Function of methylcobalamin: coenzyme M methyltransferase isoenzyme II in Methanosarcina barkeri

Long-term performance of UASB in treating N, N-dimethylformamide-containing wastewater with a rapid start-up by inoculating mixed sludge

Wastewater containing N, N-dimethylformamide (DMF) was treated by artificially mixing the anaerobic granular sludge (AGS) with DMF-degrading activated sludge (DAS) in this study. An up-flow anaerobic sludge blanket (UASB) successfully treated wastewater containing approximately 2000 mg L^-1 DMF during an operation period of 215 days. An inoculation of DAS brought about remarkable results: a rapid start-up with effective DMF methanogenic degradation on the first day, and under a low organic loading rate (OLR) of 1.63-4.22 g COD L^-1 day^-1, the UASB maintained excellent DMF removal efficiency at over 96% along with the high methane production rate (MPR). However, when the OLR increased to 9.24 g COD L^-1 day^-1, DMF removal efficiency and MPR dropped to 47.36% and 1.05 L L^-1 day^-1. A further increase in the OLR to 13.25 g COD L^-1 day^-1 resulted in a sharp deterioration in the DMF-degrading ability, at merely 19.19% and a low MPR of 0.38 L L^-1 day^-1. The excessive elevation of OLR resulted in the insufficient hydrolysis of the DMF, and the further weakening of the conversion from DMF to intermediates and an acceleration the decaying of DMF-hydrolyzing bacteria. Methane-producing archaea was starved of intermediates when hydrolysis was inadequate. Since the DAS can be massively domesticated, and the OLR should be kept lower than 6.17 g COD L^-1 day^-1, the timely replenishing of the DAS to the UASB may be a solution to maintain a stable and effective DMF hydrolysis for long-term operation. The results of this study provide insight for the development of a new concept and an improved method for the effective treatment of wastewater containing degradation-resistant organics.

Characterization and variation of microbial community structure during the anaerobic treatment of N, N-dimethylformamide-containing wastewater by UASB with artificially mixed consortium

A lab-scale UASB was operated successfully to anaerobically treat wastewater containing approximately 2000 mg L^-1N, N-dimethylformamide (DMF) by artificially mixing anaerobic granular sludge with DMF-degrading activated sludge. DMF was effectively degraded by the UASB under a low OLR of 1.63-4.22 g COD L^-1 d^-1, with over 96% DMF removal efficiency and a high methane production rate. However, the DMF-degrading ability gradually weakened along with increases in the OLR. The analysis of the microbial community structure by high-throughput sequencing revealed a decline in the abundance of the facultatively anaerobic DMF-hydrolyzing bacteria originating from activated sludge with increasing OLR, further deteriorating the methanogenic degradation of DMF. When the OLR was lowered again, the slow growth of those facultative anaerobes recovered, and slight improvements in the removal were noted. Methylotrophic methanogens utilized the intermediate products from the hydrolysis of DMF, which kept increasing in abundance throughout the entire experimental period.

Anaerobic treatment of N, N-dimethylformamide-containing wastewater by co-culturing two sources of inoculum

The complete methanogenic degradation of N, N-dimethylformamide (DMF) was achieved in this study. Initially, DMF was found to be feebly degradable by a lab-scale submerged anaerobic membrane bioreactor (SAnMBR) using normal anaerobic digestion sludge (ADS) even after 120-day's culturing. However, aerobic DMF-degrading activated sludge (AS) was rapidly cultivated in a continuous aeration reactor (CAR). A specially designed anaerobic co-cultured sludge (ACS) made by artificially mixing AS with ADS was successfully domesticated by a long term repeated batch experiment. The results demonstrated that ACS could effectively degrade over 5000 mg L^-1 DMF for methane recovery. The metabolic pathway and stoichiometric equation of DMF methanogenic degradation were also revealed and verified in detail. It was confirmed that under the anaerobic condition, with the help of enzyme, DMF converts into dimethylamine and formic acid, and the intermediates are effectively fermented through methylotrophic/hydrogenotrophic methanogenesis. Analysis of the microbial community suggested that some facultatively anaerobic bacteria played the key roles in methanogenic degradation due to their DMF-hydrolyzing ability. By co-culturing two sources of inoculum under the anaerobic condition, the symbiosis of facultatively anaerobic DMF-hydrolyzing bacteria and methylotrophic/hydrogenotrophic methanogens makes methanogenic degradation of DMF available. This study also provides a novel sludge cultivation method for anaerobic treatment of degradation-resistant organics.

