Carbon monoxide:methylene blue oxidoreductase from Pseudomonas carboxydovorans

Microbial oxidation of atmospheric trace gases

The atmosphere has recently been recognized as a major source of energy sustaining life. Diverse aerobic bacteria oxidize the three most abundant reduced trace gases in the atmosphere, namely hydrogen (H₂), carbon monoxide (CO) and methane (CH₄). This Review describes the taxonomic distribution, physiological role and biochemical basis of microbial oxidation of these atmospheric trace gases, as well as the ecological, environmental, medical and astrobiological importance of this process. Most soil bacteria and some archaea can survive by using atmospheric H₂ and CO as alternative energy sources, as illustrated through genetic studies on Mycobacterium cells and Streptomyces spores. Certain specialist bacteria can also grow on air alone, as confirmed by the landmark characterization of Methylocapsa gorgona, which grows by simultaneously consuming atmospheric CH₄, H₂ and CO. Bacteria use high-affinity lineages of metalloenzymes, namely hydrogenases, CO dehydrogenases and methane monooxygenases, to utilize atmospheric trace gases for aerobic respiration and carbon fixation. More broadly, trace gas oxidizers enhance the biodiversity and resilience of soil and marine ecosystems, drive primary productivity in extreme environments such as Antarctic desert soils and perform critical regulatory services by mitigating anthropogenic emissions of greenhouse gases and toxic pollutants.

Exploiting Aerobic Carboxydotrophic Bacteria for Industrial Biotechnology

Aerobic carboxydotrophic bacteria are a group of microorganisms which possess the unique trait to oxidize carbon monoxide (CO) as sole energy source with molecular oxygen (O₂) to produce carbon dioxide (CO₂) which subsequently is used for biomass formation via the Calvin-Benson-Bassham cycle. Moreover, most carboxydotrophs are also able to oxidize hydrogen (H₂) with hydrogenases to drive the reduction of carbon dioxide in the absence of CO. As several abundant industrial off-gases contain significant amounts of CO, CO₂, H₂ as well as O₂, these bacteria come into focus for industrial application to produce chemicals and fuels from such gases in gas fermentation approaches. Since the group of carboxydotrophic bacteria is rather unknown and not very well investigated, we will provide an overview about their lifestyle and the underlying metabolic characteristics, introduce promising members for industrial application, and give an overview of available genetic engineering tools. We will point to limitations and discuss challenges, which have to be overcome to apply metabolic engineering approaches and to utilize aerobic carboxydotrophs in the industrial environment.

Role of Carbon Monoxide in Host-Gut Microbiome Communication

Nature is full of examples of symbiotic relationships. The critical symbiotic relation between host and mutualistic bacteria is attracting increasing attention to the degree that the gut microbiome is proposed by some as a new organ system. The microbiome exerts its systemic effect through a diverse range of metabolites, which include gaseous molecules such as H₂, CO₂, NH₃, CH₄, NO, H₂S, and CO. In turn, the human host can influence the microbiome through these gaseous molecules as well in a reciprocal manner. Among these gaseous molecules, NO, H₂S, and CO occupy a special place because of their widely known physiological functions in the host and their overlap and similarity in both targets and functions. The roles that NO and H₂S play have been extensively examined by others. Herein, the roles of CO in host-gut microbiome communication are examined through a discussion of (1) host production and function of CO, (2) available CO donors as research tools, (3) CO production from diet and bacterial sources, (4) effect of CO on bacteria including CO sensing, and (5) gut microbiome production of CO. There is a large amount of literature suggesting the "messenger" role of CO in host-gut microbiome communication. However, much more work is needed to begin achieving a systematic understanding of this issue.

