Effect of redox mediators on nitrogenase and hydrogenase activities in Azotobacter vinelandii

In bioelectrochemical studies, redox mediators such as methylene blue, natural red, and thionine are used to studying the redox characteristics of enzymes in the living cell. Here we show that nitrogenase activity in Azotobacter vinelandii is completely inhibited by oxidized methylene blue (MBo) when the concentration of this mediator in the medium is increased up to 72 microM. This activity in A. vinelandii is somewhat inhibited by a coenzyme, ascorbic acid (AA). However, the nitrogenase activity within the A. vinelandii cell is unchanged even for a high concentration of oxidized natural red (NRo) alone. Interestingly, these mediators and AA do not have the capacity to inhibit the H2 uptake activity of the hydrogenase in A. vinelandii. Average active rates of 66 nM H2 evolved/mg cell protein/min from the nitrogenase and 160 nM H2-uptake/mg cell protein/min from the hydrogenase in A. vinelandii are found in aid of the activities of the enzymes for H2 evolution and for H2 uptake are compared. The activities of both enzymes in A. vinelandii are strongly inhibited by thionine having high oxidative potential. Mechanisms of various mediators acting in vivo for both enzymes in A. vinelandii are discussed.

Diphenylene iodonium as an inhibitor for the hydrogenase complex of Rhodobacter capsulatus. Evidence for two distinct electron donor sites

The photosynthetic bacterium Rhodobacter capsulatus synthesises a membrane-bound [NiFe] hydrogenase encoded by the H2 uptake hydrogenase (hup)SLC structural operon. The hupS and hupL genes encode the small and large subunits of hydrogenase, respectively; hupC encodes a membrane electron carrier protein which may be considered as the third subunit of the uptake hydrogenase. In Wolinella succinogenes, the hydC gene, homologous to hupC, has been shown to encode a low potential cytochrome b which mediates electron transfer from H2 to the quinone pool of the bacterial membrane. In whole cells of R. capsulatus or intact membrane preparation of the wild type strain B10, methylene blue but not benzyl viologen can be used as acceptor of the electrons donated by H2 to hydrogenase; on the other hand, membranes of B10 treated with Triton X-100 or whole cells of a HupC- mutant exhibit both benzyl viologen and methylene blue reductase activities. We report the effect of diphenylene iodonium (Ph2I), a known inhibitor of mitochondrial complex I and of various monooxygenases on R. capsulatus hydrogenase activity. With H2 as electron donor, Ph2I inhibited partially the methylene blue reductase activity in an uncompetitive manner, and totally benzyl viologen reductase activity in a competitive manner. Furthermore, with benzyl viologen as electron acceptor, Ph2I increased dramatically the observed lagtime for dye reduction. These results suggest that two different sites exist on the electron donor side of the membrane-bound [NiFe] hydrogenase of R. capsulatus, both located on the small subunit. A low redox potential site which reduces benzyl viologen, binds Ph2I and could be located on the distal [Fe4S4] cluster. A higher redox potential site which can reduce methylene blue in vitro could be connected to the high potential [Fe3S4] cluster and freely accessible from the periplasm.

Binding of exogenously added carbon monoxide at the active site of the iron-only hydrogenase (CpI) from Clostridium pasteurianum

A site for the binding of exogenously added carbon monoxide has been identified at the active site of the Fe-only hydrogenase (CpI) from Clostridium pasteurianum. The binding and inhibition of carbon monoxide have been exploited in biochemical and spectroscopic studies to gain mechanistic insights. In the present study, we have taken advantage of the ability to generate an irreversibly carbon monoxide bound state of CpI. The crystallization and structural characterization of CpI inhibited in the presence of carbon monoxide indicates the addition of a single molecule of carbon monoxide. The ability to generate crystals of the carbon monoxide bound state of the hydrogenase that are isomorphous to those of the native enzyme has allowed for a direct comparison of the crystallographic data and an unambiguous identification of the site of carbon monoxide binding at the active site of CpI. Carbon monoxide binds to an Fe atom of the 2Fe subcluster at the site of a terminally bound water molecule in the as crystallized native state of CpI that has been previously suggested to be a potential site of reversible hydrogen oxidation. Binding of carbon monoxide at this site results in an active site that is coordinately saturated with strong ligands (S, CO, and CN), providing a rational potential mechanism for inhibition of reversible hydrogen oxidation at the active site of CpI.