RamA, a protein required for reductive activation of corrinoid-dependent methylamine methyltransferase reactions in methanogenic archaea

Archaeal methane formation from methylamines is initiated by distinct methyltransferases with specificity for monomethylamine, dimethylamine, or trimethylamine. Each methylamine methyltransferase methylates a cognate corrinoid protein, which is subsequently demethylated by a second methyltransferase to form methyl-coenzyme M, the direct methane precursor. Methylation of the corrinoid protein requires reduction of the central cobalt to the highly reducing and nucleophilic Co(I) state. RamA, a 60-kDa monomeric iron-sulfur protein, was isolated from Methanosarcina barkeri and is required for in vitro ATP-dependent reductive activation of methylamine:CoM methyl transfer from all three methylamines. In the absence of the methyltransferases, highly purified RamA was shown to mediate the ATP-dependent reductive activation of Co(II) corrinoid to the Co(I) state for the monomethylamine corrinoid protein, MtmC. The ramA gene is located near a cluster of genes required for monomethylamine methyltransferase activity, including MtbA, the methylamine-specific CoM methylase and the pyl operon required for co-translational insertion of pyrrolysine into the active site of methylamine methyltransferases. RamA possesses a C-terminal ferredoxin-like domain capable of binding two tetranuclear iron-sulfur proteins. Mutliple ramA homologs were identified in genomes of methanogenic Archaea, often encoded near methyltrophic methyltransferase genes. RamA homologs are also encoded in a diverse selection of bacterial genomes, often located near genes for corrinoid-dependent methyltransferases. These results suggest that RamA mediates reductive activation of corrinoid proteins and that it is the first functional archetype of COG3894, a family of redox proteins of unknown function.

Genetic analysis of the methanol- and methylamine-specific methyltransferase 2 genes of Methanosarcina acetivorans C2A

The entry of methanol into the methylotrophic pathway of methanogenesis is mediated by the concerted effort of two methyltransferases, namely, methyltransferase 1 (MT1) and methyltransferase 2 (MT2). The mtaA1, mtaA2, and mtbA genes of Methanosarcina acetivorans C2A encode putative methanol- or methylamine-specific MT2 enzymes. To address the in vivo roles of these genes in growth and methanogenesis from known substrates, we constructed and characterized mutants with deletions of each of these genes. The mtaA1 gene is required for growth on methanol, whereas mtaA2 was dispensable. However, the mtaA2 mutant had a reduced rate of methane production from methanol. Surprisingly, deletion of mtaA1 in combination with deletions of the genes encoding three methanol-specific MT1 isozymes led to lack of growth on acetate, suggesting that MT1 and MT2 enzymes might play an important role during growth on this substrate. The mtbA gene was required for dimethylamine and monomethylamine (MMA) utilization and was important, but not required, for trimethylamine utilization. Analysis of reporter gene fusions revealed that both mtaA1 and mtbA were expressed on all methanogenic substrates tested. However, mtaA1 expression was induced on methanol, while mtbA expression was down-regulated on MMA and acetate. mtaA2 was expressed at very low levels on all substrates. The mtaA1 transcript had a large 5' untranslated region (UTR) (275 bp), while the 5' UTR of the mtbA transcript was only 28 bp long.

Genomic and proteomic analyses reveal multiple homologs of genes encoding enzymes of the methanol:coenzyme M methyltransferase system that are differentially expressed in methanol- and acetate-grown Methanosarcina thermophila

Each of the genomic sequences of Methanosarcina acetivorans, Methanosarcina mazei, and Methanosarcina thermophila revealed two homologs of mtaA, three homologs of mtaB, and three homologs of mtaC encoding enzymes specific for methanogenesis from methanol. Two-dimensional gel electrophoretic analyses of polypeptides from M. thermophila established that methanol induces the expression of mtaA-1, mtaB-1, mtaB-2, mtaB-3, mtaC-1, mtaC-2, and mtaC-3 whereas mtaB-3 and mtaC-3 are constitutively expressed in acetate-grown cells. The gene product of one of three mttC homologs, encoding trimethylamine-specific methyltransferase I, was detected in methanol- but not acetate-grown M. thermophila. A postulated role for the multiple homologs is discussed.

The role of zinc in the methylation of the coenzyme M thiol group in methanol:coenzyme M methyltransferase from Methanosarcina barkeri

Methanol:coenzyme M methyltransferase from methanogenic archaea is a cobalamin-dependent enzyme composed of three different subunits: MtaA, MtaB and MtaC. MtaA is a zinc protein that catalyzes the methylation of coenzyme M (HS-CoM) with methylcob(III)alamin. We report zinc XAFS (X-ray absorption fine structure) results indicating that, in the absence of coenzyme M, zinc is probably coordinated by a single sulfur ligand and three oxygen or nitrogen ligands. In the presence of coenzyme M, one (N/O)-ligand was replaced by sulfur, most likely due to ligation of the thiol group of coenzyme M. Mutations in His237 or Cys239, which are proposed to be involved in ligating zinc, resulted in an over 90% loss in enzyme activity and in distinct changes in the zinc ligands. In the His237-->Ala and Cys239-->Ala mutants, coenzyme M also seemed to bind efficiently by ligation to zinc indicating that some aspects of the zinc ligand environment are surprisingly uncritical for coenzyme M binding.

