Methanogenic cleavage of acetate by lysates of Methanosarcina barkeri

Identification of a cysteine involved in the interaction between carbon monoxide dehydrogenase and corrinoid/Fe-S protein from Clostridium thermoaceticum

In Clostridium thermoaceticum, the synthesis of acetyl-CoA from methyl tetrahydrofolate occurs via a series of enzymatic reactions involving methyl transferase, corrinoid/Fe-S protein (corrinoid), carbon monoxide dehydrogenase (CODH) and ferredoxin. We have investigated the possibility of one or more of these proteins existing as multi-enzyme complexes in vivo with higher catalytic activity. A protein complex consisting of CODH and corrinoid was isolated from the cell-free extracts of Clostridium thermoaceticum. The acetyl-CoA synthesis was found to be approximately 1.8-fold higher with the complex than that observed with the isolated protein components. HPLC gel filtration analyses of the native and DTE reduced complex suggested that the CODH:corrinoid complex is held together primarily by an inter disulfide bond. By differential labeling of thiols with [14C]N-ethylmaleimide it was found that Cys-506 of the alpha subunit of CODH was involved in the disulfide linkage with the corrinoid of the complex.

Bioenergetics of methanogenesis from acetate by Methanosarcina barkeri

Methane formation from acetate by resting cells of Methanosarcina barkeri was accompanied by an increase in the intracellular ATP content from 0.9 to 4.0 nmol/mg of protein. Correspondingly, the proton motive force increased to a steady-state level of -120 mV. The transmembrane pH gradient however, was reversed under these conditions and amounted to +20 mV. The addition of the protonophore 3,5,3',4'-tetrachlorosalicylanilide led to a drastic decrease in the proton motive force and in the intracellular ATP content and to an inhibition of methane formation. The ATPase inhibitor N,N'-dicyclohexylcarbodiimide stopped methanogenesis, and the intracellular ATP content decreased. The proton motive force decreased also under these conditions, indicating that the proton motive force could not be generated from acetate without ATP. The overall process of methane formation from acetate was dependent on the presence of sodium ions; upon addition of acetate to cell suspensions of M. barkeri, a transmembrane Na+ gradient in the range of 4:1 (Na+ out/Na+ in) was established. Possible sites of involvement of the Na+ gradient in the conversion of acetate to methane and carbon dioxide are discussed. Na+ is not involved in the CO dehydrogenase reaction.

Isolation of an enzyme complex with carbon monoxide dehydrogenase activity containing corrinoid and nickel from acetate-grown Methanosarcina thermophila

Fast protein liquid chromatography of cell extract from methanol- or acetate-grown Methanosarcina thermophila resolved two peaks of CO dehydrogenase activity. The activity of one of the CO dehydrogenases was sixfold greater in acetate-grown compared with methanol-grown cells. This CO dehydrogenase was purified to apparent homogeneity (70 mumol of methyl viologen reduced per min per mg of protein) and made up greater than 10% of the cellular protein of acetate-grown cells. The native enzyme (Mr 250,000) formed aggregates with an Mr of approximately 1,000,000. The enzyme contained five subunits (Mrs 89,000, 71,000, 60,000, 58,000, and 19,000), suggesting a multifunctional enzyme complex. Nickel, iron, cobalt, zinc, inorganic sulfide, and a corrinoid were present in the complex. The UV-visible spectrum suggested the presence of iron-sulfur centers. The electron paramagnetic resonance spectrum contained g values of 2.073, 2.049, and 2.028; these features were broadened in enzyme that was purified from cells grown in the presence of medium enriched with 61Ni, indicating the involvement of this metal in the spectrum. The pattern of potassium cyanide inhibition indicated that cyanide binds at or near the CO binding site. The properties of the enzyme imply an involvement in the dissimilation of acetate to methane, possibly by cleavage of acetate or activated acetate.

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.

Methanosarcina mutant unable to produce methane or assimilate carbon from acetate

Mutants of Methanosarcina barkeri 227 resistant to monofluoroacetate were isolated from monofluoroacetate-treated cultures. Mutant strain FAr9 was 100 times more resistant to monofluoroacetate than the wild-type strain and was deficient in carbon uptake and CH4 and CO2 production from methyl-labeled acetate. Methanol was assimilated at increased levels. Strain FAr9 was unable to shift from using methanol to using acetate for growth and exhibited increased sensitivity to growth inhibition by NaCN in methanol-containing complex medium. Unlike parent strain 227, acetate addition to methanol-containing media did not prevent NaCN inhibition. The specific activities of enzymes of exogenous acetate assimilation, CO dehydrogenase, and enzymes of the tricarboxylic acid cycle were similar for mutant and parent strain cell extracts. Mutation to monofluoroacetate resistance did not confer simultaneous resistance to 2-bromoethanesulfonate or pyruvate or alter propionate uptake. We conclude that strain FAr9 is either an acetate permeability mutant or is defective in an activation step required for the catabolism and anabolism of acetate.