The oxidation-reduction and electrocatalytic properties of CO dehydrogenase from Oligotropha carboxidovorans

CO dehydrogenase (CODH) from the Gram-negative bacterium Oligotropha carboxidovorans is a complex metalloenzyme from the xanthine oxidase family of molybdenum-containing enzymes, bearing a unique binuclear Mo-S-Cu active site in addition to two [2Fe-2S] clusters (FeSI and FeSII) and one equivalent of FAD. CODH catalyzes the oxidation of CO to CO₂ with the concomitant introduction of reducing equivalents into the quinone pool, thus enabling the organism to utilize CO as sole source of both carbon and energy. Using a variety of EPR monitored redox titrations and spectroelectrochemistry, we report the redox potentials of CO dehydrogenase at pH 7.2 namely Mo^VI/V, Mo^V/IV, FeSI^2+/+, FeSII^2+/+, FAD/FADH and FADH/FADH^-. These potentials are systematically higher than the corresponding potentials seen for other members of the xanthine oxidase family of Mo enzymes, and are in line with CODH utilising the higher potential quinone pool as an electron acceptor instead of pyridine nucleotides. CODH is also active when immobilised on a modified Au working electrode as demonstrated by cyclic voltammetry in the presence of CO.

Revisiting the catalytic mechanism of Mo-Cu carbon monoxide dehydrogenase using QM/MM and DFT calculations

Previous density functional theory (DFT) studies have shown that the release of the produced carbon dioxide (CO2) from an active-site cluster is a thermodynamically or kinetically difficult step in the enzymatic carbon monoxide (CO) oxidation catalyzed by Mo-Cu carbon monoxide dehydrogenase (Mo-Cu CODH). To better understand the effect of the protein environment on this difficult CO2 release step as well as other reaction steps, we applied hybrid quantum mechanics and molecular mechanics (QM/MM) calculations to the Mo-Cu CODH enzyme. The results show that in the first step, the equatorial Mo[double bond, length as m-dash]O group in the active-site cluster attacks the nearby CO molecule bound to the Cu site. Afterward, a stable thiocarbonate intermediate is formed in which the CO2 molecule is embedded and the copper-S(μ-sulfido) bond is broken. A free CO2 molecule, i.e., the final product, is then released from the active-site cluster, not directly from the thiocarbonate intermediate but via a previously formed intermediate that also contains CO2 but retains the Cu-S(μ-sulfido) bond. In contrast to the previous DFT results, the calculated barrier for this process was low in our QM/MM calculations. An additional QM/MM analysis of the barrier height showed that the effect of the protein environment on this barrier lowering is not very large. We found that the reason for the low barrier obtained by QM/MM is that the barrier for CO2 release is already not high at the DFT level. These results allow us to conclude that the CO oxidation reaction passes through the formation of a thiocarbonate intermediate, and that the subsequent CO2 release is kinetically not difficult. Nevertheless, the protein environment has an important role to play in making the latter process thermodynamically favored. No low-barrier pathway for the product release could be obtained for the reaction of n-butylisocyanide, which is consistent with the experimental fact that n-butylisocyanide inhibits Mo-Cu CODH.

The hydrogenase activity of the molybdenum/copper-containing carbon monoxide dehydrogenase of Oligotropha carboxidovorans

The reaction of the air-tolerant CO dehydrogenase from Oligotropha carboxidovorans with H2 has been examined. Like the Ni-Fe CO dehydrogenase, the enzyme can be reduced by H2 with a limiting rate constant of 5.3 s(-1) and a dissociation constant Kd of 525 μM; both kred and kred/Kd, reflecting the breakdown of the Michaelis complex and the reaction of free enzyme with free substrate in the low [S] regime, respectively, are largely pH-independent. During the reaction with H2, a new EPR signal arising from the Mo/Cu-containing active site of the enzyme is observed which is distinct from the signal seen when the enzyme is reduced by CO, with greater g anisotropy and larger hyperfine coupling to the active site (63,65)Cu. The signal also exhibits hyperfine coupling to at least two solvent-exchangeable protons of bound substrate that are rapidly exchanged with solvent. Proton coupling is also evident in the EPR signal seen with the dithionite-reduced native enzyme, and this coupling is lost in the presence of bicarbonate. We attribute the coupled protons in the dithionite-reduced enzyme to coordinated water at the copper site in the native enzyme and conclude that bicarbonate is able to displace this water from the copper coordination sphere. On the basis of our results, a mechanism for H2 oxidation is proposed which involves initial binding of H2 to the copper of the binuclear center, displacing the bound water, followed by sequential deprotonation through a copper-hydride intermediate to reduce the binuclear center.