Reconstitution of dimethylamine:coenzyme M methyl transfer with a discrete corrinoid protein and two methyltransferases purified from Methanosarcina barkeri

Methyl transfer from dimethylamine to coenzyme M was reconstituted in vitro for the first time using only highly purified proteins. These proteins isolated from Methanosarcina barkeri included the previously unidentified corrinoid protein MtbC, which copurified with MtbA, the methylcorrinoid:Coenzyme M methyltransferase specific for methanogenesis from methylamines. MtbC binds 1.0 mol of corrinoid cofactor/mol of 24-kDa polypeptide and stimulated dimethylamine:coenzyme M methyl transfer 3.4-fold in a cell extract. Purified MtbC and MtbA were used to assay and purify a dimethylamine:corrinoid methyltransferase, MtbB1. MtbB1 is a 230-kDa protein composed of 51-kDa subunits that do not possess a corrinoid prosthetic group. Purified MtbB1, MtbC, and MtbA were the sole protein requirements for in vitro dimethylamine:coenzyme M methyl transfer. An MtbB1:MtbC ratio of 1 was optimal for coenzyme M methylation with dimethylamine. MtbB1 methylated either corrinoid bound to MtbC or free cob(I)alamin with dimethylamine, indicating MtbB1 carries an active site for dimethylamine demethylation and corrinoid methylation. Experiments in which different proteins of the resolved monomethylamine:coenzyme M methyl transfer reaction replaced proteins involved in dimethylamine:coenzyme M methyl transfer indicated high specificity of MtbB1 and MtbC in dimethylamine:coenzyme M methyl transfer activity. These results indicate MtbB1 demethylates dimethylamine and specifically methylates the corrinoid prosthetic group of MtbC, which is subsequently demethylated by MtbA to methylate coenzyme M during methanogenesis from dimethylamine.

Methanol:coenzyme M methyltransferase from Methanosarcina barkeri -- substitution of the corrinoid harbouring subunit MtaC by free cob(I)alamin

Methyl-coenzyme M formation from coenzyme M and methanol in Methanosarcina barkeri is catalysed by an enzyme system composed of three polypeptides MtaA, MtaB and MtaC, the latter of which harbours a corrinoid prosthetic group. We report here that MtaC can be substituted by free cob(I)alamin which is methylated with methanol in an MtaB-catalysed reaction and demethylated with coenzyme M in an MtaA-catalysed reaction. Methyl transfer from methanol to coenzyme M was found to proceed at a relatively high specific activity at micromolar concentrations of cob(I)alamin. This finding was surprising because the methylation of cob(I)alamin catalysed by MtaB alone and the demethylation of methylcob(III)alamin catalysed by MtaA alone exhibit apparent Km for cob(I)alamin and methylcob(III)alamin of above 1 mm. A possible explanation is that MtaA positively affects the MtaB catalytic efficiency and vice versa by decreasing the apparent Km for their corrinoid substrates. Activation of MtaA by MtaB was methanol-dependent. In the assay for methanol:coenzyme M methyltransferase activity cob(I)alamin could be substituted by cob(I)inamide which is devoid of the nucleotide loop. Substitution was, however, only possible when the assays were supplemented with imidazole: approximately 1 mm imidazole being required for half-maximal activity. Methylation of cob(I)inamide with methanol was found to be dependent on imidazole but not on the demethylation of methylcob(III)inamide with coenzyme M. The demethylation reaction was even inhibited by imidazole. The structure and catalytic mechanism of the MtaABC complex are compared with the cobalamin-dependent methionine synthase.

Enzymology of one-carbon metabolism in methanogenic pathways

Methanoarchaea, the largest and most phylogenetically diverse group in the Archaea domain, have evolved energy-yielding pathways marked by one-carbon biochemistry featuring novel cofactors and enzymes. All of the pathways have in common the two-electron reduction of methyl-coenzyme M to methane catalyzed by methyl-coenzyme M reductase but deviate in the source of the methyl group transferred to coenzyme M. Most of the methane produced in nature derives from acetate in a pathway where the activated substrate is cleaved by CO dehydrogenase/acetyl-CoA synthase and the methyl group is transferred to coenzyme M via methyltetrahydromethanopterin or methyltetrahydrosarcinapterin. Electrons for reductive demethylation of the methyl-coenzyme M originate from oxidation of the carbonyl group of acetate to carbon dioxide by the synthase. In the other major pathway, formate or H2 is oxidized to provide electrons for reduction of carbon dioxide to the methyl level and reduction of methyl-coenzyme to methane. Methane is also produced from the methyl groups of methanol and methylamines. In these pathways specialized methyltransferases transfer the methyl groups to coenzyme M. Electrons for reduction of the methyl-coenzyme M are supplied by oxidation of the methyl groups to carbon dioxide by a reversal of the carbon dioxide reduction pathway. Recent progress on the enzymology of one-carbon reactions in these pathways has raised the level of understanding with regard to the physiology and molecular biology of methanogenesis. These advances have also provided a foundation for future studies on the structure/function of these novel enzymes and exploitation of the recently completed sequences for the genomes from the methanoarchaea Methanobacterium thermoautotrophicum and Methanococcus jannaschii.

