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

Proton translocation in methanogens

Methanogenic archaea of the genus Methanosarcina possess a unique type of metabolism because they use H(2)+CO(2), methylated C(1)-compounds, or acetate as energy and carbon source for growth. The process of methanogenesis is fundamental for the global carbon cycle and represents the terminal step in the anaerobic breakdown of organic matter in freshwater sediments. Moreover, methane is an important greenhouse gas that directly contributes to climate change and global warming. Methanosarcina species convert the aforementioned substrates to CH(4) via the CO(2)-reducing, the methylotrophic, or the aceticlastic pathway. All methanogenic processes finally result in the oxidation of two thiol-containing cofactors (HS-CoM and HS-CoB), leading to the formation of the so-called heterodisulfide (CoM-S-S-CoB) that contains an intermolecular disulfide bridge. This molecule functions as the terminal electron acceptor of a branched respiratory chain. Molecular hydrogen, reduced coenzyme F(420), or reduced ferredoxin are used as electron donors. The key enzymes of the respiratory chain (Ech hydrogenase, F(420)-nonreducing hydrogenase, F(420)H(2) dehydrogenase, and heterodisulfide reductase) couple the redox reactions to proton translocation across the cytoplasmic membrane. The resulting electrochemical proton gradient is the driving force for ATP synthesis. Here, we describe the methods and techniques of how to analyze electron transfer reactions, the process of proton translocation, and the formation of ATP.

Involvement of Ech hydrogenase in energy conservation of Methanosarcina mazei

Methanosarcina mazei belongs to the group of aceticlastic methanogens and converts acetate into the potent greenhouse gases CO(2) and CH(4). The aceticlastic respiratory chain involved in methane formation comprises the three transmembrane proteins Ech hydrogenase, F(420) nonreducing hydrogenase and heterodisulfide reductase. It has been shown that the latter two contribute to the proton motive force. The data presented here clearly demonstrate that Ech hydrogenase is also involved in energy conservation. ATP synthesis was observed in a cytoplasm-free vesicular system of Ms. mazei that was dependent on the oxidation of reduced ferredoxin and the formation of molecular hydrogen (as catalysed by Ech hydrogenase). Such an ATP formation was not observed in a Deltaech mutant strain. The protonophore 3,5-di-tert-butyl-4-hydroxybenzylidene-malononitrile (SF6847) led to complete inhibition of ATP formation in the Ms. mazei wild-type without inhibiting hydrogen production by Ech hydrogenase, whereas the sodium ion ionophore ETH157 did not affect ATP formation in this system. Thus, we conclude that Ech hydrogenase acts as primary proton pump in a ferredoxin-dependent electron transport system.

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.

The Na(+)-translocating methyltransferase complex from methanogenic archaea

Methanogenic archaea are dependent on sodium ions for methane formation. A sodium ion-dependent step has been shown to be methyl transfer from N(5)-methyltetrahydromethanopterin to coenzyme M. This exergonic reaction (DeltaG degrees '=-30 kJ/mol) is catalyzed by a Na(+)-translocating membrane-associated multienzyme complex composed of eight different subunits, MtrA-H. Subunit MtrA harbors a cob(I)amide prosthetic group which is methylated and demethylated in the catalytic cycle, demethylation being sodium ion-dependent. Based on the finding that in the cob(II)amide oxidation state the corrinoid is bound in a base-off/His-on configuration it is proposed that methyl transfer from MtrA to coenzyme M is associated with a conformational change of the protein and that this change drives the electrogenic translocation of the sodium ions.

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

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

Electron transport phosphorylation driven by glyoxylate respiration with hydrogen as electron donor in membrane vesicles of a glyoxylate-fermenting bacterium

The syntrophically glycolate-fermenting bacterium in the methanogenic binary coculture FlGlyM was isolated in pure culture (strain FlGlyR) with glyoxylate as sole substrate. This strain disproportionated 12 glyoxylate to 7 glycolate, 10 CO2, and 3 hydrogen. Glyoxylate was oxidized via the malyl-CoA pathway. All enzymes of this pathway, i.e. malyl-CoA lyase/malate: CoA ligase, malic enzyme, and pyruvate synthase, were demonstrated in cell-free extracts. Glycolate dehydrogenase, hydrogenase, and ATPase, as well as menaquinones as potential electron carriers, were present in the membranes. Everted membrane vesicles catalyzed hydrogen-dependent glyoxylate reduction to glycolate [86-207 nmol min-1 (mg protein)-1] coupled to ATP synthesis from ADP and Pi [38-82 nmol min-1 (mg protein)-1)]. ATP synthesis was abolished entirely by protonophores or ATPase inhibitors (up to 98 and 94% inhibition, respectively) indicating the involvement of proton-motive force in an electron transport phosphorylation driven by a new glyoxylate respiration with hydrogen as electron donor. Measured reaction rates in vesicle preparations revealed a stoichiometry of ATP formation of 0.2-0.5 ATP per glyoxylate reduced.