Microbiology and genetics of CO utilization in mycobacteria

Although extensive studies on the oxidation of carbon monoxide (CO) in aerobic carboxydotrophic bacteria have been carried out for over 30 years, utilization of CO as a source of carbon and energy by mycobacteria was recognized only recently. Studies on pathogenic and nonpathogenic mycobacteria have revealed that the basis for CO utilization in these bacteria is different in many aspects from that of other aerobic carboxydobacteria. We review the basis for CO utilization in mycobacterial carboxydobacteria, which is unique from physiological, biochemical, molecular, genetic and phylogenetic points of view.

Reaction of the molybdenum- and copper-containing carbon monoxide dehydrogenase from Oligotropha carboxydovorans with quinones

Carbon monoxide dehydrogenase (CODH) from Oligotropha carboxydovorans catalyzes the oxidation of carbon monoxide to carbon dioxide, providing the organism both a carbon source and energy for growth. In the oxidative half of the catalytic cycle, electrons gained from CO are ultimately passed to the electron transport chain of the Gram-negative organism, but the proximal acceptor of reducing equivalents from the enzyme has not been established. Here we investigate the reaction of the reduced enzyme with various quinones and find them to be catalytically competent. Benzoquinone has a k(ox) of 125.1 s(-1) and a K(d) of 48 μM. Ubiquinone-1 has a k(ox)/K(d) value of 2.88 × 10(5) M(-1) s(-1). 1,4-Naphthoquinone has a k(ox) of 38 s(-1) and a K(d) of 140 μM. 1,2-Naphthoquinone-4-sulfonic acid has a k(ox)/K(d) of 1.31 × 10(5) M(-1) s(-1). An extensive effort to identify a cytochrome that could be reduced by CO/CODH was unsuccessful. Steady-state studies with benzoquinone indicate that the rate-limiting step is in the reductive half of the reaction (that is, the reaction of oxidized enzyme with CO). On the basis of the inhibition of CODH by diphenyliodonium chloride, we conclude that quinone substrates interact with CODH at the enzyme's flavin site. Our results strongly suggest that CODH donates reducing equivalents directly to the quinone pool without using a cytochrome as an intermediary.

Role of carboxydobacteria in consumption of atmospheric carbon monoxide by soil

The carbon monoxide consumption rates of the carboxydobacteria Pseudomonas (Seliberia) carboxydohydrogena, P. carboxydovorans, and P. carboxydoflava were measured at high (50%) and low (0.5 mul liter) mixing ratios of CO in air. CO was only consumed when the bacteria had been grown under CO-autotrophic conditions. As an exception, P. carboxydoflava consumed CO also after heterotrophic growth on pyruvate. At low cell densities the CO consumption rates measured at low CO mixing ratios were similar in cell suspensions and in mixtures of bacteria in soil. CO consumption observed in natural soil (loess, eolian sand, chernozem) as well as in suspensions or soil mixtures of carboxydobacteria showed Michaelis-Menten kinetics. The K(m) values for CO of the carboxydobacteria (K(m) = 465 to 1,110 mul of CO liter) were much higher than those of the natural soils (K(m) = 5 to 8 mul of CO liter). Considering the difference of the K(m) values and the observed V(max) values, carboxydobacteria cannot contribute significantly to the consumption of atmospheric CO.

Kinetic and spectroscopic studies of the molybdenum-copper CO dehydrogenase from Oligotropha carboxidovorans

Carbon monoxide dehydrogenase from the aerobic bacterium Oligotropha carboxidovorans catalyzes the oxidation of CO to CO(2), yielding two electrons and two H(+). The steady-state kinetics of the enzyme exhibit a pH optimum of 7.2 with a k(cat) of 93.3 s(-1) and K(m) of 10.7 microM at 25 degrees C. k(red) for the reductive half-reaction agrees well with k(cat) and exhibits a similar pH optimum, indicating that the rate-limiting step of overall turnover is likely in the reductive half-reaction. No dependence on CO concentration was observed in the rapid reaction kinetics, however, suggesting that CO initially binds rapidly to the enzyme, possibly at the Cu(I) of the active site, prior to undergoing oxidation. A Mo(V) species that exhibits strong coupling to the copper of the active center (I = 3/2) has been characterized by EPR. The signal is further split when [(13)C]CO is used to generate it, demonstrating that substrate (or product) is a component of the signal-giving species. Finally, resonance Raman spectra of CODH reveal the presence of FAD, Fe/S clusters, and a [CuSMoO(2)] coordination in the active site, consistent with earlier x-ray absorption and crystallographic results.