Biochemistry of methanogenesis: a tribute to Marjory Stephenson. 1998 Marjory Stephenson Prize Lecture

Max-Planck-Institut für terrestrische Mikrobiologie, Karl-von-Frisch-Straße, D-35043 Marburg, and Laboratorium für Mikrobiologie, Fachbereich Biologie, Philipps-Universität, Karl-von-Frisch-Straße, D-35032 Marburg, Germany

In 1933, Stephenson & Stickland (1933a) published that they had isolated from river mud, by the single cell technique, a methanogenic organism capable of growth in an inorganic medium with formate as the sole carbon source.

Clustered genes encoding the methyltransferases of methanogenesis from monomethylamine

Coenzyme M (CoM) is methylated during methanogenesis from monomethyamine in a reaction catalyzed by three proteins. Using monomethylamine, a 52-kDa polypeptide termed monomethylamine methyltransferase (MMAMT) methylates the corrinoid cofactor bound to a second polypeptide, monomethylamine corrinoid protein (MMCP). Methylated MMCP then serves as a substrate for MT2-A, which methylates CoM. The genes for these proteins are clustered on 6.8 kb of DNA in Methanosarcina barkeri MS. The gene encoding MMCP (mtmC) is located directly upstream of the gene encoding MMAMT (mtmB). The gene encoding MT2-A (mtbA) was found 1.1 kb upstream of mtmC, but no obvious open reading frame was found in the intergenic region between mtbA and mtmC. A single monocistronic transcript was found for mtbA that initiated 76 bp from the translational start. Separate transcripts of 2.4 and 4.7 kb were detected, both of which carried mtmCB. The larger transcript also encoded mtmP, which is homologous to the APC family of cationic amine permeases and may therefore encode a methylamine permease. A single transcriptional start site was found 447 bp upstream of the translational start of mtmC. MtmC possesses the corrinoid binding motif found in corrinoid proteins involved in dimethylsulfide- and methanol-dependent methanogenesis, as well as in methionine synthase. The open reading frame of mtmB was interrupted by a single in-frame, midframe, UAG codon which was also found in mtmB from M. barkeri NIH. A mechanism that circumvents UAG-directed termination of translation must operate during expression of mtmB in this methanogen.

Methylthiol:coenzyme M methyltransferase from Methanosarcina barkeri, an enzyme of methanogenesis from dimethylsulfide and methylmercaptopropionate

During growth on acetate, Methanosarcina barkeri expresses catabolic enzymes for other methanogenic substrates such as monomethylamine. The range of substrates used by cells grown on acetate was further explored, and it was found that cells grown on acetate also converted dimethylsulfide (DMS) and methylmercaptopropionate (MMPA) to methane. Cells or extracts of cells grown on trimethylamine or methanol did not utilize either DMS or MMPA. During growth on acetate, cultures demethylated MMPA, producing methane and mercaptopropionate. Extracts of acetate-grown cells possessed DMS- and MMPA-dependent coenzyme M (CoM) methylation activities. The activity peaks of CoM methylation with either DMS or MMPA coeluted upon gel permeation chromatography of extracts of acetate-grown cells consistent with an apparent molecular mass of 470 kDa. A 480-kDa corrinoid protein, previously demonstrated to be a CoM methylase but otherwise of unknown physiological function, was found to methylate CoM with either DMS or MMPA. MMPA was demethylated by the purified 480-kDa CoM methylase, consuming 1 mol of CoM and producing 1 mol of mercaptopropionate. DMS was demethylated by the purified protein, consuming 1 mol of CoM and producing 1 mol of methanethiol. The methylthiol:CoM methyltransferase reaction could be initiated only with the enzyme-bound corrinoid in the methylated state. CoM could demethylate, and DMS and MMPA could remethylate, the corrinoid cofactor. The monomethylamine corrinoid protein and the A isozyme of methylcobamide:CoM methyltransferase (proteins homologous to the two subunits comprising the 480-kDa CoM methylase) did not catalyze CoM methylation with methylated thiols. These results indicate that the 480-kDa corrinoid protein functions as a CoM methylase during methanogenesis from DMS or MMPA.