Characterization of the region encoding the CO-induced hydrogenase of Rhodospirillum rubrum

In the photosynthetic bacterium Rhodospirillum rubrum, the presence of carbon monoxide (CO) induces expression of several proteins. These include carbon monoxide dehydrogenase (CODH) and a CO-tolerant hydrogenase. Together these enzymes catalyze the following conversion: CO + H2O --> CO2 + H2. This system enables R. rubrum to grow in the dark on CO as the sole energy source. Expression of this system has been shown previously to be regulated at the transcriptional level by CO. We have now identified the remainder of the CO-regulated genes encoded in a contiguous region of the R. rubrum genome. These genes, cooMKLXU, apparently encode proteins related to the function of the CO-induced hydrogenase. As seen before with the gene for the large subunit of the CO-induced hydrogenase (cooH), most of the proteins predicted by these additional genes show significant sequence similarity to subunits of Escherichia coli hydrogenase 3. In addition, all of the newly identified coo gene products show similarity to subunits of NADH-quinone oxidoreductase (energy-conserving NADH dehydrogenase I) from various eukaryotic and prokaryotic organisms. We have found that dicyclohexylcarbodiimide, an inhibitor of mitochondrial NADH dehydrogenase I (also called complex I), inhibits the CO-induced hydrogenase as well. We also show that expression of the cooMKLXUH operon is regulated by CO and the transcriptional activator CooA in a manner similar to that of the cooFSCTJ operon that encodes the subunits of CODH and related proteins.

Inhibition by iodoacetamide and acetylene of the H-D-exchange reaction catalyzed by Thiocapsa roseopersicina hydrogenase

The kinetics of H-D isotope exchange catalyzed by the thermostable hydrogenase from Thiocapsa roseopersicina have been studied by analysis of the exchange between D2 and H2O. The pH dependence of the exchange reaction was examined between pH 2.5 and pH 11. Over the whole pH range, HD was produced at a higher initial velocity than H2, with a marked optimum at pH 5.5; a second peak in the pH profile was observed at around pH 8.5. The rapid formation of H2 with respect to HD in the D2/H2O system is consistent with a heterolytic cleavage of D2 into D+ and an enzyme hydride that can both exchange with the solvent. The H-D-exchange activity was lower in the H2/D2O system than in the D2/H2O system. The other reactions catalyzed by the hydrogenase, H2 oxidation and H2 evolution, are pH dependent; the optimal pH were 9.5 for H2 uptake and 4.0 for H2 production. Treatment of the active form of hydrogenase by iodoacetamide led to a slow and irreversible inhibition of the H-D exchange. When iodo[1-14C]acetamide was incubated with hydrogenase, the radioactive labeling of the large subunit was higher for the enzyme activated under H2 than for the inactive oxidized form. Cysteine residues were identified as the alkylated derivative by amino acid analysis. Acetylene, which inhibits H-D exchange and abolishes the Ni-C EPR signal, protected the enzyme from irreversible inhibition by iodoacetamide. These data indicate that iodoacetamide can reach the active site of the H2-activated hydrogenase from T. roseopersicina. This was not found to be the case with the seleno hydrogenase from Desulfovibrio baculatus (now Desulfomicrobium baculatus). Cysteine modification by iodoacetamide upon activation of the enzyme concomitant with loss of H-D exchange indicates that reductive activation makes at least one Cys residue of the active site available for alkylation.

Reversible hydrogenase of Anabaena variabilis ATCC 29413: catalytic properties and characterization of redox centres

The catalytic and spectroscopic properties of the reversible hydrogenase from the cyanobacterium Anabaena variabilis have been examined. The hydrogenase required reductive activation in order to elicit hydrogen-oxidation activity. Carbon monoxide was a weak (Ki=35 microM), reversible and competitive inhibitor. A flavin with the chromatographic properties of FMN, and nickel were detected in the purified enzyme. A. variabilis hydrogenase exhibited electron paramagnetic resonance (EPR) spectra in its hydrogen-reduced state, indicative of [2Fe-2S] and [4Fe-4S] clusters. Although no EPR signals due to nickel were detected, the results are consistent with the enzyme being a flavin-containing hydrogenase of the nickel-iron type.

