The crystal structure of methanogen McrD, a methyl-coenzyme M reductase-associated protein.

Methyl-coenzyme M reductase (MCR) is a multi-subunit (α2ß2γ2) enzyme responsible for methane formation via its unique F430 cofactor. The genes responsible for producing MCR (mcrA, mcrB and mcrG) are typically colocated with two other highly conserved genes mcrC and mcrD. We present here the high-resolution crystal structure for McrD from a human gut methanogen Methanomassiliicoccus luminyensis strain B10. The structure reveals that McrD comprises a ferredoxin-like domain assembled into an α + ß barrel-like dimer with conformational flexibility exhibited by a functional loop. The description of the M. luminyensis McrD crystal structure contributes to our understanding of this key conserved methanogen protein typically responsible for promoting MCR activity and the production of methane, a greenhouse gas.

Complete genome sequence of Methanosphaera sp. ISO3-F5, a rumen methylotrophic methanogen.

Methanosphaera spp. are methylotrophic methanogenic archaea and members of the order Methanobacteriales with few cultured representatives. Methanosphaera sp. ISO3-F5 was isolated from sheep rumen contents in New Zealand. Here, we report its complete genome, consisting of a large chromosome and a megaplasmid (GenBank accession numbers CP118753 and CP118754, respectively).

Crystal Structures of Bacterial Pectin Methylesterases Pme8A and PmeC2 from Rumen Butyrivibrio.

Pectin is a complex polysaccharide that forms a substantial proportion of the plant's middle lamella of forage ingested by grazing ruminants. Methanol in the rumen is derived mainly from methoxy groups released from pectin by the action of pectin methylesterase (PME) and is subsequently used by rumen methylotrophic methanogens that reduce methanol to produce methane (CH4). Members of the genus Butyrivibrio are key pectin-degrading rumen bacteria that contribute to methanol formation and have important roles in fibre breakdown, protein digestion, and the biohydrogenation of fatty acids. Therefore, methanol release from pectin degradation in the rumen is a potential target for CH4 mitigation technologies. Here, we present the crystal structures of PMEs belonging to the carbohydrate esterase family 8 (CE8) from Butyrivibrio proteoclasticus and Butyrivibrio fibrisolvens, determined to a resolution of 2.30 Å. These enzymes, like other PMEs, are right-handed ß-helical proteins with a well-defined catalytic site and reaction mechanisms previously defined in insect, plant, and other bacterial pectin methylesterases. Potential substrate binding domains are also defined for the enzymes.

Structural characterisation of methanogen pseudomurein cell wall peptide ligases homologous to bacterial MurE/F murein peptide ligases.

Archaea have diverse cell wall types, yet none are identical to bacterial peptidoglycan (murein). Methanogens Methanobacteria and Methanopyrus possess cell walls of pseudomurein, a structural analogue of murein. Pseudomurein differs from murein in containing the unique archaeal sugar N-acetyltalosaminuronic acid instead of N-acetylmuramic acid, ß-1,3 glycosidic bonds in place of ß-1,4 bonds and only l-amino acids in the peptide cross-links. We have determined crystal structures of methanogen pseudomurein peptide ligases (termed pMurE) from Methanothermus fervidus (Mfer762) and Methanothermobacter thermautotrophicus (Mth734) that are structurally most closely related to bacterial MurE peptide ligases. The homology of the archaeal pMurE and bacterial MurE enzymes is clear both in the overall structure and at the level of each of the three domains. In addition, we identified two UDP-binding sites in Mfer762 pMurE, one at the exterior surface of the interface of the N-terminal and middle domains, and a second site at an inner surface continuous with the highly conserved interface of the three domains. Residues involved in ATP binding in MurE are conserved in pMurE, suggesting that a similar ATP-binding pocket is present at the interface of the middle and the C-terminal domains of pMurE. The presence of pMurE ligases in members of the Methanobacteriales and Methanopyrales, that are structurally related to bacterial MurE ligases, supports the idea that the biosynthetic origins of archaeal pseudomurein and bacterial peptidoglycan cell walls are evolutionarily related.

Tailored Nanoparticles With the Potential to Reduce Ruminant Methane Emissions.

