, P = 0.0007), and shorter hospital stay (weight mean difference (WMD): -1.40, 95% CI: -1.71 to -1.10, P < 0.00001) than those who underwent LA. No significant differences were found in the complication rate (odds ratio (OR): 0.67, 95% CI: 0.27-1.68, P = 0.39), the incidence of hypertensive crisis (OR: 3.16, 95% CI: 0.36-27.94, P = 0.30), the conversion rate (OR: 0.44, 95% CI: 0.04-4.32, P = 0.48) or the treatment success rate (OR: 0.72, 95% CI: 0.22-2.39, P = 0.59) between the two groups. RFA could achieve clinical outcomes that approach LA for patients with APA but result in shorter operative time, less intraoperative blood loss, and shorter hospital stay. However, RFA does not appear to be able to replace the LA. Future prospective randomized trials are needed to validate these results.

Electron Bifurcation: A Long-Hidden Energy-Coupling Mechanism

A decade ago, a novel mechanism to drive thermodynamically unfavorable redox reactions was discovered that is used in prokaryotes to drive endergonic electron transfer reactions by a direct coupling to an exergonic redox reaction in one soluble enzyme complex. This process is referred to as flavin-based electron bifurcation, or FBEB. An important function of FBEB is that it allows the generation of reduced low-potential ferredoxin (Fd_red) from comparably high-potential electron donors such as NADH or molecular hydrogen (H₂). Fd_red is then the electron donor for anaerobic respiratory chains leading to the synthesis of ATP. In many metabolic scenarios, Fd is reduced by metabolic oxidoreductases and Fd_red then drives endergonic metabolic reactions such as H₂ production by the reverse, electron confurcation. FBEB is energetically more economical than ATP hydrolysis or reverse electron transport as a driving force for endergonic redox reactions; thus, it does "save" cellular ATP. It is essential for autotrophic growth at the origin of life and also allows for heterotrophic growth on certain low-energy substrates.

pNEB193-derived suicide plasmids for gene deletion and protein expression in the methane-producing archaeon, Methanosarcina acetivorans

Gene deletion and protein expression are cornerstone procedures for studying metabolism in any organism, including methane-producing archaea (methanogens). Methanogens produce coenzymes and cofactors not found in most bacteria, therefore it is sometimes necessary to express and purify methanogen proteins from the natural host. Protein expression in the native organism is also useful when studying post-translational modifications and their effect on gene expression or enzyme activity. We have created several new suicide plasmids to complement existing genetic tools for use in the methanogen, Methanosarcina acetivorans. The new plasmids are derived from the commercially available Escherichia coli plasmid, pNEB193, and cannot replicate autonomously in methanogens. The designed plasmids facilitate markerless gene deletion, gene transcription, protein expression, and purification of proteins with cleavable affinity tags from the methanogen, M. acetivorans.

The many roles of glutamate in metabolism

The amino acid glutamate is a major metabolic hub in many organisms and as such is involved in diverse processes in addition to its role in protein synthesis. Nitrogen assimilation, nucleotide, amino acid, and cofactor biosynthesis, as well as secondary natural product formation all utilize glutamate in some manner. Glutamate also plays a role in the catabolism of certain amines. Understanding glutamate's role in these various processes can aid in genome mining for novel metabolic pathways or the engineering of pathways for bioremediation or chemical production of valuable compounds.

Identification of the Final Two Genes Functioning in Methanofuran Biosynthesis in Methanocaldococcus jannaschii

All methanofuran structural variants contain a basic core structure of 4-[N-(γ-l-glutamyl)-p-(β-aminoethyl)phenoxymethyl]-(aminomethyl)furan (APMF-Glu) but have different side chains depending on the source organism. Recently, we identified four genes (MfnA, MfnB, MfnC, and MfnD) that are responsible for the biosynthesis of the methanofuran precursor γ-glutamyltyramine and 5-(aminomethyl)-3-furanmethanol-phosphate (F1-P) from tyrosine, glutamate, glyceraldehyde-3-P, and alanine in Methanocaldococcus jannaschii. How γ-glutamyltyramine and F1-P couple together to form the core structure of methanofuran was previously unknown. Here, we report the identification of two enzymes encoded by the genes mj0458 and mj0840 that catalyze the formation of F1-PP from ATP and F1-P and the condensation of F1-PP with γ-glutamyltyramine, respectively, to form APMF-Glu. We have annotated these enzymes as MfnE and MfnF, respectively, representing the fifth and sixth enzymes in the methanofuran biosynthetic pathway to be identified. Although MfnE was previously reported as an archaeal adenylate kinase, our present results show that MfnE is a promiscuous enzyme and that its possible physiological role is to produce F1-PP. Unlike other enzymes catalyzing coupling reactions involving pyrophosphate as the leaving group, MfnF exhibits a distinctive α/β two-layer sandwich structure. By comparing MfnF with thiamine synthase and dihydropteroate synthase, a substitution nucleophilic unimolecular (SN-1) reaction mechanism is proposed for MfnF. With the identification of MfnE and MfnF, the biosynthetic pathway for the methanofuran core structure APMF-Glu is complete.