Substitution of Azotobacter vinelandii hydrogenase small-subunit cysteines by serines can create insensitivity to inhibition by O2 and preferentially damages H2 oxidation over H2 evolution

Mutants in which conserved cysteines 294, 297 or 64 and 65 of the Azotobacter vinelandii hydrogenase small subunit were replaced by serines were studied. Cysteines 294 and 297 are homologous to cysteines 246 and 249 of the Desulfovibrio gigas hydrogenase, and these cysteines are ligands to the [3Fe-4S] clusters (A. Volbeda, M.-H. Charon, C. Piras, E. C. Hatchikian, M. Frey, and J. C. Fontecilla-Camps, Nature (London) 373:580-587, 1995). Cysteine 65 is homologous to cysteine 20 of the D. gigas hydrogenase, and this cysteine is a ligand to the proximal [4Fe-4S] cluster. All three mutants retained some hydrogenase activity. All three mutants studied had H2 oxidation-to-H2 evolution activity ratios with whole cells of approximately 1.5, compared with 46 for the wild type. The changes preferentially deplete H2 oxidation activity, while having less effect on evolution. The K64,65C-->S hydrogenase was partially purified and had a specific activity for the evolution reaction that was 22% that of the wild type, while the oxidation-specific activity was 2% that of the wild type. Because cysteine 65 provides a ligand to the proximal [4Fe-4S] cluster, this cluster can be altered without entirely eliminating enzyme activity. Likewise, the detection of H2 evolution and H2 oxidation activities with whole cells and membranes of the K294C-->S and K297C-->S mutants indicates that the [3Fe-4S] cluster can also be altered or possibly eliminated without entirely eliminating enzyme activity. Membranes with K294C-->S or K297C-->S hydrogenase were uninhibited by O2 in H2 oxidation and uninhibited by H2 in H2 evolution. Wild-type membranes and membranes with K64,65C-->S hydrogenase were both sensitive to these inhibitors. These data indicate that the [3Fe-4S] cluster controls the reversible inhibition of hydrogenase activity by O2 or H2.

Effects of thiols and mercurials on the periplasmic hydrogenase from Desulfovibrio vulgaris (Hildenborough)

The H2-oxidation, H2-production and H-3H-exchange activities of the periplasmic hydrogenase from Desulfovibrio vulgaris (Hildenborough) were almost completely abolished by Hg(II) and the organic mercurials p-chloromercuribenzoate (pCMB) and p-hydroxymercuriphenylsulphonate. The thiol-modifying reagents N-ethylmaleimide, iodoacetate, dithionitrobenzoate and 2-nitro-5-thiocyanobenzoate had no effect on the activities. Kinetic and spectroscopic measurements suggest that inactivation by pCMB involves at least two reactions; a rapid reaction that is reversed by thiols, and a second, slower and irreversible reaction that occurs at high concentrations of the mercurial. The irreversible reaction was associated with loss of visible absorbance, indicative of a disrupted iron sulphur cluster(s). The effects on the H-3H-exchange activity indicate that the reversible modification affects the H2-activating site. Enzyme that had lost activity due to pCMB treatment, or during long-term storage, was reactivated by thiols. This reactivation was followed by a slower irreversible inactivation, as also occurred with native enzyme; the inactivation was O2 dependent and it was partly prevented by catalase, suggesting that H2O2 may be involved.

Evidence for an [Fe]-type hydrogenase in the parasitic protozoan Trichomonas vaginalis

The hydrogenase of the pathogenic protozoan Trichomonas vaginalis was extracted and partially purified. The catalytic and spectroscopic properties of the enzyme indicate that it belongs to the class of [Fe]-hydrogenases, rather than the [NiFe]-hydrogenases. The hydrogenase activity was highly sensitive to carbon monoxide, 50% inhibition being attained by 1 microM CO. The EPR spectrum of the most active fractions from chromatography, after reduction by hydrogen and partial reoxidation under argon, showed an EPR spectrum at g = 2.10, 2.04, 2.00. This unusual spectrum is characteristic of the 'H-cluster', as seen in [Fe]-hydrogenases of anaerobic bacteria such as Clostridium spp.