The role of Se, Mo and Fe in the structure and function of carbon monoxide dehydrogenase

CO dehydrogenase (EC 1.2.99.2) catalyzes the oxidation of CO according to the following equation: CO + H2O-->CO2 + 2 e- + 2 H+. It is a selenium-containing molybdo-iron-sulfur-flavoenzyme, which has been crystallized and structurally characterized in its oxidized state from the aerobic CO utilizing bacteria Oligotropha carboxidovorans and Hydrogenophaga pseudoflava. Both CO dehydrogenase structures show only minor differences, and the enzymes are dimers of two heterotrimers. Each heterotrimer is composed of a molybdoprotein, a flavoprotein, and an iron-sulfur protein. CO oxidation takes place at the molybdoprotein which contains a 1:1 mononuclear complex of molybdopterin-cytosine dinucleotide and a Mo-ion, along with a catalytically essential S-selanylcysteine. The latter is appropriately positioned in the SeMo-active site by a unique VAYRCSFR active site loop. In H. pseudoflava the arginine preceeding the cysteine in the active site loop is modified to a Cgamma-hydroxy arginine residue which has no obvious function. The substituents in the first coordination sphere of the Mo-ion are the enedithiolate sulfur atoms of the molybdopterin-cytosine dinucleotide, two oxo- and a sulfido-group. Extended X-ray absorption fine structure spectroscopy (EXAFS), along with the crystal structure of CO dehydrogenase (23.2 U mg(-1)) at 1.85 A resolution, have identified a sulfur atom at 2.3 A from the Mo-ion. The sulfur reacts with cyanide yielding thiocyanate. The corresponding inactive desulfo-CO dehydrogenase shows a typical desulfo inhibited-type of Mo-electron paramagnetic resonance (EPR) spectrum. Structural changes at the SeMo-site during catalysis are suggested by the Mo to Se distance of 3.7 A and the Mo-S-Se angle of 113 degrees in the oxidized enzyme which increase to 4.1 A, and 121 degrees, respectively, in the reduced enzyme. The intramolecular electron transport chain in CO dehydrogenase involves the following prosthetic groups and minimal distances: CO-->[Mo of the molybdenum cofactor] - 14.6 A - [2Fe-2S]I - 12.4 A - [2Fe-2S]II - 8.7 A - [FAD].

Carbon monoxide dehydrogenase activity in Bradyrhizobium japonicum

Bradyrhizobium japonicum strain 110spc4 was capable of chemolithoautotrophic growth with carbon monoxide (CO) as a sole energy and carbon source under aerobic conditions. The enzyme carbon monoxide dehydrogenase (CODH; EC 1.2.99.2) has been purified 21-fold, with a yield of 16% and a specific activity of 58 nmol of CO oxidized/min/mg of protein, by a procedure that involved differential ultracentrifugation, anion-exchange chromatography, hydrophobic interaction chromatography, and gel filtration. The purified enzyme gave a single protein and activity band on nondenaturing polyacrylamide gel electrophoresis and had a molecular mass of 230,000 Da. The 230-kDa enzyme was composed of large (L; 75-kDa), medium (M; 28.4-kDa), and small (S; 17.2-kDa) subunits occurring in heterohexameric (LMS)(2) subunit composition. The 75-kDa polypeptide exhibited immunological cross-reactivity with the large subunit of the CODH of Oligotropha carboxidovorans. The B. japonicum enzyme contained, per mole, 2.29 atoms of Mo, 7.96 atoms of Fe, 7.60 atoms of labile S, and 1.99 mol of flavin. Treatment of the enzyme with iodoacetamide yielded di(carboxamidomethyl)molybdopterin cytosine dinucleotide, identifying molybdopterin cytosine dinucleotide as the organic portion of the B. japonicum CODH molybdenum cofactor. The absorption spectrum of the purified enzyme was characteristic of a molybdenum-containing iron-sulfur flavoprotein.