Reconstitution of Monomethylamine:Coenzyme M methyl transfer with a corrinoid protein and two methyltransferases purified from Methanosarcina barkeri

Methanogenesis from methylamines requires the intermediate methylation of 2-mercaptoethanesulfonate (CoM). In vitro reconstitution of CoM methylation with monomethylamine was achieved with three purified proteins: a monomethylamine corrinoid protein (MMCP), the "A" isozyme of methylcobamide:CoM methyltransferase (MT2-A), and a newly isolated protein termed monomethylamine methyltransferase (MMAMT).MMAMT is a 170-kDa protein with 52-kDa subunits. The MMAMT polypeptide was rate-limiting for methyl transfer until at a 2-fold molar excess over MMCP. MMAMT is a monomethylamine:MMCP methyltransferase, since methylation of MMCP required MMAMT but not MT2-A. MMCP and MMAMT formed a complex detectable by size exclusion high pressure liquid chromatography. Methyl group transfer from methyl-MMCP to CoM was mediated by MT2-A, since methyl iodide:CoM methyl transfer by MMCP and MT2-A did not require MMAMT. MT2-M, an isozyme of MT2-A, was inactive in MMCP-dependent methyl transfer. Immunodepletion of MMCP from the extract inhibited CoM methylation with monomethylamine but not dimethylamine. Purified MMCP reconstituted activity in immunodepleted extracts. These results show that MMCP is the major corrinoid protein for methanogenesis from monomethylamine detectable in extracts and that it interacts with two methyltransferases. MMAMT functions as a MMA:MMCP methyltransferase, while MT2-A functions as a methyl-MMCP:CoM methyltransferase.

Reconstitution of trimethylamine-dependent coenzyme M methylation with the trimethylamine corrinoid protein and the isozymes of methyltransferase II from Methanosarcina barkeri

Reconstitution of trimethylamine-dependent coenzyme M (CoM) methylation was achieved with three purified polypeptides. Two of these polypeptides copurified as a trimethylamine methyl transfer (TMA-MT) activity detected by stimulation of the TMA:CoM methyl transfer reaction in cell extracts. The purified TMA-MT fraction stimulated the rate of methyl-CoM formation sevenfold, up to 1.7 micromol/min/mg of TMA-MT protein. The TMA-MT polypeptides had molecular masses of 52 and 26 kDa. Gel permeation of the TMA-MT fraction demonstrated that the 52-kDa polypeptide eluted with an apparent molecular mass of 280 kDa. The 26-kDa protein eluted primarily as a monomer, but some 26-kDa polypeptides also eluted with the 280-kDa peak, indicating that the two proteins weakly associate. The two polypeptides could be completely separated using gel permeation in the presence of sodium dodecyl sulfate. The corrinoid remained associated with the 26-kDa polypeptide at a molar ratio of 1.1 corrin/26-kDa polypeptide. This polypeptide was therefore designated the TMA corrinoid protein, or TCP. The TMA-MT polypeptides, when supplemented with purified methylcorrinoid:CoM methyltransferase (MT2), could effect the demethylation of TMA with the subsequent methylation of CoM and the production of dimethylamine at specific activities of up to 600 nmol/min/mg of TMA-MT protein. Neither dimethylamine nor monomethylamine served as the substrate, and the activity required Ti(III) citrate and methyl viologen. TMA-MT could interact with either isozyme of MT2 but had the greatest affinity for the A isozyme. These results suggest that TCP is uniquely involved in TMA-dependent methanogenesis, that this corrinoid protein is methylated by the substrate and demethylated by either isozyme of MT2, and that the predominant isozyme of MT2 found in TMA-grown cells is the favored participant in the TMA:CoM methyl transfer reaction.

Methanol:coenzyme M methyltransferase from Methanosarcina barkeri. Purification, properties and encoding genes of the corrinoid protein MT1

In Methanosarcina barkeri, methanogenesis from methanol is initiated by the formation of methylcoenzyme M from methanol and coenzyme M. This methyl transfer reaction is catalyzed by two enzymes, designated MT1 and MT2. Transferase MT1 is a corrinoid protein. The purification, catalytic properties and encoding genes of MT2 (MtaA) have been described previously [Harms, U. and Thauer, R.K. (1996) Eur. J. Biochem. 235, 653-659]. We report here on the corresponding analysis of MT1. The corrinoid protein MT1 was purified to apparent homogeneity and showed a specific activity of 750 mumol min-1 mg-1. The enzyme catalyzed the methylation of its bound corrinoid in the cob(I)amide oxidation state by methanol. In addition to this automethylation, the purified enzyme was found to catalyze the methylation of free cob(I)alamin to methylcob(III)alamin. It was composed of two different subunits designated MtaB and MtaC, with apparent molecular masses of 49 kDa and 24 kDa, respectively. The subunit MtaC was shown to harbour the corrinoid prosthetic group. The genes mtaB and mtaC were cloned and sequenced. They were found to be juxtapositioned and to form a transcription unit mtaCB. The corrinoid-harbouring subunit MtaC exhibits 35% sequence similarity to the cobalamin-binding domain of methionine synthase from Escherichia coli.

