Flavin-based electron bifurcation is a recently discovered mechanism of coupling endergonic to exergonic redox reactions in the cytoplasm of anaerobic bacteria and archaea. Among the five electron-bifurcating enzyme complexes characterized to date, one is a heteromeric ferredoxin- and NAD-dependent [FeFe]-hydrogenase. We report here a novel electron-bifurcating [FeFe]-hydrogenase that is NADP rather than NAD specific and forms a complex with a formate dehydrogenase. The complex was found in high concentrations (6% of the cytoplasmic proteins) in the acetogenic Clostridium autoethanogenum autotrophically grown on CO, which was fermented to acetate, ethanol, and 2,3-butanediol. The purified complex was composed of seven different subunits. As predicted from the sequence of the encoding clustered genes (fdhA/hytA-E) and from chemical analyses, the 78.8-kDa subunit (FdhA) is a selenocysteine- and tungsten-containing formate dehydrogenase, the 65.5-kDa subunit (HytB) is an iron-sulfur flavin mononucleotide protein harboring the NADP binding site, the 51.4-kDa subunit (HytA) is the [FeFe]-hydrogenase proper, and the 18.1-kDa (HytC), 28.6-kDa (HytD), 19.9-kDa (HytE1), and 20.1-kDa (HytE2) subunits are iron-sulfur proteins. The complex catalyzed both the reversible coupled reduction of ferredoxin and NADP+ with H2 or formate and the reversible formation of H2 and CO2 from formate. We propose the complex to have two functions in vivo, namely, to normally catalyze CO2 reduction to formate with NADPH and reduced ferredoxin in the Wood-Ljungdahl pathway and to catalyze H2 formation from NADPH and reduced ferredoxin when these redox mediators get too reduced during unbalanced growth of C. autoethanogenum on CO (E0′ = −520 mV).
Cell extracts of uric acid-grown Clostridium acidurici catalyzed the coupled reduction of NAD+ and ferredoxin with formate at a specific activity of 1.3 U/mg. The enzyme complex catalyzing the electron-bifurcating reaction was purified 130-fold and found to be composed of four subunits encoded by the gene cluster hylCBA-fdhF2.
Moorella thermoacetica was long the only model organism used to study the biochemistry of acetogenesis from CO2. Depending on the growth substrate, this Gram-positive bacterium can either form H2 or consume it. Despite the importance of H2 in its metabolism, a hydrogenase from the organism has not yet been characterized. We report here the purification and properties of an electron-bifurcating [FeFe]-hydrogenase from M. thermoacetica and show that the cytoplasmic enzyme efficiently catalyzes both H2 formation and H2 uptake. The purified heterotrimeric iron-sulfur flavoprotein (HydABC) catalyzed the coupled reduction of ferredoxin (Fd) and NAD+ with H2 at 55°C at pH 7.5 at a specific rate of about 100 μmol min−1 mg protein−1 and the reverse reaction, the coupled reduction of protons to H2 with reduced ferredoxin and NADH, at a specific rate of about 10 μmol min−1 mg protein−1 in the stoichiometry Fdox + NAD+ + 2H2 ⇋ Fdred2− + NADH + 3H+. When ferredoxin from Clostridium pasteurianum, NAD+, and the enzyme were incubated at pH 7.0 under 100% H2 in the gas phase (E0′ = −414 mV), more than 95% of the ferredoxin (E0′ = −400 mV) was reduced, which indicated that ferredoxin reduction with H2 is driven by the exergonic reduction of NAD+ (E0′ = −320 mV) with H2. In the absence of NAD+, ferredoxin was not reduced. We identified the genes encoding HydABC within the transcriptional unit hydCBAX and mapped the transcription start site.