The structural genes encoding CO dehydrogenase subunits (cox L, M and S) in Pseudomonas carboxydovorans OM5 reside on plasmid pHCG3 and are, with the exception of Streptomyces thermoautotrophicus, conserved in carboxydotrophic bacteria

Employing deoxyoligonucleotide probes and Southern hybridizations, we have examined in carboxydotrophic bacteria the localization on the genome of genes encoding the large, medium and small subunits of CO dehydrogenase (coxL, M and S, respectively). In Pseudomonas carboxydovorans OM5 coxL, M and S were identified on the plasmid pHCG3; they were absent on the chromosome. This was evident from positive hybridizations with plasmid DNA of the wild-type strain OM5 and the absence of hybridizations with chromosomal DNA from the plasmid cured mutant strain OM5-12. The genes coxL, M and S were found on plasmids in all other plasmid-containing carboxydotrophic bacteria e.g. Alcaligenes carboxydus, Azomonas B1, Pseudomonas carboxydoflava, Pseudomonas carboxydovorans OM2 and OM4. Cox L, M and S could be identified on the chromosome of the plasmid-free bacteria Arthrobacter 11/x, Bacillus schlegelii, Pseudomonas carboxydohydrogena, and Pseudomonas carboxydovorans OM3. These results essentially confirm and extend former reports that cox genes are rather conserved among carboxydotrophic bacteria of distinct taxonomic position. However, Streptomyces thermoautotrophicus is an noteworthy exception since none of the three cox genes could be detected. This refers to a new type of CO dehydrogenase and is in accord with results indicating that the S. thermoautotrophicus CO dehydrogenase has an unusual electron acceptor specificity and some other properties setting it apart from the 'classical' CO dehydrogenases.

Removal of CO dehydrogenase from Pseudomonas carboxydovorans cytoplasmic membranes, rebinding of CO dehydrogenase to depleted membranes, and restoration of respiratory activities

In Pseudomonas carboxydovorans, CO dehydrogenase and hydrogenase were found in association with the cytoplasmic membrane in a weakly bound and a tightly bound pool. The pools could be experimentally distinguished on the basis of resistance to removal by washes in low-ionic-strength buffer. The tightly bound pool of the enzymes could be differentially solubilized under conditions leaving the electron transport system intact and with the nondenaturing zwitterionic detergent 3-(3-cholamidopropyl) dimethylammonio 1-propane-sulfonic acid (CHAPS) and the nonionic detergent dodecyl beta-D-maltoside. In vitro reconstitution of depleted membranes with the corresponding supernatants containing CO dehydrogenase led to binding of the enzyme and to reactivation of respiratory activities with CO. The reconstitution reaction required cations with effectiveness which increased with increasing ionic charge: monovalent (Li+), divalent (Mg2+, Mn2+), or trivalent (Cr3+, La3+). Reconstitution of depleted membranes with CO dehydrogenase was specific for CO-grown bacteria. Cytoplasmic membranes from H2- or heterotrophically grown Pseudomonas carboxydovorans had no affinity for CO dehydrogenase at all, indicating the absence of the physiological electron acceptor of the enzyme, which presumably is cytochrome b561, or another membrane anchor.

Homology and distribution of CO dehydrogenase structural genes in carboxydotrophic bacteria

The 17 (S), 30 (M) and 87 kDa (L) subunits of CO dehydrogenases from the CO-oxidizing bacteria Pseudomonas carboxydoflava, Pseudomonas carboxydohydrogena and Pseudomonas carboxydovorans OM5 were isolated and purified. The N-terminal sequences of same subunits from different bacteria showed distinct homologies. Dot blot hybridization employing oligonucleotide probes derived from the sequences of the S-subunit of P. carboxydovorans OM5 and the M-subunit of P. carboxydohydrogena and DNA of the plasmid-containing CO-oxidizing bacteria Alcaligenes carboxydus, Azomonas B1, P. carboxydoflava, P. carboxydovorans OM2, OM4 and OM5 indicated that all genes encoding these subunits reside on plasmids. That in P. carboxydovorans OM5 CO dehydrogenase structural genes are located entirely on plasmid pHCG3 was evident from the absence of hybridization employing DNA from the cured mutant strain OM5-12. CO dehydrogenase structural genes could be identified on the chromosome of the plasmid-free bacteria Arthrobacter 11/x, Bacillus schlegelii, P. carboxydohydrogena and P. carboxydovorans OM3. There was no example of a plasmid-harboring carboxydotrophic bacterium that did not carry CO dehydrogenase structural genes on the plasmid. The N-terminal sequences of CO dehydrogenase structural genes were found to be conserved among carboxydotrophic bacteria of distinct taxonomic position, independent of the presence of plasmids. It is discussed whether this might be the consequence of horizontal gene transfer.