IMPORTANCE

This work describes the identification of the final two enzymes responsible for catalyzing the biosynthesis of the core structure of methanofuran. The gene products of mj0458 and mj0840 catalyze the formation of F1-PP and the coupling of F1-PP with γ-glutamyltyramine, respectively, to form APMF-Glu. Although the chemistry of such a coupling reaction is widespread in biochemistry, we provide here the first evidence that such a mechanism is used in methanofuran biosynthesis. MfnF belongs to the hydantoinase A family (PF01968) and exhibits a unique α/β two-layer sandwich structure that is different from the enzymes catalyzing similar reactions. Our results show that MfnF catalyzes the formation of an ether bond during methanofuran biosynthesis. Therefore, this work further expands the functionality of this enzyme family.

Identification of structurally diverse methanofuran coenzymes in methanococcales that are both N-formylated and N-acetylated

Methanofuran (MF) is a coenzyme necessary for the first step of methanogenesis from CO2. The well-characterized MF core structure is 4-[N-(γ-l-glutamyl-γ-l-glutamyl)-p-(β-aminoethyl)phenoxymethyl]-2-(aminomethyl)furan (APMF-γ-Glu2). Three different MF structures that differ on the basis of the composition of their side chains have been determined previously. Here, we use liquid chromatography coupled with high-resolution mass spectrometry and a variety of biochemical methods to deduce the unique structures of MFs present in four different methanogens in the order Methanococcales. This is the first detailed characterization of the MF occurring in methanogens of this order. MF in each of these organisms contains the expected APMF-γ-Glu2; however, the composition of the side chain is different from that of the previously described MF structures. In Methanocaldococcus jannaschii, additional γ-linked glutamates that range from 7 to 12 residues are present. The MF coenzymes in Methanococcus maripaludis, Methanococcus vannielii, and Methanothermococcus okinawensis also have additional glutamate residues but interestingly also contain a completely different chemical moiety in the middle of the side chain that we have identified as N-(3-carboxy-2- or 3-hydroxy-1-oxopropyl)-l-aspartic acid. This addition results in the terminal γ-linked glutamates being incorporated in the opposite orientation. In addition to these nonacylated MF coenzymes, we also identified the corresponding N-formyl-MF and, surprisingly, N-acetyl-MF derivatives. N-Acetyl-MF has never been observed or implied to be functioning in nature and may represent a new route for acetate formation in methanogens.

Promiscuous anaerobes: new and unconventional metabolism in methanogenic archaea

The development of an oxygenated atmosphere on earth resulted in the polarization of life into two major groups, those that could live in the presence of oxygen and those that could not-the aerobes and the anaerobes. The evolution of aerobes from the earliest anaerobic prokaryotes resulted in a variety of metabolic adaptations. Many of these adaptations center on the need to sustain oxygen-sensitive reactions and cofactors to function in the new oxygen-containing atmosphere. Still other metabolic pathways that were not sensitive to oxygen also diverged. This is likely due to the physical separation of the organisms, based on their ability to live in the presence of oxygen, which allowed for the independent evolution of the pathways. Through the study of metabolic pathways in anaerobes and comparison to the more established pathways from aerobes, insight into metabolic evolution can be gained. This, in turn, can allow for extra- polation to those metabolic pathways occurring in the Last Universal Common Ancestor (LUCA). Some of the unique and uncanonical metabolic pathways that have been identified in the archaea with emphasis on the biochemistry of an obligate anaerobic methanogen, Methanocaldococcus jannaschii are reviewed.

