Methanosphaera stadtmanae has the most restricted energy metabolism of all methanogenic archaea. This human intestinal inhabitant can generate methane only by reduction of methanol with H 2 and is dependent on acetate as a carbon source. We report here the genome sequence of M. stadtmanae, which was found to be composed of 1,767,403 bp with an average G؉C content of 28% and to harbor only 1,534 protein-encoding sequences (CDS). The genome lacks 37 CDS present in the genomes of all other methanogens. Among these are the CDS for synthesis of molybdopterin and for synthesis of the carbon monoxide dehydrogenase/acetylcoenzyme A synthase complex, which explains why M. stadtmanae cannot reduce CO 2 to methane or oxidize methanol to CO 2 and why this archaeon is dependent on acetate for biosynthesis of cell components. Four sets of mtaABC genes coding for methanol:coenzyme M methyltransferases were found in the genome of M. stadtmanae. These genes exhibit homology to mta genes previously identified in Methanosarcina species. The M. stadtmanae genome also contains at least 323 CDS not present in the genomes of all other archaea. Seventythree of these CDS exhibit high levels of homology to CDS in genomes of bacteria and eukaryotes. These 73 CDS include 12 CDS which are unusually long (>2,400 bp) with conspicuous repetitive sequence elements, 13 CDS which exhibit sequence similarity on the protein level to CDS encoding enzymes involved in the biosynthesis of cell surface antigens in bacteria, and 5 CDS which exhibit sequence similarity to the subunits of bacterial type I and III restriction-modification systems.There are two types of methanogenic archaea, those belonging to the order Methanosarcinales, which contain cytochromes and which can use methanol, methyl amines, acetate, and/or CO 2 plus H 2 as methanogenic substrates, and those belonging to the orders Methanobacteriales, Methanomicrobiales, Methanococcales, and Methanopyrales, which are devoid of cytochromes and which can use CO 2 plus H 2 and/or formate only to fuel anaerobic growth (95, 102). The energy metabolism of both types of methanogens has been investigated in detail (17). However, there are still a few pertinent questions. For example, why is the growth yield on H 2 and CO 2 of methanogens lacking cytochromes considerably lower (Ͻ50%) than that of cytochrome-containing methanogens? The growth yield on H 2 and CO 2 of Methanobrevibacter arboriphilus is 1.3 g/mol methane, whereas that of Methanosarcina barkeri is 7.3 g/mol (101). Could the reason for this be that in cytochrome-containing methanogens two steps in the reduction of CO 2 to methane, methyl transfer from methyl-tetrahydromethanopterin (methyl-H 4 MPT) to coenzyme M and reduction of the heterodisulfide coenzyme M-S-S-coenzyme B (CoM-S-S-CoB) with H 2 , are coupled with energy conservation, whereas in methanogens without cytochromes only one step, the methyltransfer reaction, is coupled? Indeed, methanogens with cytochromes contain a heterodisulfide reductase (HdrDE) that is anchored via a cytoc...
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 fi Ala and Cys239 fi 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. Keywords:1 1 zinc enzymes; methanogenic archaea; methyl transferases; thiol group alkylation; EXAFS.Methanosarcina barkeri and other Methanosarcina species can grow on methanol as carbon source which is disproportionated to CH 4 and CO 2 [1]. The first step in this metabolic pathway is the formation of methyl-coenzyme M (CH 3 -S-CoM) from methanol and coenzyme M (HS-CoM) [2].The reaction is catalyzed by methanol:coenzyme M methyltransferase which is composed of the three subunits MtaA (35.9 kDa), MtaB (50.7 kDa) and MtaC (27.9 kDa), of which MtaC is a corrinoid protein. They catalyze the following partial reactions [3][4][5][6][7].MtaA is a zinc protein [3,7,8] that also catalyzes the methylation of coenzyme M with methylcob(III)alamin [9]. Several isoenzymes of MtaA, designated MtbA and MtsA have been found [9][10][11].The methylation of coenzyme M to methyl-coenzyme M is a reaction in which a thiol group is alkylated. Enzymes catalyzing alkyl transfers to thiols have all been found to be zinc proteins [12] [19]. The postulated role of zinc in these enzymes is that of a Lewis acid that activates the thiol group to be alkylated. Coordination of the thiol group to the active site zinc has been shown by extended X-ray absorption fine structure (EXAFS) spectroscopy [14,15], by UV spectroscopy [20] and in the case of protein farnesyl transferase by crystal structure analysis [19]. It results in a decrease in the pK value of the thiol group as shown by the release of a proton upon binding of the substrate to the zinc enzyme [21].MtaA does not share sequence similarity to any of the other zinc enzymes catalyzing thiol group alkylation [8,22]. Abbreviations: HS-CoM, coenzyme M; CH 3 -S-CoM, methyl-coenzyme M; EXAFS, extended X-ray absorption fine structure; Mta, methanol:coenzyme M methyltransferase; MtaA, protein subunit of Mta; XANES, X-ray absorption near edge structure; XAS, X-ray absorption spectroscopy.
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