Agricultural methane produced by archaea in the forestomach of ruminants is a key contributor to rising levels of greenhouse gases leading to climate change. Functionalized biological polyhydroxybutyrate (PHB) nanoparticles offer a new concept for the reduction of enteric methane emissions by inhibiting rumen methanogens. Nanoparticles were functionalized in vivo with an archaeal virus lytic enzyme, PeiR, active against a range of rumen Methanobrevibacter species. The impact of functionalized nanoparticles against rumen methanogens was demonstrated in pure cultures, in rumen batch and continuous flow rumen models yielding methane reduction of up to 15% over 11 days in the most complex system. We further present evidence of biological nanoparticle fermentation in a rumen environment. Elevated levels of short-chain fatty acids essential to ruminant nutrition were recorded, giving rise to a promising new strategy combining methane mitigation with a possible increase in animal productivity.

Using genome comparisons of wild-type and resistant mutants of Methanococcus maripaludis to help understand mechanisms of resistance to methane inhibitors.

Methane emissions from enteric fermentation in the ruminant digestive system generated by methanogenic archaea are a significant contributor to anthropogenic greenhouse gas emissions. Additionally, methane produced as an end-product of enteric fermentation is an energy loss from digested feed. To control the methane emissions from ruminants, extensive research in the last decades has been focused on developing viable enteric methane mitigation practices, particularly, using methanogen-specific inhibitors. We report here the utilization of two known inhibitors of methanogenic archaea, neomycin and chloroform, together with a recently identified inhibitor, echinomycin, to produce resistant mutants of Methanococcus maripaludis S2 and S0001. Whole-genome sequencing at high coverage (> 100-fold) was performed subsequently to investigate the potential targets of these inhibitors at the genomic level. Upon analysis of the whole-genome sequencing data, we identified mutations in a number of genetic loci pointing to potential mechanisms of inhibitor action and their underlying mechanisms of resistance.

Archaeal pseudomurein and bacterial murein cell wall biosynthesis share a common evolutionary ancestry.

Bacteria near-universally contain a cell wall sacculus of murein (peptidoglycan), the synthesis of which has been intensively studied for over 50 years. In striking contrast, archaeal species possess a variety of other cell wall types, none of them closely resembling murein. Interestingly though, one type of archaeal cell wall termed pseudomurein found in the methanogen orders Methanobacteriales and Methanopyrales is a structural analogue of murein in that it contains a glycan backbone that is cross-linked by a L-amino acid peptide. Here, we present taxonomic distribution, gene cluster and phylogenetic analyses that confirm orthologues of 13 bacterial murein biosynthesis enzymes in pseudomurein-containing methanogens, most of which are distantly related to their bacterial counterparts. We also present the first structure of an archaeal pseudomurein peptide ligase from Methanothermus fervidus DSM1088 (Mfer336) to a resolution of 2.5 Å and show that it possesses a similar overall tertiary three domain structure to bacterial MurC and MurD type murein peptide ligases. Taken together the data strongly indicate that murein and pseudomurein biosynthetic pathways share a common evolutionary history.

Inhibition of Rumen Methanogens by a Novel Archaeal Lytic Enzyme Displayed on Tailored Bionanoparticles.

Methane is a potent greenhouse gas, 25 times more efficient at trapping heat than carbon dioxide. Ruminant methane emissions contribute almost 30% to anthropogenic sources of global atmospheric methane levels and a reduction in methane emissions would significantly contribute to slowing global temperature rises. Here we demonstrate the use of a lytic enyzme, PeiR, from a methanogen virus that infects Methanobrevibacter ruminantium M1 as an effective agent inhibiting a range of rumen methanogen strains in pure culture. We determined the substrate specificity of soluble PeiR and demonstrated that the enzyme is capable of hydrolysing the pseudomurein cell walls of methanogens. Subsequently, peiR was fused to the polyhydroxyalkanoate (PHA) synthase gene phaC and displayed on the surface of PHA bionanoparticles (BNPs) expressed in Eschericia coli via one-step biosynthesis. These tailored BNPs were capable of lysing not only the original methanogen host strain, but a wide range of other rumen methanogen strains in vitro. Methane production was reduced by up to 97% for 5 days post-inoculation in the in vitro assay. We propose that tailored BNPs carrying anti-methanogen enzymes represent a new class of methane inhibitors. Tailored BNPs can be rapidly developed and may be able to modulate the methanogen community in vivo with the aim to lower ruminant methane emissions without impacting animal productivity.

