The pH dependence of proton-deuterium exchange, hydrogen production and uptake catalyzed by hydrogenases from sulfate-reducing bacteria

Selenium-More than Just a Fortuitous Sulfur Substitute in Redox Biology

Living organisms use selenium mainly in the form of selenocysteine in the active site of oxidoreductases. Here, selenium's unique chemistry is believed to modulate the reaction mechanism and enhance the catalytic efficiency of specific enzymes in ways not achievable with a sulfur-containing cysteine. However, despite the fact that selenium/sulfur have different physicochemical properties, several selenoproteins have fully functional cysteine-containing homologues and some organisms do not use selenocysteine at all. In this review, selected selenocysteine-containing proteins will be discussed to showcase both situations: (i) selenium as an obligatory element for the protein's physiological function, and (ii) selenium presenting no clear advantage over sulfur (functional proteins with either selenium or sulfur). Selenium's physiological roles in antioxidant defence (to maintain cellular redox status/hinder oxidative stress), hormone metabolism, DNA synthesis, and repair (maintain genetic stability) will be also highlighted, as well as selenium's role in human health. Formate dehydrogenases, hydrogenases, glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases will be herein featured.

Studies on hydrogenase

Hydrogenases are microbial enzymes which catalyze uptake and production of H(2). Hydrogenases are classified into 10 classes based on the electron carrier specificity, or into 3 families, [NiFe]-family (including [NiFeSe]-subfamily), [FeFe]-family and [Fe]-family, based on the metal composition of the active site. H(2) is heterolytically cleaved on the enzyme (E) to produce EH(a)H(b), where H(a) and H(b) have different rate constants for exchange with the medium hydron. X-ray crystallography unveiled the three-dimensional structures of hydrogenases. The simplest [NiFe]-hydrogenase is a heterodimer, in which the large subunit bears the Ni-Fe center buried deep in the protein, and the small subunit bears iron-sulfur clusters, which mediate electron transfer between the Ni-Fe center and the protein surface. Some hydrogenases have additional subunit(s) for interaction with their electron carriers. Various redox states of the enzyme were characterized by EPR, FTIR, etc. Based on the kinetic, structural and spectroscopic studies, the catalytic mechanism of [NiFe]-hydrogenase was proposed to explain H(2)-uptake, H(2)-production and isotopic exchange reactions.(Communicated by Shigekazu NAGATA, M.J.A.).

Hydrogen production and deuterium-proton exchange reactions catalyzed by Desulfovibrio nickel(II)-substituted rubredoxins

The nickel tetrahedral sulfur-coordinated core formed upon metal replacement of the native iron in Desulfovibrio sp. rubredoxins is shown to mimic the reactivity pattern of nickel-containing hydrogenases with respect to hydrogen production, deuterium-proton exchange, and inhibition by carbon monoxide.

The organometallic active site of [Fe]hydrogenase: models and entatic states

The simple organometallic, (mu-S(2))Fe(2)(CO)(6), serves as a precursor to synthetic analogues of the chemically rudimentary iron-only hydrogenase enzyme active site. The fundamental properties of the (mu-SCH(2)CH(2)CH(2)S)[Fe(CO)(3)](2) compound, including structural mobility and regioselectivity in cyanidecarbon monoxide substitution reactions, relate to the enzyme active site in the form of transition-state structures along reaction paths rather than ground-state structures. Even in the absence of protein-based active-site organization, the ground-state structural model complexes are shown to serve as hydrogenase enzyme reaction models, H(2) uptake and H(2) production, with the input of photo- or electrochemical energy, respectively.

Enzyme electrokinetics: hydrogen evolution and oxidation by Allochromatium vinosum [NiFe]-hydrogenase

The mechanism of catalytic hydrogen evolution and oxidation by Allochromatium vinosum [NiFe]-hydrogenase has been studied by protein film voltammetry (PFV) with the enzyme adsorbed at a pyrolytic graphite edge electrode. By analyzing the entire shapes of catalytic voltammograms, the energetics of the catalytic cycles (reduction potentials and acidity constants of the active states), including the detailed profiles of activity against pH and the sequences of proton and electron transfers, have been determined, and these are discussed with respect to the mechanism. PFV, which probes rates as a continuous function of the electrochemical potential (i.e., in the "potential domain"), is proven to be an invaluable tool for determining the redox properties of an active site in the presence of its substrate, at room temperature, and during turnover. This is especially relevant in the case of the active states of hydrogenase, since one of its substrates (the proton) is always present at significant levels in the titration medium at physiological pH values.