The hydrogen-tritium exchange activity of Megasphaera elsdenii hydrogenase

The hydrogenase of Megasphaera elsdenii was purified to a specific activity of 350 units/mg. The hydrogen-tritium exchange assay of Hallahan et al. [Hallahan, D.L., Fernandez, V. M., Hatchikian, E. C. and Hall, D. O. (1986) Biochimie (Paris) 68, 49-54] was adapted to allow its use in the study of the M. elsdenii hydrogenase preparation. Under the assay conditions routinely employed, the enzyme's exchange activity was inhibited by Tris/HCl and MgCl2; it was stimulated by ethylene glycol. Maximal activity in this standard assay occurred at pH 7.1. The effect of the concentration of molecular hydrogen (1H2 plus 3H1H) on the exchange activity was studied. The resulting double-reciprocal plot was linear; its slope and its intercepts on the ordinate and abscissa were pH-dependent. The rate equations for a number of models of the exchange activity were derived. Each model gave rise to a linear double-reciprocal plot at constant pH, but none could explain fully the observed effects of varying pH. The experimental data corresponded most closely to the predictions of models in which protons were treated both as substrates and as regulators of the enzyme's activity.

Acetylene inhibition of Azotobacter vinelandii hydrogenase: acetylene binds tightly to the large subunit

Acetylene is a slow-binding inhibitor of the Ni- and Fe-containing dimeric hydrogenase isolated from Azotobacter vinelandii. Acetylene was released from hydrogenase during the recovery from inhibition. This indicates that no transformation of acetylene to another compound occurred as a result of the interaction with hydrogenase. However, the release of C2H2 proceeds more rapidly than the recovery of activity, which indicates that release of C2H2 is not sufficient for recovery of activity. Acetylene binds tightly to native hydrogenase; hydrogenase and radioactivity coelute from a gel permeation column following inhibition with 14C2H2. Acetylene, or a derivative, remains bound to the large 65,000 MW subunit (and not to the small 35,000 MW subunit) of hydrogenase following denaturation as evidenced by SDS-PAGE and fluorography of 14C2H2-inhibited hydrogenase. This result suggests that C2H2, and by analogy H2, binds to and is activated by the large subunit of this dimeric hydrogenase. Radioactivity is lost from 14C2H2-inhibited protein during recovery. The inhibition is remarkably specific for C2H2: propyne, butyne, and ethylene are not inhibitors.

The extremely thermophilic eubacterium, Thermotoga maritima, contains a novel iron-hydrogenase whose cellular activity is dependent upon tungsten

Thermotoga maritima is the most thermophilic eubacterium currently known and grows up to 90 degrees C by a fermentative metabolism in which H2, CO2, and organic acids are end products. It was shown that the production of H2 is catalyzed by a single hydrogenase located in the cytoplasm. The addition of tungsten to the growth medium was found to increase both the cellular concentration of the hydrogenase and its in vitro catalytic activity by up to 10-fold, but the purified enzyme did not contain tungsten. It is a homotetramer of Mr 280,000 and contains approximately 20 atoms of Fe and 18 atoms of acid-labile sulfide/monomer. Other transition metals, including nickel (and also selenium), were present in only trace amounts (less than 0.1 atoms/monomer). The hydrogenase was unstable at both 4 and 23 degrees C, even under anaerobic conditions, but no activity was lost in anaerobic buffer containing glycerol and dithiothreitol. Under these conditions the enzyme was also quite thermostable (t50% approximately 1 h at 90 degrees C) but extremely sensitive to irreversible inactivation by O2 (t50% approximately 10 s in air). The optimum pH ranges for H2 evolution and H2 oxidation were 8.6-9.5 and greater than or equal to 10.4, respectively, and the optimum temperature for catalytic activity was above 95 degrees C. In contrast to mesophilic Fe hydrogenases, the T. maritima enzyme had very low H2 evolution activity, did not use T. maritima ferredoxin as an electron donor for H2 evolution, was inhibited by acetylene but not by nitrite, and exhibited EPR signals typical of [2Fe-2S]1+ clusters. Moreover, the oxidized enzyme did not exhibit the rhombic EPR signal that is characteristic of the catalytic iron-sulfur cluster of mesophilic Fe hydrogenases. These data suggest that T. maritima hydrogenase has a different FeS site and/or mechanism for catalyzing H2 production. The potential role of tungsten in regulating the activity of this enzyme is discussed.