Structural determination of archaeal UDP-N-acetylglucosamine 4-epimerase from Methanobrevibacter ruminantium M1 in complex with the bacterial cell wall intermediate UDP-N-acetylmuramic acid.

The crystal structure of UDP-N-acetylglucosamine 4-epimerase (UDP-GlcNAc 4-epimerase; WbpP; EC 5.1.3.7), from the archaeal methanogen Methanobrevibacter ruminantium strain M1, was determined to a resolution of 1.65 Å. The structure, with a single monomer in the crystallographic asymmetric unit, contained a conserved N-terminal Rossmann-fold for nucleotide binding and an active site positioned in the C-terminus. UDP-GlcNAc 4-epimerase is a member of the short-chain dehydrogenases/reductases superfamily, sharing sequence motifs and structural elements characteristic of this family of oxidoreductases and bacterial 4-epimerases. The protein was co-crystallized with coenzyme NADH and UDP-N-acetylmuramic acid, the latter an unintended inclusion and well known product of the bacterial enzyme MurB and a critical intermediate for bacterial cell wall synthesis. This is a non-native UDP sugar amongst archaea and was most likely incorporated from the E. coli expression host during purification of the recombinant enzyme.

Expression, Purification, and Characterization of (R)-Sulfolactate Dehydrogenase (ComC) from the Rumen Methanogen Methanobrevibacter millerae SM9.

(R)-Sulfolactate dehydrogenase (EC 1.1.1.337), termed ComC, is a member of an NADH/NADPH-dependent oxidoreductase family of enzymes that catalyze the interconversion of 2-hydroxyacids into their corresponding 2-oxoacids. The ComC reaction is reversible and in the biosynthetic direction causes the conversion of (R)-sulfolactate to sulfopyruvate in the production of coenzyme M (2-mercaptoethanesulfonic acid). Coenzyme M is an essential cofactor required for the production of methane by the methyl-coenzyme M reductase complex. ComC catalyzes the third step in the first established biosynthetic pathway of coenzyme M and is also involved in methanopterin biosynthesis. In this study, ComC from Methanobrevibacter millerae SM9 was cloned and expressed in Escherichia coli and biochemically characterized. Sulfopyruvate was the preferred substrate using the reduction reaction, with 31% activity seen for oxaloacetate and 0.2% seen for α-ketoglutarate. Optimal activity was observed at pH 6.5. The apparent KM for coenzyme (NADH) was 55.1 µM, and for sulfopyruvate, it was 196 µM (for sulfopyruvate the Vmax was 93.9 µmol min-1 mg-1 and kcat was 62.8 s-1). The critical role of ComC in two separate cofactor pathways makes this enzyme a potential means of developing methanogen-specific inhibitors for controlling ruminant methane emissions which are increasingly being recognized as contributing to climate change.

A review on comparative mechanistic studies of antimicrobial peptides against archaea.

Archaea was until recently considered as a third domain of life in addition to bacteria and eukarya but recent studies support the existence of only two superphyla (bacteria and archaea). The fundamental differences between archaeal, bacterial, and eukaryal cells are probably the main reasons for the comparatively lower susceptibility of archaeal strains to current antimicrobial agents. The possible emerging pathogenicity of archaea and the role of archaeal methanogens in methane emissions, a potent greenhouse gas, has led many researchers to examine the sensitivity patterns of archaea and make attempts to find agents that have significant anti-archaeal activity. Even though antimicrobial peptides (AMPs) are well known with several published reviews concerning their mode of action against bacteria and eukarya, to our knowledge, to date no reviews are available that focus on the action of these peptides against archaea. Herein, we present a review on all the peptides that have been tested against archaea. In addition, in an attempt to shed more light on possible future work that needs to be performed we have included a brief overview of the chemical characteristics, spectrum of activity, and the known mechanism of action of each of these peptides against bacteria and/or fungi. We also discuss the nature of and key physiological differences between Archaea, Bacteria, and Eukarya that are relevant to the development of anti-archaeal peptides. Despite our relatively limited knowledge about archaea, available data suggest that AMPs have an even broader spectrum of activity than currently recognized.