Mechanism of dihydrogen cleavage by high-valent metal oxo compounds: experimental and computational studies

The oxidation of dihydrogen by metal tetraoxo compounds was investigated. Kinetic measurements of the oxidations of H(2) by MnO(4)(-) and RuO(4), performed by UV-vis spectroscopy, showed these reactions to be quite rapid at 25 degrees C (k(1) approximately (3-6) x 10(-2) M(-1) s(-1)). Rates measured for H(2) oxidation by MnO(4)(-) in aqueous solution (using KMnO(4)) and in chlorobenzene (using (n)Bu(4)NMnO(4)) revealed only a minor solvent effect on the reaction rate. Substantial kinetic isotope effects [(k(H)2/k(D)2 = 3.8(2) (MnO(4)(-), aq), 4.5(5) (MnO(4)(-), C(6)H(5)Cl soln), and 1.8(6) (RuO(4), CCl(4) soln)] indicated that H-H bond cleavage is rate determining and that the mechanism of dihydrogen cleavage is likely similar in aqueous and organic solutions. Third-row transition-metal oxo compounds, such as OsO(4), ReO(4)(-), and MeReO(3), were found to be completely unreactive toward H(2). Experiments were performed to probe for a catalytic hydrogen/deuterium exchange between D(2) and H(2)O as possible evidence of dihydrogen sigma-complex intermediates, but no H/D exchange was observed in the presence of various metal oxo compounds at various pH values. In addition, no inhibition of RuO(4)-catalyzed hydrocarbon oxidation by H(2) was observed. On the basis of the available evidence, a concerted mechanism for the cleavage of H(2) by metal tetraoxo compounds is proposed. Theoretical models were developed for pertinent MnO(4)(-) + H(2) transition states using density functional theory in order to differentiate between concerted [2 + 2] and [3 + 2] scissions of H(2). The density functional theory calculations strongly favor the [3 + 2] mechanism and show that the H(2) cleavage shares some mechanistic features with related hydrocarbon oxidation reactions. The calculated activation energy for the [3 + 2] pathway (DeltaH(++) = 15.4 kcal mol(-1)) is within 2 kcal mol(-1) of the experimental value.

Transition Metal Complexes with Sulfur Ligands. 130.(1) Synthesis, Structure, and Reactivity of the Sulfur-Rich Ruthenium Hydride Complexes [Ru(H)(PR(3))('S(4)')](-) and the eta(2)-H(2) Complex [Ru(H(2))(PCy(3))('S(4)')] (R = Ph, (i)Pr, Cy; 'S(4)'(2-) = 1,2-Bis((2-mercaptophenyl)thio)ethane(2-))

Hydride and eta(2)-H(2) ruthenium complexes with sulfur-rich coordination spheres were synthesized. Substitution of either DMSO or PPh(3) in [Ru(DMSO)(PR(3))('S(4)')] and [Ru(PPh(3))(2)('S(4)')] by hydride anions from LiAlH(4) or NaBEt(3)H yielded [Ru(H)(PR(3))('S(4)')](-) complexes (R = (i)Pr, Ph, Cy; 'S(4)'(2)(-) = 1,2-bis((2-mercaptophenyl)thio)ethane(2-)). They were isolated as [Li(THF)(Et(2)O)][Ru(H)(PR(3))('S(4)')] (R = (i)Pr (1a), Cy (1b), Na[Ru(H)(PCy(3))('S(4)')].2BEt(3).0.5DMSO (2a), and the solvent-free Na[Ru(H)(PPh(3))('S(4)')].2BEt(3) (2b). X-ray structure determinations of 1a.0.5Et(2)O and 1b.Et(2)O showed that in both complexes pseudooctahedral [Ru(H)(PR(3))('S(4)')](-) anions are bridged to pseudotetrahedral [Li(THF)(Et(2)O)] cations via the hydride ligand and one thiolate donor of the 'S(4)'(2)(-) ligand (crystal data: 1a, monoclinic, P2(1)/n, a = 1401.6(2) pm, b = 1045.2(3) pm, c = 2590.6(4) pm, beta = 95.04(1) degrees, V = 3.780(1) nm(3), Z = 4; 1b, triclinic, P&onemacr;, a = 1264.2(1) pm, b = 1322.9(3) pm, c = 1569.5(2) pm, alpha = 88.96(1) degrees, beta = 83.48(1) degrees, gamma = 62.16(1) degrees, V = 2.3042(6) nm(3), Z = 2). Short intramolecular C-H.H-Ru contacts ( approximately 230 pm) between the hydride ligands, phosphine substituents, and lithium-coordinated Et(2)O molecules indicate "unconventional" hydrogen bonds. They potentially help to decrease the hydridic character of the hydride ligand to such an extent that no structural hydride trans influence can be observed in the solid state. In solution at room temperature, all hydride complexes 1a-2b rapidly release H(2) or HD, when treated with CH(3)OH or CD(3)OD. Low-temperature (1)H and (2)H NMR spectroscopy between -20 and -80 degrees C showed that initially eta(2)-H(2) or eta(2)-HD complexes form. Their formation explains the observed scrambling between protons and hydride ligands, which requires a heterolytic cleavage of dihydrogen. A 1:1:1 triplet at delta = -6.5 ppm ((1)J(HD) = 32 Hz, (2)J(PH) = 5 Hz) and a relaxation time of T(1)(min) = 4 ms (-60 degrees C, 270 MHz) firmly established the formation of the eta(2)-dihydrogen complexes. The reversibility of H(2) release and uptake by [Ru(PCy(3))('S(4)')] fragments and the heterolytic cleavage of H(2) in [Ru(eta(2)-H(2))(PCy(3))('S(4)')] was further ascertained by the reaction of [Ru(DMSO)(PCy(3))('S(4)')] with H(2) in the presence of NaOMe, yielding the [Ru(H)(PCy(3))('S(4)')](-) anion. The relevance of the complexes and their reactions for the heterolytic H(2) activation at the transition metal sulfur sites of hydrogenases is discussed.