Reduction of the amount of periplasmic hydrogenase in Desulfovibrio vulgaris (Hildenborough) with antisense RNA: direct evidence for an important role of this hydrogenase in lactate metabolism

To establish the function of the periplasmic Fe-only hydrogenase in the anaerobic sulfate reducer Desulfovibrio vulgaris (Hildenborough), derivatives with a reduced content of this enzyme were constructed by introduction of a plasmid that directs the synthesis of antisense RNA complementary to hydrogenase mRNA. It was demonstrated that the antisense RNA technique allowed specific suppression of the synthesis of this hydrogenase in D. vulgaris by decreasing the amount of hydrogenase mRNA but did not result in the complete elimination of the enzyme, as is usual with most conventional mutagenesis techniques. The hydrogenase content in these antisense RNA-producing D. vulgaris clones was two- to threefold lower than in the parental strain when the strains were grown in batch cultures with lactate as a substrate and sulfate as a terminal electron acceptor. Under these conditions, several differences in growth parameters were measured between the hydrogenase-suppressed clones and wild-type D. vulgaris: growth rates of the clones decreased two- to threefold, and at excess lactate, growth yields were reduced by 20%. Furthermore, the amount of hydrogen measured in the culture headspaces was reduced three- to fivefold for the clones. These observations indicate that this hydrogenase has an important function during growth on lactate and is involved in hydrogen production from protons and electrons originating from at least one of the two oxidation reactions in the conversion of lactate to acetate. The implications for the energy metabolism of D. vulgaris are discussed.

Electron paramagnetic resonance spectroscopic investigation of the inhibition of the phosphoroclastic system of Clostridium sporogenes by nitrite

The proposal that nitrite exerts its inhibitory effect on anaerobic bacteria by direct interaction with the iron-sulphur proteins of the phosphoroclastic system was investigated. The effects of nitrate, nitrite with or without ascorbate, and nitric oxide on the growth of Clostridium sporogenes in liquid cultures at pH 7.4, on the rates of hydrogen production, and on the activities of the enzymes pyruvate-ferredoxin oxidoreductase and hydrogenase, and of ferredoxin were investigated. In agreement with previous studies, nitrate was the least effective inhibitor of cell growth, and nitric oxide the most effective. Nitrite reductase activity was very low in C. sporogenes, indicating that the presence of external reducing agents would be necessary for the reduction of nitrite to nitric oxide. Inhibition by nitrite was enhanced by ascorbate; 0.5 mM-nitrite with 10 mM-ascorbate stopped growth completely. In partially-purified preparations 4.1 mM-NaNO2 and equimolar ascorbate caused complete inactivation of hydrogenase activity but only partial (up to 78%) inactivation of pyruvate-ferredoxin oxidoreductase. This agreed with the loss of hydrogen production observed with nitrite in vivo. Inhibition occurred within 5 min, and was irreversible in each case. Electron paramagnetic resonance (EPR) spectroscopy showed that paramagnetic [Fe(NO)2(SR)2] species were formed during growth in the presence of nitrite, and were associated with cells. However, the intensity of these EPR signals did not correlate with the inhibition of cell growth. The [4Fe-4S] clusters in ferredoxin were shown by EPR spectroscopy to be resistant to treatment with 3.6 mM-NaNO2 and 3.6 mM-ascorbate. It is concluded that the effects of nitrite on pre-formed iron-sulphur proteins are not convincing as a basis for the lethal effects on bacterial cells.