The unique biochemistry of methanogenesis

Methanogenic archaea have an unusual type of metabolism because they use H2 + CO2, formate, methylated C1 compounds, or acetate as energy and carbon sources for growth. The methanogens produce methane as the major end product of their metabolism in a unique energy-generating process. The organisms received much attention because they catalyze the terminal step in the anaerobic breakdown of organic matter under sulfate-limiting conditions and are essential for both the recycling of carbon compounds and the maintenance of the global carbon flux on Earth. Furthermore, methane is an important greenhouse gas that directly contributes to climate changes and global warming. Hence, the understanding of the biochemical processes leading to methane formation are of major interest. This review focuses on the metabolic pathways of methanogenesis that are rather unique and involve a number of unusual enzymes and coenzymes. It will be shown how the previously mentioned substrates are converted to CH4 via the CO2-reducing, methylotrophic, or aceticlastic pathway. All catabolic processes finally lead to the formation of a mixed disulfide from coenzyme M and coenzyme B that functions as an electron acceptor of certain anaerobic respiratory chains. Molecular hydrogen, reduced coenzyme F420, or reduced ferredoxin are used as electron donors. The redox reactions as catalyzed by the membrane-bound electron transport chains are coupled to proton translocation across the cytoplasmic membrane. The resulting electrochemical proton gradient is the driving force for ATP synthesis as catalyzed by an A1A0-type ATP synthase. Other energy-transducing enzymes involved in methanogenesis are the membrane-integral methyltransferase and the formylmethanofuran dehydrogenase complex. The former enzyme is a unique, reversible sodium ion pump that couples methyl-group transfer with the transport of Na+ across the membrane. The formylmethanofuran dehydrogenase is a reversible ion pump that catalyzes formylation and deformylation of methanofuran. Furthermore, the review addresses questions related to the biochemical and genetic characteristics of the energy-transducing enzymes and to the mechanisms of ion translocation.

Generation of formate by the formyltransferase/hydrolase complex (Fhc) from Methylobacterium extorquens AM1

Methylobacterium extorquens AM1 possesses a formyltransferase (Ftr) complex that is essential for growth in the presence of methanol and involved in formaldehyde oxidation to CO(2). One of the subunits of the complex carries the catalytic site for transfer of the formyl group from tetrahydromethanopterin to methanofuran (MFR). We now found via nuclear magnetic resonance-based studies that the Ftr complex also catalyzes the hydrolysis of formyl-MFR and generates formate. The enzyme was therefore renamed Ftr/hydrolase complex (Fhc). FhcA shares a sequence pattern with amidohydrolases and is assumed to be the catalytic site where the hydrolysis takes place.

The active species of 'CO2' utilized by formylmethanofuran dehydrogenase from methanogenic Archaea

Formylmethanofuran dehydrogenase from methanogenic Archaea catalyzes the reversible conversion of CO2 and methanofuran to formylmethanofuran, which is an intermediate in methanogenesis from CO2, a biological process yielding approximately 0.3 billion tons of CH4 per year. With the enzyme from Methanosarcina barkeri, it is shown that CO2 rather than HCO3- is the active species of 'CO2' utilized by the dehydrogenase. Evidence is also presented that the enzyme catalyzes a methanofuran-dependent exchange between CO2 and the formyl group of formylmethanofuran. The results are consistent with N-carboxymethanofuran being an intermediate in CO2 reduction to formylmethanofuran.

Thermodynamics of the formylmethanofuran dehydrogenase reaction in Methanobacterium thermoautotrophicum

Purified formylmethanofuran dehydrogenase from Methanobacterium thermoautotrophicum, which is a thermophilic methanogenic Archaeon growing on H2 and CO2, was shown to catalyze the reversible reduction of CO2 to N-formylmethanofuran with 1,1',2,2'-tetramethylviologen (E'0 = -550 mV) as electron donor. The rate of CO2 reduction was approximately 25 times higher than the rate of N-formylmethanofuran dehydrogenation. From determinations of equilibrium concentrations at 60 degrees C and pH 7.0 a midpoint potential (E'0) for the CO2 + methanofuran/formylmethanofuran couple of approximately -530 mV was estimated. The initial step of methanogenesis from CO2 thus has a midpoint potential considerably more negative than that of the H+/H2 couple (E'0 = -460 mV at 60 degrees C). Evidence is described indicating that the as-yet unidentified physiological electron donor of the formylmethanofuran dehydrogenase is present in the soluble cell fraction.