Moorella thermoacetica ferments glucose to three acetic acids. In the oxidative part of the fermentation, the hexose is converted to 2 acetic acids and 2 CO2 molecules with the formation of 2 NADH and 2 reduced ferredoxin (Fdred2−) molecules. In the reductive part, 2 CO2 molecules are reduced to acetic acid, consuming the 8 reducing equivalents generated in the oxidative part. An open question is how the two parts are electronically connected, since two of the four oxidoreductases involved in acetogenesis from CO2 are NADP specific rather than NAD specific. We report here that the 2 NADPH molecules required for CO2 reduction to acetic acid are generated by the reduction of 2 NADP+ molecules with 1 NADH and 1 Fdred2− catalyzed by the electron-bifurcating NADH-dependent reduced ferredoxin:NADP+ oxidoreductase (NfnAB). The cytoplasmic iron-sulfur flavoprotein was heterologously produced in Escherichia coli, purified, and characterized. The purified enzyme was composed of 30-kDa (NfnA) and 50-kDa (NfnB) subunits in a 1-to-1 stoichiometry. NfnA harbors a [2Fe2S] cluster and flavin adenine dinucleotide (FAD), and NfnB harbors two [4Fe4S] clusters and FAD. M. thermoacetica contains a second electron-bifurcating enzyme. Cell extracts catalyzed the coupled reduction of NAD+ and Fd with 2 H2 molecules. The specific activity of this cytoplasmic enzyme was 3-fold higher in H2-CO2-grown cells than in glucose-grown cells. The function of this electron-bifurcating hydrogenase is not yet clear, since H2-CO2-grown cells additionally contain high specific activities of an NADP+-dependent hydrogenase that catalyzes the reduction of NADP+ with H2. This activity is hardly detectable in glucose-grown cells.
The hydrogenotrophic methanogens Methanothermobacter marburgensis and Methanothermobacter thermautotrophicus can easily be mass cultured. They have therefore been used almost exclusively to study the biochemistry of methanogenesis from H2 and CO2, and the genomes of these two model organisms have been sequenced. The close relationship of the two organisms is reflected in their genomic architecture and coding potential. Within the 1,607 protein coding sequences (CDS) in common, we identified approximately 200 CDS required for the synthesis of the enzymes, coenzymes, and prosthetic groups involved in CO2 reduction to methane and in coupling this process with the phosphorylation of ADP. Approximately 20 additional genes, such as those for the biosynthesis of F430 and methanofuran and for the posttranslational modifications of the two methyl-coenzyme M reductases, remain to be identified.
Synthesis of acetate from carbon dioxide and molecular hydrogen is considered to be the first carbon assimilation pathway on earth. It combines carbon dioxide fixation into acetyl-CoA with the production of ATP via an energized cell membrane. How the pathway is coupled with the net synthesis of ATP has been an enigma. The anaerobic, acetogenic bacterium Acetobacterium woodii uses an ancient version of this pathway without cytochromes and quinones. It generates a sodium ion potential across the cell membrane by the sodium-motive ferredoxin:NAD oxidoreductase (Rnf). The genome sequence of A. woodii solves the enigma: it uncovers Rnf as the only ion-motive enzyme coupled to the pathway and unravels a metabolism designed to produce reduced ferredoxin and overcome energetic barriers by virtue of electron-bifurcating, soluble enzymes.
The circular genome sequence of the chemolithoautotrophic euryarchaeon Methanothermobacter marburgensis, with 1,639,135 bp, was determined and compared with that of Methanothermobacter thermautotrophicus. The genomes of the two model methanogens differ substantially in protein coding sequences, in insertion sequence (IS)-like elements, and in clustered regularly interspaced short palindromic repeats (CRISPR) loci.
It was recently found that the cytoplasmic butyryl-coenzyme A (butyryl-CoA) dehydrogenase-EtfAB complex from Clostridium kluyveri couples the exergonic reduction of crotonyl-CoA to butyryl-CoA with NADH and the endergonic reduction of ferredoxin with NADH via flavin-based electron bifurcation. We report here on a second cytoplasmic enzyme complex in C. kluyveri capable of energetic coupling via this novel mechanism. It was found that the purified iron-sulfur flavoprotein complex NfnAB couples the exergonic reduction of NADP+ with reduced ferredoxin (Fdred) and the endergonic reduction of NADP+ with NADH in a reversible reaction: Fdred2− + NADH + 2 NADP+ + H+ = Fdox + NAD+ + 2 NADPH. The role of this energy-converting enzyme complex in the ethanol-acetate fermentation of C. kluyveri is discussed.