Interactions of iron-thiol-nitrosyl compounds with the phosphoroclastic system of Clostridium sporogenes

Certain reagents, such as ascorbate or iron salts and thiols, enhance the bacteriostatic action of nitrite on food-spoilage bacteria. This may be due to the formation of nitric oxide and iron-thiol-nitrosyl [( Fe-S-NO]) complexes. The minimum concentrations of these reagents required to inhibit growth of Clostridium sporogenes were investigated. A mixture of nitrite (0.72 mM) with iron (1.44 mM) and cysteine (2.16 mM) was found to be extremely inhibitory when autoclaved and diluted into the culture medium. This mixture caused rapid cessation of growth and loss of cell viability at a final concentration corresponding to 40 microM-nitrite. If added to the initial culture medium, it prevented growth at 5 microM-nitrite. The mixture was more inhibitory, on the basis of the nitrite concentration used, than the 'Perigo factor', obtained by autoclaving nitrite in growth medium. [Fe-S-NO] compounds of known chemical structure were tested to determine if they were responsible for this effect. Total inhibition of cell growth was observed with the tetranuclear clusters [Fe4S3(NO)7] (Roussin's black salt), [Fe4S4(NO)4] or [Fe4Se3(NO)7], added at concentrations equivalent to 10 microM-nitrite, or with [Fe2(SMe)2(NO)4] (methyl ester of Roussin's red salt), equivalent to 200 microM-nitrite. The rate of hydrogen production in growing cell cultures was inhibited by [Fe4S3(NO)7] at levels equivalent to 2.5 microM-nitrite. EPR spectra of the inhibited cells showed features with g-values of 2.03, characteristic of mononuclear iron-nitrosyl species, and, under non-reducing conditions, an unusual signal at g = 1.65. There was no correlation between growth inhibition and the g = 2.03 signal, though there was a better correlation between inhibition and the g = 1.65 signal. The direct effects of the compounds were tested on the iron-sulphur proteins of the phosphoroclastic system, namely ferredoxin, pyruvate-ferredoxin oxidoreductase and hydrogenase. EPR spectra and enzyme assays showed that these proteins were not destroyed by [Fe4S3(NO)7], [Fe4S4(NO)4], [Fe2(SMe)2(NO)4], [Fe(SPh)2(NO)2], or M2 (an autoclaved mixture of 66 mM-cysteine, 3.6 mM-FeSO4 and 0.72 mM-NaNO2) at concentrations higher than those that caused total inhibition of cell growth. Inhibition of cells by [Fe-S-NO] compounds is unlikely to be due to interaction with the preformed enzymes. The possible formation of iron-nitrosyl complexes in vivo, and their inhibitory actions, are discussed.

Inhibition of Desulfovibrio gigas hydrogenase with copper salts and other metal ions

The effect of several transition metals on the activity of Desulfovibrio gigas hydrogenase has been studied. Co(II) and Ni(II) at a concentration of 1 mM did not modify the activity of the enzyme; nor did they affect the pattern of activation/deactivation. Cu(II) inhibited the active hydrogenase, prepared by treatment with hydrogen, but had little effect on the 'unready' enzyme unless a reductant such as ascorbate was present, in which case inactivation took place either in air or under argon. Hg(II) also inactivated the enzyme irreversible in the 'unready' state without the requirement for reductants. The reaction of H2 uptake with methyl viologen was much more sensitive to inhibition than the H2/tritium exchange activity. EPR spectra of this preparation showed that the rates of decline were [3Fe-4S] signal greater than H2-uptake activity greater than Ni-A signal. Similar results were obtained when the protein was treated with Hg(II). The results demonstrate that the [3Fe-4S] cluster is not essential for H2-uptake activity with methyl viologen, but the integrity of [4Fe-4S] clusters is probably necessary to catalyze the reduction of methyl viologen with hydrogen. D. gigas hydrogenase was found to be highly resistant to digestion by proteases.