The molybdoenzyme formylmethanofuran dehydrogenase from Methanosarcina barkeri contains a pterin cofactor

Recently formylmethanofuran dehydrogenase from the archaebacterium Methanosarcina barkeri has been shown to be a novel molybdo-iron-sulfur protein. We report here that the enzyme contains one mol of a bound pterin cofactor/mol molybdenum, similar but not identical to the molybdopterin of milk xanthine oxidase. The two pterins, after oxidation with I2 at pH 2.5, showed identical fluorescence spectra and, after oxidation with permanganate at pH 13, yielded pterin 6-carboxylic acid. They differed, however, in their apparent molecular mass: the pterin of formylmethanofuran dehydrogenase was 400 Da larger than that of milk xanthine oxidase, a property also exhibited by the pterin cofactor of eubacterial molybdoenzymes. A homogeneous formylmethanofuran dehydrogenase preparation was used for these investigations. The enzyme, with a molecular mass of 220 kDa, contained 0.5-0.8 mol molybdenum, 0.6-0.9 mol pterin, 28 +/- 2 mol non-heme iron and 28 +/- 2 mol acid-labile sulfur/mol based on a protein determination with bicinchoninic acid. The specific activity was 175 mumol.min-1.mg-1 (kcat = 640 s-1) assayed with methylviologen (app. Km = 0.02 mM) as artificial electron acceptor. The apparent Km for formylmethanofuran was 0.02 mM.

N-furfurylformamide as a pseudo-substrate for formylmethanofuran converting enzymes from methanogenic bacteria

Methanofuran (4-[N-(4,5,7-tricarboxyheptanoyl-gamma-L-glutamyl)-gamma-L- glutamyl)-p-(beta-aminoethyl)phenoxymethyl]-2-(aminomethyl)furan is a coenzyme involved in methanogenesis. The N-formyl derivative is an intermediate in the reduction of CO2 to CH4 and the disproportionation of methanol to CO2 and CH4. Formylmethanofuran dehydrogenase and formylmethanofuran:tetrahydromethanopterin formyltransferase are the enzymes catalyzing its conversions. We report here that the two enzymes from Methanosarcina barkeri and the formyltransferase from Methanobacterium thermoautotrophicum can also use N-furfurylformamide as a pseudo-substrate albeit with higher apparent Km and lower apparent Vmax values. N-Methylformamide, formamide, and formate were not converted indicating that the furfurylamine moiety of methanofuran is the minimum structure required for the correct binding of the coenzyme.

Formate auxotroph of Methanobacterium thermoautotrophicum Marburg

A formate-requiring auxotroph of Methanobacterium thermoautotrophicum Marburg was isolated after hydroxylamine mutagenesis and bacitracin selection. The requirement for formate is unique and specific; combined pools of other volatile fatty acids, amino acids, vitamins, and nitrogen bases did not substitute for formate. Compared with those of the wild type, cell extracts of the formate auxotroph were deficient in formate dehydrogenase activity, but cells of all of the strains examined catalyzed a formate-carbon dioxide exchange activity. All of the strains examined took up a small amount (200 to 260 mumol/liter) of formate (3 mM) added to medium. The results of the study of this novel auxotroph indicate a role for formate in biosynthetic reactions in this methanogen. Moreover, because methanogenesis from H2-CO2 is not impaired in the mutant, free formate is not an intermediate in the reduction of CO2 to CH4.

Activation of formylmethanofuran synthesis in cell extracts of Methanobacterium thermoautotrophicum

In cell extracts of Methanobacterium thermoautotrophicum, formylmethanofuran (formyl-MFR) synthesis (an essential CO2 fixation reaction that is an early step in CO2 reduction to methane) is subject to a complex activation that involves a heterodisulfide of coenzyme M and N-(7-mercaptoheptanoyl)threonine O3-phosphate (CoM-S-S-HTP). In this paper we report that titanium(III) citrate, a low-potential reducing agent, stimulated CO2 reduction to methane and activated formyl-MFR synthesis in cell extracts. Titanium(III) citrate functioned as the sole source of electrons for formyl-MFR synthesis and enabled this reaction to occur independently of CoM-S-S-HTP. In addition, CoM-S-S-HTP was found to activate an unknown electron carrier that reduced metronidazole. The activation of formyl-MFR synthesis by CoM-S-S-HTP may involve the activation of a low-potential electron carrier.

Structural diversity among methanofurans from different methanogenic bacteria

An examination of the methanofurans isolated from a wide range of methanogenic bacteria and from Archaeoglobus fulgidus has revealed at least five chromatographically distinct methanofurans. Bacteria from each major genus of methanogenic bacteria have been found to contain a chemically different methanofuran. The nature of the differences in the methanofurans appears to lie in the modification of the side chain attached to the basic core structure of 4-[N-(gamma-L-glutamyl-gamma-L-glutamyl)-p-(beta-aminoethyl)phenoxyme thy l]-2-(amino-methyl)furan. This was supported by the structural elucidation of the methanofuran isolated from Methanobrevibacter smithii, designated methanofuran-c, which was the same as the originally characterized methanofuran except for a hydroxy group at the 2 position of the 4,5-dicarboxyoctanedioic acid moiety of the molecule.