[Fe] hydrogenase (iron–sulfur-cluster-free hydrogenase) catalyzes the reversible reduction of methenyltetrahydromethanopterin (methenyl-H4MPT+) with H2 to methylene-H4MPT, a reaction involved in methanogenesis from H2 and CO2 in many methanogenic archaea. The enzyme harbors an iron-containing cofactor, in which a low-spin iron is complexed by a pyridone, two CO and a cysteine sulfur. [Fe] hydrogenase is thus similar to [NiFe] and [FeFe] hydrogenases, in which a low-spin iron carbonyl complex, albeit in a dinuclear metal center, is also involved in H2 activation. Like the [NiFe] and [FeFe] hydrogenases, [Fe] hydrogenase catalyzes an active exchange of H2 with protons of water; however, this activity is dependent on the presence of the hydride-accepting methenyl-H4MPT+. In its absence the exchange activity is only 0.01% of that in its presence. The residual activity has been attributed to the presence of traces of methenyl-H4MPT+ in the enzyme preparations, but it could also reflect a weak binding of H2 to the iron in the absence of methenyl-H4MPT+. To test this we reinvestigated the exchange activity with [Fe] hydrogenase reconstituted from apoprotein heterologously produced in Escherichia coli and highly purified iron-containing cofactor and found that in the absence of added methenyl-H4MPT+ the exchange activity was below the detection limit of the tritium method employed (0.1 nmol min−1 mg−1). The finding reiterates that for H2 activation by [Fe] hydrogenase the presence of the hydride-accepting methenyl-H4MPT+ is essentially required. This differentiates [Fe] hydrogenase from [FeFe] and [NiFe] hydrogenases, which actively catalyze H2/H2O exchange in the absence of exogenous electron acceptors.
Hydrogenase; Exchange reactions; Methanogenic archaea
We have used 57Fe nuclear resonance vibrational spectroscopy (NRVS) to study the iron site in the iron-sulfur-cluster-free hydrogenase Hmd from the methanogenic archaeon Methanothermobacter marburgensis. The spectra have been interpreted by comparison with a cis-(CO)2-ligated Fe model compound, Fe(S2C2H4)(CO)2(PMe3)2, as well as by normal mode simulations of plausible active site structures. For this model complex, normal mode analyses both from an optimized Urey-Bradley force field and from complementary density functional theory (DFT) calculations produced consistent results.
Previous IR spectroscopic studies found strong CO stretching modes at 1944 and 2011 cm−1, interpreted as evidence for cis-Fe(CO)2 ligation. The NRVS data provide further insight into the dynamics of the Fe site, revealing Fe-CO stretch and Fe-CO bend modes at 494, 562, 590, and 648 cm−1, consistent with the proposed cis-Fe(CO)2 ligation. The NRVS also reveals a band assigned to Fe-S stretching motion at ~311 cm−1, and another reproducible feature at ~380 cm−1. The 57Fe partial vibrational densities of states (PVDOS) for Hmd can be reasonably well simulated by a normal mode analysis based on a Urey-Bradley force field for a 5-coordinate cis-(CO)2-ligated Fe site with additional cysteine, water, and pyridone cofactor ligands. A final interpretation of the Hmd NRVS data, including DFT analysis, awaits a 3-dimensional structure for the active site.
hydrogenase; Hmd; 57Fe; nuclear resonant vibrational spectroscopy; NRVS; Mössbauer; synchrotron radiation; resonance Raman; normal mode; density functional theory; DFT
At the invitation of Govindjee, we reprint here the English translation of the letter, in German, that we sent, on behalf of the Senate and the Presidium as well as the members of the German Academy of Sciences Leopoldina, from Halle (Saale), to Professor Dr. Dr. h.c.mult. Achim Trebst on his 80th birthday. The original of this letter written in German will appear in Jahrbuch 2009, Deutsche Akademie der Naturforscher Leopoldina, Halle (Saale), Wissenschaftliche Verlagsgesellschaft mbH Stuttgart.