Cyanide inactivation of hydrogenase from Azotobacter vinelandii

The effects of cyanide on membrane-associated and purified hydrogenase from Azotobacter vinelandii were characterized. Inactivation of hydrogenase by cyanide was dependent on the activity (oxidation) state of the enzyme. Active (reduced) hydrogenase showed no inactivation when treated with cyanide over several hours. Treatment of reversibly inactive (oxidized) states of both membrane-associated and purified hydrogenase, however, resulted in a time-dependent, irreversible loss of hydrogenase activity. The rate of cyanide inactivation was dependent on the cyanide concentration and was an apparent first-order process for purified enzyme (bimolecular rate constant, 23.1 M-1 min-1 for CN-). The rate of inactivation decreased with decreasing pH. [14C]cyanide remained associated with cyanide-inactivated hydrogenase after gel filtration chromatography, with a stoichiometry of 1.7 mol of cyanide bound per mol of inactive enzyme. The presence of saturating concentrations of CO had no effect on the rate or extent of cyanide inactivation of hydrogenases. The results indicate that cyanide can cause a time-dependent, irreversible inactivation of hydrogenase in the oxidized, activatable state but has no effect when hydrogenase is in the reduced, active state.

Effects of acetylene on hydrogenases from the sulfate reducing and methanogenic bacteria

The effect of acetylene on the activity of the three types of hydrogenase from the anaerobic sulfate reducing bacteria has been investigated. The (Fe) hydrogenase is resistant to inhibition by acetylene while the nickel-containing hydrogenases are inhibited by acetylene with the (NiFe) hydrogenase being 10-50 fold more sensitive than the (NiFeSe) hydrogenase. In addition the Ni(III) EPR signal (g approximately 2.3) of the "as isolated" (NiFe) hydrogenase was significantly decreased in intensity upon exposure to acetylene.

Redox-dependent inactivation of the NAD-dependent hydrogenase from Alcaligenes eutrophus Z1

A novel inactivation mechanism of the NAD-dependent hydrogenase from Alcaligenes eutrophus Z1 comprising redox-dependent steps is described. The model of the hydrogenase inactivation process is proposed which implies that the enzyme may exist in several forms which differ in their stability and spectral properties. One of these forms, existing within a limited (approximately -200 +/- 30 mV) potential range, undergoes a rapid and irreversible inactivation. The dissociation of the FMN prosthetic group from the apohydrogenase appears to be the main reason for the enzyme inactivation. The rationale for the enzyme stabilization under real operational conditions based on the chemical modification of the hydrogenase molecule is suggested.

Aerobic, inactive forms of Azotobacter vinelandii hydrogenase: activation kinetics and insensitivity to C2H2 inhibition

Azotobacter vinelandii hydrogenase (EC class 1.12), either purified or membrane-associated, was obtained aerobically in an inactive state. The kinetics of activation by treatment with a reductant (H2 or dithionite) were determined. Three distinct phases of the activation were observed. Aerobically prepared, inactive hydrogenase was insensitive to acetylene inhibition, but could be rendered acetylene-sensitive by reduction with dithionite. These findings indicate that acetylene inhibition of hydrogenase requires catalytically active enzyme.

Reversible and irreversible effects of nitric oxide on the soluble hydrogenase from Alcaligenes eutrophus H16

The effects of NO on the H2-oxidizing and diaphorase activities of the soluble hydrogenase from Alcaligenes eutrophus H16 were investigated. With fully activated enzyme, NO (8-150 nM in solution) inhibited H2 oxidation in a time- and NO-concentration-dependent process. Neither H2 nor NAD+ appeared to protect the enzyme against the inhibition. Loss of activity in the absence of an electron acceptor was about 10 times slower than under turnover conditions. The inhibition was partially reversible; approx. 50% of full activity was recoverable after removal of the NO. Recovery was slower in the absence of an electron acceptor than in the presence of H2 plus an electron acceptor. The diaphorase activity of the unactivated hydrogenase was not affected by NO concentrations of up to 200 microM in solution. Exposure of the unactivated hydrogenase to NO irreversibly inhibited the ability of the enzyme to be fully activated for H2-oxidizing activity. The enzyme also lost its ability to respond to H2 during activation in the presence of NADH. The results are interpreted in terms of a complex inhibition that displays elements of (1) a reversible slow-binding inhibition of H2-oxidizing activity, (2) an irreversible effect on H2-oxidizing activity and (30 an irreversible inhibition of a regulatory component of the enzyme. Possible sites of action for NO are discussed.