Involvement of methyltransferase-activating protein and methyltransferase 2 isoenzyme II in methylamine:coenzyme M methyltransferase reactions in Methanosarcina barkeri Fusaro

The enzyme systems involved in the methyl group transfer from methanol and from tri- and dimethylamine to 2-mercaptoethanesulfonic acid (coenzyme M) were resolved from cell extracts of Methanosarcina barkeri Fusaro grown on methanol and trimethylamine, respectively. Resolution was accomplished by ammonium sulfate fractionation, anion-exchange chromatography, and fast protein liquid chromatography. The methyl group transfer reactions from tri- and dimethylamine, as well as the monomethylamine:coenzyme M methyltransferase reaction, were strictly dependent on catalytic amounts of ATP and on a protein present in the 65% ammonium sulfate supernatant. The latter could be replaced by methyltransferase-activating protein isolated from methanol-grown cells of the organism. In addition, the tri- and dimethylamine:coenzyme M methyltransferase reactions required the presence of a methylcobalamin:coenzyme M methyltransferase (MT2), which is different from the analogous enzyme from methanol-grown M. barkeri. In this work, it is shown that the various methylamine:coenzyme M methyltransfer steps proceed in a fashion which is mechanistically similar to the methanol:coenzyme M methyl transfer, yet with the participation of specific corrinoid enzymes and a specific MT2 isoenzyme.

Sequence and transcript analysis of a novel Methanosarcina barkeri methyltransferase II homolog and its associated corrinoid protein homologous to methionine synthase

The sequence and transcript of the genes encoding a recently discovered coenzyme M methylase in Methanosarcina barkeri were analyzed. This 480-kDa protein is composed of two subunits in equimolar concentrations which bind one corrinoid cofactor per alphabeta dimer. The gene for the alphabeta polypeptide, mtsA, is upstream of that encoding the beta polypeptide, mtsB. The two genes are contiguous and overlap by several nucleotides. A 1.9-kb mRNA species which reacted with probes specific for either mtsA or mtsB was detected. Three possible methanogen consensus BoxA sequences as well as two sets of direct repeats were found upstream of mtsA. The 5' end of the mts transcript was 19 nucleotides upstream of the translational start site of mtsA and was positioned 25 bp from the center of the proximal BoxA sequence. The transcript was most abundant in cells grown to the late log phase on acetate but barely detectable in cells grown on methanol or trimethylamine. The amino acid sequence of MtsB was homologous to the cobalamin-binding fragment of methionine synthase from Escherichia coli and possessed the signature residues involved in binding the corrinoid, including a histidyl residue which ligates cobalt. The sequence of MtsA is homologous to the "A" and "M" isozymes of methylcobamide:coenzyme M methyltransferases (methyltransferase II), indicating that the alpha polypeptide is a new member of the methyltransferase II family of coenzyme M methylases. All three methyltransferase II homolog sequences could be aligned with the sequences of uroporphyrinogen decarboxylase from various sources. The implications of these homologies for the mechanism of corrinoid binding by proteins involved in methylotrophic methanogenesis are discussed.

Methylcobamide:coenzyme M methyltransferase isozymes from Methanosarcina barkeri. Physicochemical characterization, cloning, sequence analysis, and heterologous gene expression

A comparative study was made on the physicochemical characteristics of two isozymes of methylcobamide:- coenzyme M methyltransferase (MT2). Both isozymes catalyzed S-methylation of 2-thioethanesulfonate (coenzyme M) and exhibited similar apparent Km values for coenzyme M of 35 microM (MT2-A) and 20 microM (MT2-M). Weak binding to methylcobalamin was indicated by the apparent Km of 14 mM for both isozymes. Cob(I)alamin was established as the major product of the reaction, demonstrating heterolytic cleavage of the methylcobamide carbon-cobalt bond. The isozymes were shown to be zinc-containing metalloproteins. Metal ion chelators strongly inhibited both isozymes. A variety of coenzyme M analogs were tested for activity and/or inhibition. One alternative substrate 3-mercaptopropionate was discovered, with apparent Km 9 mM (MT2-A) and 10 mM (MT2-M). The results suggested an active site geometry in which coenzyme M is bound both by S-coordination to zinc, and electrostatic interaction of the sulfonate with a cationic group on the enzyme. Methanosarcina barkeri genes cmtA and cmtM encoding both isozymes were cloned and sequenced. Both genes encoded proteins with 339 amino acids and predicted molecular masses of 36-37 kDa. Active forms of both isozymes were expressed in Escherichia coli. A conserved segment with the potential for metal binding was found. The possibility of zinc involvement in catalysis of coenzyme M methylation is considered.