The Structural and Functional Characterization of Mammalian ADP-dependent Glucokinase.

The enzyme-catalyzed phosphorylation of glucose to glucose-6-phosphate is a reaction central to the metabolism of all life. ADP-dependent glucokinase (ADPGK) catalyzes glucose-6-phosphate production, utilizing ADP as a phosphoryl donor in contrast to the more well characterized ATP-requiring hexokinases. ADPGK is found in Archaea and metazoa; in Archaea, ADPGK participates in a glycolytic role, but a function in most eukaryotic cell types remains unknown. We have determined structures of the eukaryotic ADPGK revealing a ribokinase-like tertiary fold similar to archaeal orthologues but with significant differences in some secondary structural elements. Both the unliganded and the AMP-bound ADPGK structures are in the "open" conformation. The structures reveal the presence of a disulfide bond between conserved cysteines that is positioned at the nucleotide-binding loop of eukaryotic ADPGK. The AMP-bound ADPGK structure defines the nucleotide-binding site with one of the disulfide bond cysteines coordinating the AMP with its main chain atoms, a nucleotide-binding motif that appears unique to eukaryotic ADPGKs. Key amino acids at the active site are structurally conserved between mammalian and archaeal ADPGK, and site-directed mutagenesis has confirmed residues essential for enzymatic activity. ADPGK is substrate inhibited by high glucose concentration and shows high specificity for glucose, with no activity for other sugars, as determined by NMR spectroscopy, including 2-deoxyglucose, the glucose analogue used for tumor detection by positron emission tomography.

Biochemical Characterisation of Phage Pseudomurein Endoisopeptidases PeiW and PeiP Using Synthetic Peptides.

Pseudomurein endoisopeptidases cause lysis of the cell walls of methanogens by cleaving the isopeptide bond Ala-ε-Lys in the peptide chain of pseudomurein. PeiW and PeiP are two thermostable pseudomurein endoisopeptidases encoded by phage ΨM100 of Methanothermobacter wolfei and phages ΨM1 and ΨM2 of Methanothermobacter marburgensis, respectively. A continuous assay using synthetic peptide substrates was developed and used in the biochemical characterisation of recombinant PeiW and PeiP. The advantages of these synthetic peptide substrates over natural substrates are sensitivity, high purity, and characterisation and the fact that they are more easily obtained than natural substrates. In the presence of a reducing agent, purified PeiW and PeiP each showed similar activity under aerobic and anaerobic conditions. Both enzymes required a divalent metal for activity and showed greater thermostability in the presence of Ca(2+). PeiW and PeiP involve a cysteine residue in catalysis and have a monomeric native conformation. The kinetic parameters, K(M) and k(cat), were determined, and the ε-isopeptide bond between alanine and lysine was confirmed as the bond lysed by these enzymes in pseudomurein. The new assay may have wider applications for the general study of peptidases and the identification of specific methanogens susceptible to lysis by specific pseudomurein endoisopeptidases.

Structure and Evolution of the Archaeal Lipid Synthesis Enzyme sn-Glycerol-1-phosphate Dehydrogenase.

One of the most critical events in the origins of cellular life was the development of lipid membranes. Archaea use isoprenoid chains linked via ether bonds to sn-glycerol 1-phosphate (G1P), whereas bacteria and eukaryotes use fatty acids attached via ester bonds to enantiomeric sn-glycerol 3-phosphate. NAD(P)H-dependent G1P dehydrogenase (G1PDH) forms G1P and has been proposed to have played a crucial role in the speciation of the Archaea. We present here, to our knowledge, the first structures of archaeal G1PDH from the hyperthermophilic methanogen Methanocaldococcus jannaschii with bound substrate dihydroxyacetone phosphate, product G1P, NADPH, and Zn(2+) cofactor. We also biochemically characterized the enzyme with respect to pH optimum, cation specificity, and kinetic parameters for dihydroxyacetone phosphate and NAD(P)H. The structures provide key evidence for the reaction mechanism in the stereospecific addition for the NAD(P)H-based pro-R hydrogen transfer and the coordination of the Zn(2+) cofactor during catalysis. Structure-based phylogenetic analyses also provide insight into the origins of G1PDH.