Regulation of carbon monoxide dehydrogenase and hydrogenase in Rhodospirillum rubrum: effects of CO and oxygen on synthesis and activity

Exposure of the photosynthetic bacterium Rhodospirillum rubrum to carbon monoxide led to increased carbon monoxide dehydrogenase and hydrogenase activities due to de novo protein synthesis of both enzymes. Two-dimensional gels of [35S]methionine-pulse-labeled cells showed that induction of CO dehydrogenase synthesis was rapidly initiated (less than 5 min upon exposure to CO) and was inhibited by oxygen. Both CO dehydrogenase and the CO-induced hydrogenase were inactivated by oxygen in vivo and in vitro. In contrast to CO dehydrogenase, the CO-induced hydrogenase was 95% inactivated by heating at 70 degrees C for 5 min. Unlike other hydrogenases, this CO-induced hydrogenase was inhibited only 60% by a 100% CO gas phase.

Purification and some properties of carbon monoxide dehydrogenase from Acinetobacter sp. strain JC1 DSM 3803

A brown carbon monoxide dehydrogenase from CO-autotrophically grown cells of Acinetobacter sp. strain JC1, which is unstable outside the cells, was purified 80-fold in seven steps to better than 95% homogeneity, with a yield of 44% in the presence of the stabilizing agents iodoacetamide (1 mM) and ammonium sulfate (100 mM). The final specific activity was 474 mumol of acceptor reduced per min per mg of protein as determined by an assay based on the CO-dependent reduction of thionin. Methyl viologen, NAD(P), flavin mononucleotide, flavin adenine dinucleotide, and ferricyanide were not reduced by the enzyme, but methylene blue, thionin, and dichlorophenolindophenol were reduced. The molecular weight of the native enzyme was determined to be 380,000. Sodium dodecyl sulfate-gel electrophoresis revealed at least three nonidentical subunits of molecular weights 16,000 (alpha), 34,000 (beta), and 85,000 (gamma). The purified enzyme contained particulate hydrogenase-like activity. Selenium did not stimulate carbon monoxide dehydrogenase activity. The isoelectic point of the native enzyme was found to be 5.8; the Km of CO was 150 microM. The enzyme was rapidly inactivated by methanol. One mole of native enzyme was found to contain 2 mol of each of flavin adenine dinucleotide and molybdenum and 8 mol each of nonheme iron and labile sulfide, which indicated that the enzyme was a molybdenum-containing iron-sulfur flavoprotein. The ratio of densities of each subunit after electrophoresis (alpha:beta:gamma = 1:2:6) and the number of each cofactor in the native enzyme suggest a alpha 2 beta 2 gamma 2 structure of the enzyme. The carbon monoxide dehydrogenase of Acinetobacter sp. strain JC1 was found to have no immunological relationship with enzymes of Pseudomonas carboxydohydrogena and Pseudomonas carboxydovorans.

CO oxidoreductase from Streptomyces strain G26 is a molybdenum hydroxylase

CO oxidoreductase was purified to 95% homogeneity from crude mycelial extracts of Streptomyces G26. The purified preparation has a specific activity of 25.7 units/mg, a 13-fold improvement on crude soluble mycelial extracts. The native enzyme (Mr 282,000) is composed of non-identical subunits of Mr 110,000 and 33,000. It is a molybdenum hydroxylase containing 1.6 mol of FAD, 7.3 mol of Fe, 8.3 mol of acid-labile sulphide and 1.3 mol of Mo per mol of enzyme. Purified CO oxidoreductase catalyses the reduction of benzyl viologen, confirming the previously reported ability of this enzyme to interact with low-potential acceptors. Cytochrome c reduction cannot be accounted for entirely by non-enzymic reduction by superoxide radicals. NAD+ and NADP+ are not reduced, nor is clostridial ferredoxin.