Acetate catabolism by Methanosarcina barkeri: evidence for involvement of carbon monoxide dehydrogenase, methyl coenzyme M, and methylreductase

The pathway of acetate catabolism in Methanosarcina barkeri strain MS was studied by using a recently developed assay for methanogenesis from acetate by soluble enzymes in cell extracts. Extracts incubated with [2-14C]acetate, hydrogen, and ATP formed 14CH4 and [14C]methyl coenzyme M as products. The apparent Km for acetate conversion to methane was 5 mM. In the presence of excess acetate, both the rate and duration of methane production was dependent on ATP. Acetyl phosphate replaced the cell extract methanogenic requirement for both acetate and ATP (the Km for ATP was 2 mM). Low concentrations of bromoethanesulfonic acid and cyanide, inhibitors of methylreductase and carbon monoxide dehydrogenase, respectively, greatly reduced the rate of methanogenesis. Precipitation of CO dehydrogenase in cell extracts by antibodies raised to 95% purified enzyme inhibited both CO dehydrogenase and acetate-to-methane conversion activity. The data are consistent with a model of acetate catabolism in which methylreductase, methyl coenzyme M, CO dehydrogenase, and acetate-activating enzymes are components. These results are discussed in relation to acetate uptake and rate-limiting transformation mechanisms in methane formation.

Growth substrate effects on acetate and methanol catabolism in Methanosarcina sp. strain TM-1

When Methanosarcina sp. strain TM-1 is grown in medium in which both methanol and acetate are present, growth is biphasic, with methanol used as the primary catabolic substrate during the first phase. To better understand this phenomenon, we grew cells on methanol or on acetate or on both and examined the abilities of anaerobically washed cells to catabolize these substrates. Washed acetate-grown cells incubated with 10 mM acetate, 10 mM methanol, or both substrates together produced methane at initial rates of 325, 3, and 315 nmol min-1 mg of protein-1, respectively. Although the initial rate of methanogenesis from both substrates was nearly identical to the rate for acetate alone, after several hours of incubation the rate was greater for cells provided with both substrates. Studies with 14C-labeled methanol indicated that methanol was catabolized to methane at increasing rates by acetate-grown cells in a manner reminiscent of an induction curve, but only when cells were provided with acetate as a cosubstrate. Acetate was presumably providing energy and carbon for induction of methanol-catabolic enzymes. Methanol-grown cells showed a pattern of substrate utilization significantly different from that of acetate-grown cells, producing methane from 10 mM acetate, 10 mM methanol, or both substrates at initial rates of 10, 280, and 450 nmol min-1 mg of protein-1, respectively. There was significant oxidation of the methyl group of acetate during metabolism of both substrates. Cells grown on methanol-acetate and harvested before methanol depletion (methanol phase) showed catabolic patterns nearly identical to those of methanol-grown cells, including a low rate of methanogenesis from acetate. Cells harvested from methanol-acetate cultures in the acetate phase were capable of significant methanogenesis from either methanol or acetate alone, and the rate from both substrates together was nearly equal to the sum of the rates for the single substrates. When both 10 mM methanol and 10 mM acetate were presented to the acetate-phase cells, there was a preference for the methanol. These results are consistent with a model for regulation in Methanosarcina sp. strain TM-1 in which methanol represses acetate catabolism while methanol catabolism is inducible.

Single-carbon chemistry of acetogenic and methanogenic bacteria

Methanogenic and acetogenic bacteria metabolize carbon monoxide, methanol, formate, hydrogen and carbon dioxide gases and, in the case of certain methanogens, acetate, by single-carbon (C1) biochemical mechanisms. Many of these reactions occur while the C1 compounds are linked to pteridine derivatives and tetrapyrrole coenzymes, including corrinoids, which are used to generate, reduce, or carbonylate methyl groups. Several metalloenzymes, including a nickel-containing carbon monoxide dehydrogenase, are used in both catabolic and anabolic oxidoreductase reactions. We propose biochemical models for coupling carbon and electron flow to energy conservation during growth on C1 compounds based on the carbon flow pathways inherent to acetogenic and methanogenic metabolism. Biological catalysts are therefore available which are comparable to those currently in use in the Monsanto process. The potentials and limitations of developing biotechnology based on these organisms or their enzymes and coenzymes are discussed.