A high-throughput screening assay for identification of inhibitors of the A1AO-ATP synthase of the rumen methanogen Methanobrevibacter ruminantium M1.

We report the development of a high-throughput screening platform to identify inhibitors of the membrane-bound A1Ao-ATP synthase from the rumen methanogen Methanobrevibacter ruminantium M1. Inhibitors identified in the screen were tested against growing cultures of M. ruminantium, validating the approach to identify new inhibitors of methanogens.

The crystal structure of methenyltetrahydromethanopterin cyclohydrolase from Methanobrevibacter ruminantium.

Methenyltetrahydromethanopterin cyclohydrolase (Mch) is involved in the methanogenesis pathway of archaea as a C1 unit carrier where N(5) -formyl-tetrahydromethanopterin is converted to methenyl-tetrahydromethanopterin. Mch from Methanobrevibacter ruminantium was cloned, purified, crystallized and its crystal structure solved at 1.37 Å resolution. A biologically active trimer, the enzyme is composed of two domains including an N-terminal domain of six α-helices encompassing a series of four ß-sheets and a predominantly anti-parallel ß-sheet at the C-terminus flanked on one side by α-helices. Sequence and structural alignments have helped identify residues involved in substrate binding and trimer formation.

Zinc finger nuclease mediated knockout of ADP-dependent glucokinase in cancer cell lines: effects on cell survival and mitochondrial oxidative metabolism.

Zinc finger nucleases (ZFN) are powerful tools for editing genes in cells. Here we use ZFNs to interrogate the biological function of ADPGK, which encodes an ADP-dependent glucokinase (ADPGK), in human tumour cell lines. The hypothesis we tested is that ADPGK utilises ADP to phosphorylate glucose under conditions where ATP becomes limiting, such as hypoxia. We characterised two ZFN knockout clones in each of two lines (H460 and HCT116). All four clones had frameshift mutations in all alleles at the target site in exon 1 of ADPGK, and were ADPGK-null by immunoblotting. ADPGK knockout had little or no effect on cell proliferation, but compromised the ability of H460 cells to survive siRNA silencing of hexokinase-2 under oxic conditions, with clonogenic survival falling from 21±3% for the parental line to 6.4±0.8% (p = 0.002) and 4.3±0.8% (p = 0.001) for the two knockouts. A similar increased sensitivity to clonogenic cell killing was observed under anoxia. No such changes were found when ADPGK was knocked out in HCT116 cells, for which the parental line was less sensitive than H460 to anoxia and to hexokinase-2 silencing. While knockout of ADPGK in HCT116 cells caused few changes in global gene expression, knockout of ADPGK in H460 cells caused notable up-regulation of mRNAs encoding cell adhesion proteins. Surprisingly, we could discern no consistent effect on glycolysis as measured by glucose consumption or lactate formation under anoxia, or extracellular acidification rate (Seahorse XF analyser) under oxic conditions in a variety of media. However, oxygen consumption rates were generally lower in the ADPGK knockouts, in some cases markedly so. Collectively, the results demonstrate that ADPGK can contribute to tumour cell survival under conditions of high glycolytic dependence, but the phenotype resulting from knockout of ADPGK is cell line dependent and appears to be unrelated to priming of glycolysis in these lines.

Expression and role in glycolysis of human ADP-dependent glucokinase.