Otto Warburg; DBMIB; Artificial energy conservation; Herbicides; Singlet oxygen; Trebst-Tsujimoto-Arnon experiment; Light and dark phases of photosynthesis; Inhibitors of photosynthesis
Cell extracts of butyrate-forming clostridia have been shown to catalyze acetyl-coenzyme A (acetyl-CoA)- and ferredoxin-dependent formation of H2 from NADH. It has been proposed that these bacteria contain an NADH:ferredoxin oxidoreductase which is allosterically regulated by acetyl-CoA. We report here that ferredoxin reduction with NADH in cell extracts from Clostridium kluyveri is catalyzed by the butyryl-CoA dehydrogenase/Etf complex and that the acetyl-CoA dependence previously observed is due to the fact that the cell extracts catalyze the reduction of acetyl-CoA with NADH via crotonyl-CoA to butyryl-CoA. The cytoplasmic butyryl-CoA dehydrogenase complex was purified and is shown to couple the endergonic reduction of ferredoxin (E0′ = −410 mV) with NADH (E0′ = −320 mV) to the exergonic reduction of crotonyl-CoA to butyryl-CoA (E0′ = −10 mV) with NADH. The stoichiometry of the fully coupled reaction is extrapolated to be as follows: 2 NADH + 1 oxidized ferredoxin + 1 crotonyl-CoA = 2 NAD+ + 1 ferredoxin reduced by two electrons + 1 butyryl-CoA. The implications of this finding for the energy metabolism of butyrate-forming anaerobes are discussed in the accompanying paper.
The synthesis of citrate from acetyl-coenzyme A and oxaloacetate is catalyzed in most organisms by a Si-citrate synthase, which is Si-face stereospecific with respect to C-2 of oxaloacetate. However, in Clostridium kluyveri and some other strictly anaerobic bacteria, the reaction is catalyzed by a Re-citrate synthase, whose primary structure has remained elusive. We report here that Re-citrate synthase from C. kluyveri is the product of a gene predicted to encode isopropylmalate synthase. C. kluyveri is also shown to contain a gene for Si-citrate synthase, which explains why cell extracts of the organism always exhibit some Si-citrate synthase activity.
The methanogenic archaeon Methanosarcina barkeri synthesizes protoheme via precorrin-2, which is formed from uroporphyrinogen III in two consecutive methylation reactions utilizing S-adenosyl-l-methionine. The existence of this pathway, previously exclusively found in the sulfate-reducing δ-proteobacterium Desulfovibrio vulgaris, was demonstrated for M. barkeri via the incorporation of two methyl groups from methionine into protoheme.
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 H2 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/acetyl-coenzyme A synthase complex, which explains why M. stadtmanae cannot reduce CO2 to methane or oxidize methanol to CO2 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. Seventy-three 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.
Recently it was reported that methanogens of the genus Methanobrevibacter exhibit catalase activity. This was surprising, since Methanobrevibacter species belong to the order Methanobacteriales, which are known not to contain cytochromes and to lack the ability to synthesize heme. We report here that Methanobrevibacter arboriphilus strains AZ and DH1 contained catalase activity only when the growth medium was supplemented with hemin. The heme catalase was purified and characterized, and the encoding gene was cloned. The amino acid sequence of the catalase from the methanogens is most similar to that of Methanosarcina barkeri.
Formaldehyde is toxic for all organisms from bacteria to humans due to its reactivity with biological macromolecules. Organisms that grow aerobically on single-carbon compounds such as methanol and methane face a special challenge in this regard because formaldehyde is a central metabolic intermediate during methylotrophic growth. In the α-proteobacterium Methylobacterium extorquens AM1, we found a previously unknown enzyme that efficiently catalyzes the removal of formaldehyde: it catalyzes the condensation of formaldehyde and tetrahydromethanopterin to methylene tetrahydromethanopterin, a reaction which also proceeds spontaneously, but at a lower rate than that of the enzyme-catalyzed reaction. Formaldehyde-activating enzyme (Fae) was purified from M. extorquens AM1 and found to be one of the major proteins in the cytoplasm. The encoding gene is located within a cluster of genes for enzymes involved in the further oxidation of methylene tetrahydromethanopterin to CO2. Mutants of M. extorquens AM1 defective in Fae were able to grow on succinate but not on methanol and were much more sensitive toward methanol and formaldehyde. Uncharacterized orthologs to this enzyme are predicted to be encoded by uncharacterized genes from archaea, indicating that this type of enzyme occurs outside the methylotrophic bacteria.