Physiological importance of the heterodisulfide of coenzyme M and 7-mercaptoheptanoylthreonine phosphate in the reduction of carbon dioxide to methane in Methanobacterium

The heterodisulfide of the two coenzymes 2-mercaptoethanesulfonic acid (coenzyme M, HS-CoM) and N-(7-mercaptoheptanoyl)threonine O3-phosphate (HS-HTP) increased the rate of CO2 reduction to methane by cell extracts 42-fold. The stimulation resulted from activation of the initial step of methanogenesis, the production of formylmethanofuran from methanofuran and CO2. These results establish a role for this heterodisulfide (CoM-S-S-HTP) in the reduction of CO2 to formylmethanofuran. Evidence indicates that CoM-S-S-HTP is the labile intermediate that accounts for the coupling of the reduction of 2-(methylthio)ethanesulfonic acid by the methylreductase to formylmethanofuran biosynthesis, the "RPG effect." The heterodisulfide was found to be labile in cell extracts due to enzyme-catalyzed reduction and possibly thioldisulfide exchange.

Structure of a methanofuran derivative found in cell extracts of Methanosarcina barkeri

Cell extracts prepared from cells of Methanosarcina barkeri grown on hydrogen and carbon dioxide, acetate, or methanol contain a coenzyme structurally related to methanofuran. This modified coenzyme was highly purified and its structure assigned as 4-[N-(gamma-L-glutamyl-gamma-L-glutamyl-gamma-L-glutamyl-gamma-L-glutamy l)-p- (beta-amino-ethyl)phenoxymethyl]-2-(aminomethyl)furan. The key structural evidence was obtained by high-resolution fast atom bombardment-mass spectrometry and 1H NMR spectroscopy. Quantitative analysis of the hydrolytic fragments of the coenzyme supported the assigned structure. We propose that this coenzyme be called methanofuran-b.

Source of carbon and hydrogen in methane produced from formate by Methanococcus thermolithotrophicus

Methanococcus thermolithotrophicus is able to produce methane either from H2-CO2 or from formate. The route of formate entry into the methanogenic pathway was investigated by using 2H2O or [13C]formate and analysis by mass spectrometry. When cells (H2-CO2 or formate grown) were transferred to formate medium in 95% 2H water, the proportion of 2H in methane was 95%. When cells (H2-CO2 or formate grown) were transferred to media containing [13C]formate in the presence of H2-CO2 or He-CO2, the ratio of 13CH4 to 12CH4 increased over time parallel to the ratio of 13CO2 to 12CO2. The cells catalyzed a significant exchange of label between [13C]formate and 13CO2.

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.

Methenyl-tetrahydromethanopterin cyclohydrolase in cell extracts of Methanobacterium

Cell extracts of Methanobacterium thermoautotrophicum possess a methenyl-tetrahydromethanopterin (methenyl-H4MPT) cyclohydrolase. The enzyme catalyzes the hydrolysis of methenyl-H4MPT to formyltetrahydromethanopterin (formyl-H4MPT). The reaction is reversible and both the rate and extent of the reaction depend on the pH and the buffer used. Similarly, the nonenzymatic hydrolysis of methenyl-H4MPT is highly dependent on pH and buffer. An active derivative of methenyl-H4MPT was obtained in 94% yield by reacting H4MPT with formic acid in the presence of excess acetic acid under anoxic conditions at 80 degrees C for 3 h. H NMR spectroscopy and fast atom bombardment mass spectrometry revealed the product to be a derivative of methenyl-H4MPT which had lost the alpha-hydroxyglutarylphosphate unit. In spite of this loss, this derivative served both as a substrate for methanogenesis and for the cyclohydrolase. Comparison of the properties of the products of the enzymatic and nonenzymatic hydrolyses indicates that the enzymatic reaction yields N5-formyl-H4MPT whereas the nonenzymatic reaction yields N10-formyl-H4MPT.

Evidence of a common pathway of carbon dioxide reduction to methane in methanogens

The roles of methanofuran and tetrahydromethanopterin as carriers of C1 moieties in the reduction of carbon dioxide to methane were studied in representatives of diverse groups of methanogens, confirming that these roles, first reported for Methanobacterium thermoautotrophicum, are common for methanogenesis in general. Extracts of the methanogens tested converted formyl-methanofuran and methyl-tetrahydromethanopterin to methane; the extractable cofactors derived from the same methanogens, with one exception, complemented a methanofuran- and tetrahydromethanopterin-deficient enzyme system from M. thermoautotrophicum. The amounts of extractable methanofuran and tetrahydromethanopterin were determined for each representative methanogen.