The pterin (bactopterin) of carbon monoxide dehydrogenase from Pseudomonas carboxydoflava

Radioactively labeled carbon monoxide (CO) dehydrogenase has been obtained in good yield and purity from Pseudomonas carboxydoflava grown in the presence of [32P]phosphate. One enzyme molecule contained an average of 8.32 molecules of phosphate. The entire phosphate content was confined to 2 molecules of FAD and 2 molecules of a pterin. These were noncovalently bound. Molybdoenzyme cofactors could be extracted into N-methyl formamide; pterins were isolated by thin-layer chromatography. CO dehydrogenase contained a novel pterin, different from molybdopterin, which was also resolved in other bacterial molybdoenzymes. Therefore, it was tentatively named bactopterin. The characteristic features of bactopterin were as follows. A relative molecular mass, Mr, of 730 which was much greater than that of molybdopterin (330) (Mr values refer to molybdenum-free forms of the cofactors; presumably, the latter were also devoid of the sulfhydryl groups contained in the native compounds). A content of 2 molecules of phosphate/molecule compared to only 1 phosphate in molybdopterin. Bactopterin was three times less susceptible to air oxidation than molybdopterin. Native bactopterin was cleaved by perchloric acid into two phosphorous-containing fragments with Mr of 330 and 420. The smaller one is believed to be very similar to molybdopterin, the larger one was not a pterin but probably contained an aromatic structure.

Carbon monoxide dehydrogenase from Rhodospirillum rubrum

The carbon monoxide dehydrogenase from the photosynthetic bacterium Rhodospirillum rubrum was purified over 600-fold by DEAE-cellulose chromatography, heat treatment, hydroxylapatite chromatography, and preparative scale gel electrophoresis. In vitro, this enzyme catalyzed a two-electron oxidation of CO to form CO2 as the product. The reaction was dependent on the addition of an electron acceptor. The enzyme was oxygen labile, heat stable, and resistant to tryptic and chymotryptic digestion. Optimum in vitro activity occurred at pH 10.0. A sensitive, hemoglobin-based assay for measuring dissolved CO levels is presented. The in vitro Km for CO was determined to be 110 microM. CO, through an unknown mechanism, stimulated hydrogen evolution in whole cells, suggesting the presence of a reversible hydrogenase in R. rubrum which is CO insensitive in vivo.

Characterization and purification of carbon monoxide dehydrogenase from Methanosarcina barkeri

Carbon monoxide-dependent production of H2, CO2, and CH4 was detected in crude cell extracts of acetate-grown Methanosarcina barkeri. This metabolic transformation was associated with an active methyl viologen-linked CO dehydrogenase activity (5 to 10 U/mg of protein). Carbon monoxide dehydrogenase activity was inhibited 85% by 10 microM KCN and was rapidly inactivated by O2. The enzyme was nearly homogeneous after 20-fold purification, indicating that a significant proportion of soluble cell protein was CO dehydrogenase (ca. 5%). The native purified enzyme displayed a molecular weight of 232,000 and a two-subunit composition of 92,000 and 18,000 daltons. The enzyme was shown to contain nickel by isolation of radioactive CO dehydrogenase from cells grown in 63Ni. Analysis of enzyme kinetic properties revealed an apparent Km of 5 mM for CO and a Vmax of 1,300 U/mg of protein. The spectral properties of the enzyme were similar to those published for CO dehydrogenase from acetogenic anaerobes. The physiological functions of the enzyme are discussed.

Molybdopterin in carbon monoxide oxidase from carboxydotrophic bacteria

The carbon monoxide oxidases (COXs) purified from the carboxydotrophic bacteria Pseudomonas carboxydohydrogena and Pseudomonas carboxydoflava were found to be molybdenum hydroxylases, identical in cofactor composition and spectral properties to the recently characterized enzyme from Pseudomonas carboxydovorans (O. Meyer, J. Biol. Chem. 257:1333-1341, 1982). All three enzymes exhibited a cofactor composition of two flavin adenine dinucleotides, two molybdenums, eight irons and eight labile sulfides per dimeric molecule, typical for molybdenum-containing iron-sulfur flavoproteins. The millimolar extinction coefficient of the COXs at 450 nm was 72 (per two flavin adenine dinucleotides), a value similar to that of milk xanthine oxidase and chicken liver xanthine dehydrogenase at 450 nm. That molybdopterin, the novel prosthetic group of the molybdenum cofactor of a variety of molybdoenzymes (J. Johnson and K. V. Rajagopalan, Proc. Natl. Acad. Sci. U.S.A. 79:6856-6860, 1982) is also a constituent of COXs from carboxydotrophic bacteria is indicated by the formation of identical fluorescent cofactor derivatives, by complementation of the nitrate reductase activity in extracts of Neurospora crassa nit-l, and by the presence of organic phosphate additional to flavin adenine dinucleotides. Molybdopterin is tightly but noncovalently bound to the protein. COX, sulfite oxidase, xanthine oxidase, and xanthine dehydrogenase each contains 2 mol of molybdopterin per mol of enzyme. The presence of a trichloroacetic acid-releasable, so-far-unidentified, phosphorous-containing moiety in COX is suggested by the results of phosphate analysis.