The methylotrophic proteobacterium Methylobacterium extorquens AM1 possesses tetrahydromethanopterin (H4MPT)-dependent enzymes, which are otherwise specific to methanogenic and sulfate-reducing archaea and which have been suggested to be involved in formaldehyde oxidation to CO2 in M. extorquens AM1. The distribution of H4MPT-dependent enzyme activities in cell extracts of methylotrophic bacteria from 13 different genera are reported. H4MPT-dependent activities were detected in all of the methylotrophic and methanotrophic proteobacteria tested that assimilate formaldehyde by the serine or ribulose monophosphate pathway. H4MPT-dependent activities were also found in autotrophic Xanthobacter strains. However, no H4MPT-dependent enzyme activities could be detected in other autotrophic α-proteobacteria or in gram-positive methylotrophic bacteria. Genes encoding methenyl H4MPT cyclohydrolase (mch genes) were cloned and sequenced from several proteobacteria. Bacterial and archaeal Mch sequences have roughly 35% amino acid identity and form distinct groups in phylogenetic analysis.
An NADP-dependent methylene tetrahydromethanopterin (H4MPT) dehydrogenase has recently been proposed to be involved in formaldehyde oxidation to CO2 in Methylobacterium extorquens AM1. We report here on the purification of this novel enzyme to apparent homogeneity. Via the N-terminal amino acid sequence, it was identified to be the mtdA gene product. The purified enzyme catalyzed the dehydrogenation of methylene H4MPT with NADP+ rather than with NAD+, with a specific activity of approximately 400 U/mg of protein. It also catalyzed the dehydrogenation of methylene tetrahydrofolate (methylene H4F) with NADP+. With methylene H4F as the substrate, however, the specific activity (26 U/mg) and the catalytic efficiency (Vmax/Km) were approximately 20-fold lower than with methylene H4MPT. Whereas the dehydrogenation of methylene H4MPT (E0 = −390 mV) with NADP+ (E0 = −320 mV) proceeded essentially irreversibly, the dehydrogenation of methylene H4F (E0 = −300 mV) was fully reversible. Comparison of the primary structure of the NADP-dependent dehydrogenase from M. extorquens AM1 with those of methylene H4F dehydrogenases from other bacteria and eucarya and with those of methylene H4MPT dehydrogenases from methanogenic archaea revealed only marginally significant similarity (<15%).
Cultures of Clostridium formicoaceticum and C. thermoaceticum growing on fructose and glucose, respectively, were shown to rapidly oxidize CO to CO2. Rates up to 0.4 μmol min−1 mg of wet cells−1 were observed. Carbon monoxide oxidation by cell suspensions was found (i) to be dependent on pyruvate, (ii) to be inhibited by alkyl halides and arsenate, and (iii) to stimulate CO2 reduction to acetate. Cell extracts catalyzed the oxidation of carbon monoxide with methyl viologen at specific rates up to 10 μmol min−1 mg of protein−1 (35°C, pH 7.2). Nicotinamide adenine dinucleotide, nicotinamide adenine dinucleotide phosphate and ferredoxin from C. pasteurianum were ineffective as electron acceptors. The catalytic mechanism of carbon monoxide oxidation was “ping-pong,” indicating that the enzyme catalyzing carbon monoxide oxidation can be present in an oxidized and a reduced form. The oxidized form was shown to react reversibly with cyanide, and the reduced form was shown to react reversibly with alkyl halides: cyanide inactivated the enzyme only in the absence of carbon monoxide, and alkyl halides inactivated it only in the presence of carbon monoxide. Extracts inactivated by alkyl halides were reactivated by photolysis. The findings are interpreted to indicate that carbon monoxide oxidation in the two bacteria is catalyzed by a corrinoid enzyme and that in vivo the reaction is coupled with the reduction of CO2 to acetate. Cultures of C. acidi-urici and C. cylindrosporum growing on hypoxanthine were found not to oxidize CO, indicating that clostridia mediating a corrinoid-independent total synthesis of acetate from CO2 do not possess a CO-oxidizing system.