A novel murine enzyme, ADP-dependent glucokinase (ADPGK), has been shown to catalyse glucose phosphorylation using ADP as phosphoryl donor. The ancestral ADPGK gene appears to have been laterally transferred from Archaea early in metazoan evolution, but its biological role has not been established. Here, we undertake an initial investigation of the functional properties of human ADPGK in human tumour cell lines and specifically test the hypothesis that ADPGK might prime glycolysis using ADP under stress conditions such as hypoxia. Recombinant human ADPGK was confirmed to catalyse ADP-dependent glucose phosphorylation in vitro, with an apparent K (M) for glucose of 0.29 mM. Expression databases and western blotting of surgical samples demonstrated high expression in many human tissues, including tumours. Unlike hexokinase-2 (HK2), RNAi studies with exon arrays showed that ADPGK is not a transcriptional target of hypoxia inducible factor-1. Consistent with this, ADPGK protein was not upregulated by hypoxia or anoxia. Surprisingly, stable fivefold overexpression of ADPGK in H460 or HCT116 cells had no apparent effect on proliferation or glycolysis, and did not rescue clonogenicity or glycolysis when HK2 was suppressed by siRNA. Furthermore, suppression of ADPGK by siRNA did not cause detectable inhibition of glycolysis or cell killing by anoxia, although it did induce a statistically significant decrease in plating efficiency of H460 cells under aerobic conditions. Thus, human ADPGK catalyses ADP-dependent phosphorylation of glucose in vitro, but despite its high expression in human tumour cell lines it appears not to make a quantifiable contribution to glycolysis under the conditions evaluated.

A1Ao-ATP synthase of Methanobrevibacter ruminantium couples sodium ions for ATP synthesis under physiological conditions.

An unresolved question in the bioenergetics of methanogenic archaea is how the generation of proton-motive and sodium-motive forces during methane production is used to synthesize ATP by the membrane-bound A(1)A(o)-ATP synthase, with both proton- and sodium-coupled enzymes being reported in methanogens. To address this question, we investigated the biochemical characteristics of the A(1)A(o)-ATP synthase (MbbrA(1)A(o)) of Methanobrevibacter ruminantium M1, a predominant methanogen in the rumen. Growth of M. ruminantium M1 was inhibited by protonophores and sodium ionophores, demonstrating that both ion gradients were essential for growth. To study the role of these ions in ATP synthesis, the ahaHIKECFABD operon encoding the MbbrA(1)A(o) was expressed in Escherichia coli strain DK8 (Δatp) and purified yielding a 9-subunit protein with an SDS-stable c oligomer. Analysis of the c subunit amino acid sequence revealed that it consisted of four transmembrane helices, and each hairpin displayed a complete Na(+)-binding signature made up of identical amino acid residues. The purified MbbrA(1)A(o) was stimulated by sodium ions, and Na(+) provided pH-dependent protection against inhibition by dicyclohexylcarbodiimide but not tributyltin chloride. ATP synthesis in inverted membrane vesicles lacking sodium ions was driven by a membrane potential that was sensitive to cyanide m-chlorophenylhydrazone but not to monensin. ATP synthesis could not be driven by a chemical gradient of sodium ions unless a membrane potential was imposed. ATP synthesis under these conditions was sensitive to monensin but not cyanide m-chlorophenylhydrazone. These data suggest that the M. ruminantium M1 A(1)A(o)-ATP synthase exhibits all the properties of a sodium-coupled enzyme, but it is also able to use protons to drive ATP synthesis under conditions that favor proton coupling, such as low pH and low levels of sodium ions.

Methanogen community structure in the rumens of farmed sheep, cattle and red deer fed different diets.

Development of inhibitors and vaccines that mitigate rumen-derived methane by targeting methanogens relies on knowledge of the methanogens present. We investigated the composition of archaeal communities in the rumens of farmed sheep (Ovis aries), cattle (Bos taurus) and red deer (Cervus elaphus) using denaturing gradient gel electrophoresis (DGGE) to generate fingerprints of archaeal 16S rRNA genes. The total archaeal communities were relatively constant across species and diets, and were less variable and less diverse than bacterial communities. There were diet- and ruminant-species-based differences in archaeal community structure, but the same dominant archaea were present in all rumens. These were members of three coherent clades: species related to Methanobrevibacter ruminantium and Methanobrevibacter olleyae; species related to Methanobrevibacter gottschalkii, Methanobrevibacter thaueri and Methanobrevibacter millerae; and species of the genus Methanosphaera. Members of an archaeal group of unknown physiology, designated rumen cluster C (RCC), were also present. RCC-specific DGGE, clone library analysis and quantitative real-time PCR showed that their 16S rRNA gene sequences were very diverse and made up an average of 26.5% of the total archaea. RCC sequences were not readily detected in the DGGE patterns of total archaeal 16S rRNA genes because no single sequence type was abundant enough to form dominant bands.