Sodium ion translocation by N5-methyltetrahydromethanopterin: coenzyme M methyltransferase from Methanosarcina mazei Gö1 reconstituted in ether lipid liposomes

The N5-methyltetrahydromethanopterin (H,MPT):coenzyme M methyltransferase is a membrane associated, corrinoid-containing protein that uses the methylation of coenzyme M (HS-CoM) by methyl-tetrahydromethanopterin to drive an energy-conserving sodium ion pump. The enzyme was purified from acetate-grown Methanosarcina mazei Gö1 by a two-step solubilization with n-octyl-beta-glucoside, chromatography on hydroxyapatite, and by gel filtration on Superdex 200 or Sepharose CL-6B. The highly purified protein was apparently composed of six different subunits of 34, 28, 20, 13, 12, and 9 kDa. The N-terminal amino acid sequences of these polypeptides were determined. The native enzyme exhibited an apparent molecular mass of about 380 kDa. During purification, the enzyme was stabilized with 10 microM hydroxocobalamin. The highest specific activity reached during purification was 10.4 U/mg. The purified enzyme was reconstituted in monolayer liposomes prepared from ether lipids of M. mazei Gö1. In experiments with radioactive sodium ions, it was shown that the methyltransferase catalyzes the vectorial translocation of sodium ions across the membrane. Methyltransferase activity was stimulated by sodium ions. 1.7 mol Na-/mol methyl groups transferred were translocated. Methyltetrahydrofolate and methyl-cobalamin could substitute for methyl-H,MPT.

Carbon monoxide dehydrogenase from Methanosarcina frisia Gö1. Characterization of the enzyme and the regulated expression of two operon-like cdh gene clusters

Carbon monoxide dehydrogenase (Cdh) has been anaerobically purified from Methanosarcina frisia Gö1. The enzyme is a Ni2+-, Fe2+-, and S2--containing alpha2beta2 heterotetramer of 214 kDa with a pI of 5.2 and subunits of 94 and 19 kDa. It has a Vmax of 0.3 mmol of CO min-1 mg-1 and Km values for CO and methyl viologen of approximately 0.9 mM and 0.12 mM, respectively. EPR spectroscopy on the reduced enzyme showed two overlapping signals: one indicative for 2 (4Fe-4S)+ clusters and a second signal that is atypical for standard Fe/S clusters. The latter was, together with high-spin EPR signals of the oxidized enzyme tentatively assigned to an Fe/S cluster of high nuclearity.

Coenzyme M methylase activity of the 480-kilodalton corrinoid protein from Methanosarcina barkeri

Activity staining of extracts of Methanosarcina barkeri electrophoresed in polyacrylamide gels revealed an additional methylcobalamin:coenzyme M (methylcobalamin:CoM) methyltransferase present in cells grown on acetate but not in those grown on trimethylamine. This methyltransferase is the 480-kDa corrinoid protein previously identified by its methylation following inhibition of methyl-CoM reductase in otherwise methanogenic cell extracts. The methylcobalamin:CoM methyltransferase activity of the purified 480-kDa protein increased from 0.4 to 3.8 micromol/min/mg after incubation with sodium dodecyl sulfate (SDS). Following SDS-polyacrylamide gel electrophoresis analysis of unheated protein samples, a polypeptide with an apparent molecular mass of 48 kDa which possessed methylcobalamin:CoM methyltransferase activity was detected. This polypeptide migrated with an apparent mass of 41 kDa when the 480-kDa protein was heated before electrophoresis, indicating that the alpha subunit is responsible for the activity. The N-terminal sequence of this subunit was 47% similar to the N termini of the A and M isozymes of methylcobalamin:CoM methyltransferase (methyltransferase II). The endogenous methylated corrinoid bound to the beta subunit of the 480-kDa protein could be demethylated by CoM, but not by homocysteine or dithiothreitol, resulting in a Co(I) corrinoid. The Co(I) corrinoid could be remethylated by methyl iodide, and the protein catalyzed a methyl iodide:CoM transmethylation reaction at a rate of 2.3 micromol/min/mg. Methyl-CoM was stoichiometrically produced from CoM, as demonstrated by high-pressure liquid chromatography with indirect photometric detection. Two thiols, 2-mercaptoethanol and mercapto-2-propanol, were poorer substrates than CoM, while several others tested (including 3-mercaptopropanesulfonate) did not serve as methyl acceptors. These data indicate that the 480-kDa corrinoid protein is composed of a novel isozyme of methyltransferase II which remains firmly bound to a corrinoid cofactor binding subunit during isolation.