The relationship between activity and the axial g = 2.06 EPR signal induced by CO in the periplasmic (Fe) hydrogenase from Desulfovibrio vulgaris

The effect of exposure to carbon monoxide on the activity of the (Fe) hydrogenase from Desulfovibrio vulgaris has been determined. Concentrations of carbon monoxide which completely inhibit hydrogenase activity and induce formation of the axial g = 2.06 EPR signal up to 0.8 spin/molecule do not cause irreversible inhibition of the (Fe) hydrogenase.

Regulation of H2 oxidation activity and hydrogenase protein levels by H2, O2, and carbon substrates in Alcaligenes latus

Regulation of H2 oxidation activity and hydrogenase protein levels in the free-living hydrogen bacterium Alcaligenes latus was investigated. Hydrogenase activity was induced when heterotrophically grown cells were transferred to chemolithoautotrophic conditions, i.e., in the presence of H2 and absence of carbon sources, with NH4Cl as the N source. Under these conditions, H2 oxidation activity was detectable after 30 min of incubation and reached near-maximal levels by 12 h. The levels of hydrogenase protein, as measured by a Western blot (immunoblot) assay of the hydrogenase large subunit, increased in parallel with activity. This increase suggested that the increased H2 oxidation activity was due to de novo synthesis of hydrogenase protein. H2 oxidation activity was controlled over a surprisingly wide range of H2 concentrations, between 0.001 and 30% in the gas phase. H2 oxidation activity was induced to high levels between 2 and 12.5% O2, and above 12.5% O2, H2 oxidation activity was inhibited. Almost all organic carbon sources studied inhibited the expression of hydrogenase, although none repressed hydrogenase synthesis completely. In all cases examined, hydrogenase protein, as detected by Western blot, paralleled the level of H2 oxidation activity, suggesting that control of hydrogenase activity was mediated through changes in hydrogenase protein levels.

Inhibition studies of three classes of Desulfovibrio hydrogenase: application to the further characterization of the multiple hydrogenases found in Desulfovibrio vulgaris Hildenborough

The three types of hydrogenase hitherto characterized in genus Desulfovibrio exhibit distinctive inhibition patterns of their proton-deuterium exchange activity by CO, NO and NO2-. The (Fe) and (NiFeSe) hydrogenases are the most sensitive to all three inhibitors while the (NiFe) enzymes, relatively little inhibited by CO, are still very sensitive to NO but unaffected by NO2-. These differences together with some specific catalytic properties, in particular the pH profile and the H2 to HD ratio in the exchange reaction, constitute a simple means of characterizing multiple hydrogenases present in one or different species.

Reversible inactivation of the O2-labile hydrogenases from Azotobacter vinelandii and Rhizobium japonicum

Hydrogenases catalyze the reversible activation of dihydrogen. The hydrogenases from the aerobic, N2-fixing microorganisms Azotobacter vinelandii and Rhizobium japonicum are nickel- and iron-containing dimers that belong to the group of O2-labile enzymes. Exposure of these hydrogenases to O2 results in an irreversible inactivation; therefore, these enzymes are purified anaerobically in a fully active state. We describe in this paper an electron acceptor-requiring and pH-dependent, reversible inactivation of these hydrogenases. These results are the first example of an anaerobic, reversible inactivation of the O2-labile hydrogenases. The reversible inactivation required the presence of an electron acceptor. The rate of inactivation was first-order, with similar rates observed for methylene blue, benzyl viologen, and phenazine-methosulfate. The rate of inactivation was also dependent on the pH. However, increasing the pH of the enzyme in the absence of an electron acceptor did not result in inactivation. Thus, the reversible inactivation was not a result of high pH alone. The inactive enzyme could not be reactivated by H2 or other reductants at high pH. Titration of enzyme inactivated at high pH back to low pH was also ineffective at reactivating the enzyme. However, if reductants were present during this titration, the enzyme could be fully reactivated. The temperature dependence of inactivation yielded an activation energy of 44 kJ X mol-1. Gel filtration chromatography of active and inactive hydrogenase indicated that neither dissociation nor aggregation of the dimer hydrogenase was responsible for this reversible inactivation. We propose a four-state model to describe this reversible inactivation.