Delta mu Na+ drives the synthesis of ATP via an delta mu Na(+)-translocating F1F0-ATP synthase in membrane vesicles of the archaeon Methanosarcina mazei Gö1

Methanosarcina mazei Gö1 couples the methyl transfer from methyl-tetrahydromethanopterin to 2-mercaptoethanesulfonate (coenzyme M) with the generation of an electrochemical sodium ion gradient (delta mu Na+) and the reduction of the heterodisulfide of coenzyme M and 7-mercaptoheptanoylthreoninephosphate with the generation of an electrochemical proton gradient (delta muH+). Experiments with washed inverted vesicles were performed to investigate whether both ion gradients are used directly for the synthesis of ATP. delta mu Na+ and delta mu H+ were both able to drive the synthesis of ATP in the vesicular system. ATP synthesis driven by heterodisulfide reduction (delta mu H+) or an artificial delta pH was inhibited by the protonophore SF6847 but not by the sodium ionophore ETH157, whereas ETH157 but not SF6847 inhibited ATP synthesis driven by a chemical sodium ion gradient (delta pNa) as well as the methyl transfer reaction (delta mu Na+). Inhibition of the Na+/H+ antiporter led to a stimulation of ATP synthesis driven by the methyl transfer reaction (delta mu Na+), as well as by delta pNa. These experiments indicate that delta mu Na+ and delta mu H+ drive the synthesis of ATP via an Na(+)- and an H(+)-translocating ATP synthase, respectively. Inhibitor studies were performed to elucidate the nature of the ATP synthase(s) involved. delta pH-driven ATP synthesis was specifically inhibited by bafilomycin A1, whereas delta pNa-driven ATP synthesis was exclusively inhibited by 7-chloro-4-nitro-2-oxa-1,3-diazole, azide, and venturicidin. These results are evidence for the presence of an F(1)F(0)-ATP synthase in addition to the A(1)A(0)-ATP synthase in membranes of M. Mazei Gö1 and suggest that the F(1)F(0)-type enzyme is an Na+-translocating ATP synthase, whereas the A(1)A(0)-ATP synthase uses H+ as the coupling ion.

Energetics of methanogenesis studied in vesicular systems

Methanogenesis is restricted to a group of prokaryotic microorganisms which thrive in strictly anaerobic habitats where they play an indispensable role in the anaerobic food chain. Methanogenic bacteria possess a number of unique cofactors and coenzymes that play an important role in their specialized metabolism. Methanogenesis from a number of simple substrates such as H2 + CO2, formate, methanol, methylamines, and acetate is associated with the generation of transmembrane electrochemical gradients of protons and sodium ions which serve as driving force for a number of processes such as the synthesis of ATP via an ATP synthase, reverse electron transfer, and solute uptake. Several unique reactions of the methanogenic pathways have been identified that are involved in energy transduction. Their role and importance for the methanogenic metabolism are described.

Biochemistry of methanogenesis

Methane is a product of the energy-yielding pathways of the largest and most phylogenetically diverse group in the Archaea. These organisms have evolved three pathways that entail a novel and remarkable biochemistry. All of the pathways have in common a reduction of the methyl group of methyl-coenzyme M (CH3-S-CoM) to CH4. Seminal studies on the CO2-reduction pathway have revealed new cofactors and enzymes that catalyze the reduction of CO2 to the methyl level (CH3-S-CoM) with electrons from H2 or formate. Most of the methane produced in nature originates from the methyl group of acetate. CO dehydrogenase is a key enzyme catalyzing the decarbonylation of acetyl-CoA; the resulting methyl group is transferred to CH3-S-CoM, followed by reduction to methane using electrons derived from oxidation of the carbonyl group to CO2 by the CO dehydrogenase. Some organisms transfer the methyl group of methanol and methylamines to CH3-S-CoM; electrons for reduction of CH3-S-CoM to CH4 are provided by the oxidation of methyl groups to CO2.

Reduced coenzyme F420: heterodisulfide oxidoreductase, a proton- translocating redox system in methanogenic bacteria

Washed everted vesicles of the methanogenic bacterium strain Go1 were found to couple the F420H2-dependent heterodisulfide reduction with the transfer of protons across the membrane into the lumen of the everted vesicles. The transmembrane electrochemical potential of protons thereby generated was shown to be competent in driving ATP synthesis from ADP + Pi, exhibiting a stoichiometry of 2 H+ translocated or 0.4 ATP synthesized per F420H2 oxidized. This enzyme system exhibits the phenomenon of coupling and uncoupling and represents a different kind of electron transport chain with the heterodisulfide of 2-mercaptoethanesulfonate and 7-mercaptoheptanoylthreonine phosphate as terminal electron acceptor. The heterodisulfide and methane are formed in the methyl coenzyme M reductase reaction. The reducing equivalents are derived from reduced coenzyme F420, which represents an analogue of NADH + H+ in other respiratory chains. It is assumed that the proton-translocating oxidoreductase discovered in strain Go1 is of principal importance to all methanogenic bacteria not utilizing H2.

Dependence on membrane components of methanogenesis from methyl-CoM with formaldehyde or molecular hydrogen as electron donors

Methane formation from 2-(methylthio)-ethanesulfonate (methyl-CoM) and H2 by the soluble fraction from the methanogenic bacterium strain Gö1 was stimulated up to tenfold by the addition of the membrane fraction. This stimulation was observed with membranes from various methanogenic species belonging to different phylogenetic families, but not with membranes from Escherichia coli or Acetobacterium woodii. Treatment of the membranes with strong oxidants, i.e. O2 and K3[Fe(CN)6], or with SH reagents, i.e. Ag+, p-chloromercuribenzoate or iodoacetamide, caused an irreversible decrease or loss in stimulatory activity, as did heat treatment at temperatures above 78 degrees C. Methanogenesis from methyl-CoM with formaldehyde instead of H2 as electron donor depended similarly on the membrane fraction. With membranes, 1 mol HCHO was oxidized to 1 mol CO2 and allowed the formation of 2 mol CH4 from 2 mol CH3-CoM. Without membranes, per mol of HCHO oxidized 1 mol H2 was formed and 1 mol CH4 was produced from CH3-CoM; the rate was 10-20% of that in the presence of membranes. When methyl-CoM was replaced by an artificial electron acceptor system consisting of methylviologen and metronidazole, the formaldehyde-oxidizing activity was no longer stimulated by the membrane fraction. These results demonstrate for the first time an essential function of membrane components in methanogenic electron transfer.