Methylcobalamin: coenzyme M methyltransferase isoenzymes MtaA and MtbA from Methanosarcina barkeri. Cloning, sequencing and differential transcription of the encoding genes, and functional overexpression of the mtaA gene in Escherichia coli

Methanosarcina barkeri is known to contain two methyltransferase isoenzymes, here designated MtaA and MtbA, which catalyze the formation of methyl-coenzyme M from methylcobalamin and coenzyme M. The genes encoding the two soluble 34-kDa proteins have been cloned and sequenced. mtaA and mtbA wee found to be located in different parts of the genome, each forming a monocystronic transcription unit. Northern blot analysis revealed that mtaA is preferentially transcribed when M. barkeri is grown on methanol and the mtbA gene when the organism is grown on H2/CO2 or trimethylamine. Comparison of the deduced amino acid sequences revealed the sequences of the two isoenzymes to be 37% identical. Both isoenzymes showed sequence similarity to uroporphyrinogen III decarboxylase from Escherichia coli. The mtaA gene was tagged with a sequence encoding six His placed bp before the mtaA start codon, and was functionally overexpressed in E. coli. 25% of the E. coli protein was found to be active methyltransferase which could be purified in two steps to apparent homogeneity with a 70% yield.

Involvement of the "A" isozyme of methyltransferase II and the 29-kilodalton corrinoid protein in methanogenesis from monomethylamine

An assay which allowed detection of proteins involved in the trimethylamine- or monomethylamine (MMA)-dependent methylation of coenzyme M (CoM) was developed. The two activities could be separated by anion-exchange chromatography. The unresolved activity responsible for MMA:CoM methyl transfer eluted from a gel permeation column in the molecular mass range of 32 kDa. The activity was purified to two monomeric proteins of 40 and 29 kDa. The preparation contained protein-bound corrinoid in a mixture of Co(II) and Co(III) states, as well as methyl-B12:CoM methyltransferase (MT2) activity. N-terminal sequence analysis demonstrated that the polypeptides were two previously identified proteins of undefined physiological function. The smaller polypeptide was the monomeric 29-kDa corrinoid protein. The larger polypeptide was the "A" isozyme of MT2. Individually purified preparations of both proteins increased the rate of MMA-dependent CoM methylation by approximately 1.7 mumol/min/mg of purified protein above background activity in the extract of methanol-grown cells. These results indicate that the 29-kDa corrinoid protein and the "A" isozyme of MT2 function in methanogenesis from MMA. A likely mechanism is that the 29-kDa corrinoid is methylated by MMA and the methyl group is then transferred by the "A" isozyme of MT2 to CoM.

The energetics and sodium-ion dependence of N5-methyltetrahydromethanopterin:coenzyme M methyltransferase studied with cob(I)alamin as methyl acceptor and methylcob(III)alamin as methyl donor

N5-Methyltetrahydromethanopterin:coenzyme M methyltransferase from methanogenic Archaea is a membrane-associated enzyme complex that uses a methyl-transfer reaction to drive an energy-conserving sodium-ion pump. Methyl transfer occurs in two steps, first from N5-methyltetrahydromethanopterin (CH3-H4MPT) to an enzyme-bound cob(I)amide prosthetic group, and secondly from the methylated cobamide to coenzyme M (H-S-CoM). In this study, we report that methyltransferase can also use exogenous cob(I)alamin and methylcob(III)alamin as methyl acceptor and methyl donor, respectively. The enzyme catalyzes methylcob(III)alamin formation from CH3-H4MPT and cob(I)alamin (reaction a), and methyl-coenzyme M formation from methylcob(III)alamin and H-S-CoM (reaction b). Both reactions were shown to be reversible. Reaction a was catalyzed at approximately the same rate (3 U/mg) and reaction b at approximately 10% the rate (0.3 U/mg) of the physiological reaction, namely methyl transfer from CH3-H4MPT to H-S-CoM. The free energy changes (delta G0') associated with reactions a and b were both between -10 kJ/mol and -20 kJ/mol, consistent with a free energy change of approximately -30 kJ/mol determined for the physiological reaction. Reaction b but not reaction a was sodium-ion dependent. Assuming that methylation of exogenous cob(I)alamin and demethylation of exogenous methylcob(III)alamin mimic methylation and demethylation of the enzyme-bound corrinoid prosthetic group, it can be inferred that methyl transfer from the methylated cobamide prosthetic group to H-S-CoM is a site of coupling with sodium-ion translocation.