Isolation of carbon monoxide dehydrogenase from Acetobacterium woodii and comparison of its properties with those of the Clostridium thermoaceticum enzyme

An oxygen-labile carbon monoxide dehydrogenase was purified to at least 98% homogeneity from fructose-grown cells of Acetobacterium woodii. Gel filtration and electrophoresis experiments gave molecular weights of 480,000 and 153,000, respectively, of the active enzyme. The molecular weights for the subunits are 80,000 and 68,000; the subunits occur in equal proportion. The small subunit of the A. woodii enzyme differs in size from that of the Clostridium thermoaceticum enzyme; however, the large subunits are similar. The specific activity of the A. woodii enzyme, measured at 30 degrees C and pH 7.6, is 500 mumol of CO oxidized min-1 mg-1 with 20 mM methyl viologen as the electron acceptor. Analysis revealed (number per dimer) iron (9), acid-labile sulfide (12), nickel (1.4), and magnesium or zinc (1). This metal content is quite similar to that of the C. thermoaceticum enzyme (Ragsdale et al., J. Biol. Chem. 258:2364-2369, 1983). The nickel as well as the iron-sulfur clusters are redox-active, as was found for the C. thermoaceticum enzyme (Ragsdale et al., Biochem. Biophys. Res. Commun. 108:658-663, 1982). CO can reduce and CO2 can oxidize the iron-sulfur clusters. The enzyme is inhibited by cyanide, but CO2 in the presence of reduced methyl viologen or CO alone can reverse or prevent this inhibition. Several ferredoxins, flavodoxin, and rubredoxin and some artificial electron carriers were tested for their relative rates of reaction with the CO dehydrogenases from A. woodii, C. thermoaceticum, and Clostridium formicoaceticum. Rubredoxin was by far the most reactive acceptor and is proposed to be the primary natural electron carrier for the acetogenic CO dehydrogenases.

Studies by e.p.r. spectroscopy of carbon monoxide oxidases from Pseudomonas carboxydovorans and Pseudomonas carboxydohydrogena

E.p.r. spectra were obtained at 8-120 K for carbon monoxide oxidases isolated from the carboxydotrophic bacteria Pseudomonas carboxydovorans and Pseudomonas carboxydohydrogena. Spectra from the two enzymes are extremely similar to one another. Under appropriate conditions each enzyme shows signals from Mo(V) atoms in two different chemical environments, as well as showing signals from two distinct iron-sulphur centres, presumed to be [2Fe-2S] clusters, and weak FADH X free-radical signals. Parameters of most of the signals were measured, and they show considerable similarities to those of the corresponding signals from xanthine oxidase and related enzymes. Though the signals from carbon monoxide oxidases appear and disappear under reducing and oxidizing conditions, we have so far failed to demonstrate the kinetic competence of any of them. It seems likely that this was due to the presence in the enzyme preparation examined of high amounts of desulpho carbon monoxide oxidase together with another non-functional form of the enzyme giving a stable 'Resting' Mo(V) e.p.r. signal.

Occurrence of nickel in carbon monoxide dehydrogenase from Clostridium pasteurianum and Clostridium thermoaceticum

The carbon monoxide (CO) dehydrogenase activity band from Clostridium pasteurianum was shown to contain nickel by in situ activity staining of polyacrylamide gels. However, the majority of the nickel in cell extracts was found to electrophorese independently of CO dehydrogenase. Comparative analysis with Clostridium thermoaceticum demonstrated that, although the majority of nickel was accounted for in CO dehydrogenase in anaerobic extracts, the metal dissociated from the enzyme when inactivated by oxidation.