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.

Comparison of isotope exchange, H2 evolution, and H2 oxidation activities of Azotobacter vinelandii hydrogenase

Azotobacter vinelandii hydrogenase was purified aerobically with a 35% yield. The purified enzyme catalyzed H2 oxidation at much greater velocity than H2 evolution. There was a large difference in activation energy for the two reactions. EA was 10 kcal/mol for H2 oxidation and 22 kcal/mol for evolution. This difference in activation energies between the two reactions means that the ratio of oxidation velocity to evolution velocity drops from 70 at 33 degrees C to 8 at 48 degrees C. With D2 and H2O as substrates, both membranes and purified enzyme produced only H2 and no HD in the isotope exchange reaction. The velocity of isotope exchange was equal to the velocity of H2 evolution from reduced methyl viologen, indicating that the two reactions share the same rate-limiting step. D2 and H2 inhibited H2 evolution, but D2 did not inhibit isotope exchange. We conclude that H2 and D2 do not inhibit H2 evolution by competing with H+ for the active site of the reduced enzyme. The Km for D2 in isotope exchange is 40-times greater than its Km in D2 oxidation. The difference in Km cannot be accounted for by differences in kcat. We propose that redox environment regulates hydrogenase's affinity for D2 (and likely H2 as well).

Coupling H2 to electron transfer

A catalytic transformation of dihydrogen into two protons and two electrons has been discovered with a ruthenium/iron complex. The chemical reactions of complexes between transition metals and dihydrogen give insights into the function of biological hydrogenases.

Influence of illumination on the electronic interaction between 77Se and nickel in active F420-non-reducing hydrogenase from Methanococcus voltae

The selenium-containing F420-non-reducing hydrogenase from Methanococcus voltae was anaerobically purified. The enzyme as isolated showed an EPR spectrum with gx,y,z = 2.21, 2.15 and 2.01. Upon illumination this spectrum disappeared and a new signal with the lowest g value at 2.05 arose. EPR studies were carried out either with the enzyme containing natural selenium or enriched in the nuclear isotope 77Se. The hyperfine splitting caused by 77Se in the 'dark' signal is shown to be highly anisotropic. In contrast the splitting is nearly isotropic after illumination. A new model for the nickel site is proposed to explain these observations.

Reactivity of [Fe] and [Ni-Fe-Se] hydrogenases with their oxido-reduction partner: the tetraheme cytochrome c3

In order to understand the electron transfer mechanisms for the [Fe] and [Ni-Fe] hydrogenases, a kinetic study of cytochrome c3 reduction has been undertaken. Cyclic voltammetry and controlled-potential amperometry techniques have been used to investigate the intermolecular electron-transfer reaction between cytochrome c3 and [Fe] hydrogenase from Desulfovibrio vulgaris Hildenborough. Electron-transfer cross-reactions between [Fe] or [Ni-Fe-Se] hydrogenase and cytochrome c3 from Desulfovibrio vulgaris Hildenborough or Desulfovibrio desulfuricans Norway have been studied. Some structural implications are considered from these experimental data.

Partial purification and characterization of the first hydrogenase isolated from a thermophilic sulfate-reducing bacterium

A soluble [NiFe] hydrogenase has been partially purified from the obligate thermophilic sulfate-reducing bacterium Thermodesulfobacterium mobile. A 17% purification yield was obtained after four chromatographic steps and the hydrogenase presents a purity index (A398 nm/A277 nm) equal to 0.21. This protein appears to be 75% pure on SDS-gel electrophoresis showing two major bands of molecular mass around 55 and 15 kDa. This hydrogenase contains 0.6-0.7 nickel atom and 7-8 iron atoms per mole of enzyme and has a specific activity of 783 in the hydrogen uptake reaction, of 231 in the hydrogen production assay and of 84 in the deuterium-proton exchange reaction. The H2/HD ratio is lower than one in the D2-H+ exchange reaction. The enzyme is very sensitive to NO, relatively little inhibited by CO but unaffected by NO2-. The EPR spectrum of the native hydrogenase shows the presence of a [3Fe-4S] oxidized cluster and of a Ni(III) species.

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.

A gas chromatographic-mass spectrometric technique for studying simultaneous hydrogen-deuteron exchange and para-orthohydrogen conversion in hydrogenases of Desulfovibrio vulgaris Hildenborough

An original gas chromatographic-mass spectrometric technique is described for studying simultaneous dihydrogen-deuteron exchange and para-ortho H2 conversion catalyzed by different Desulfovibrio hydrogenases. Para and orthohydrogens are separated on an alumina column at the temperature of liquid nitrogen, but if both HD and ortho H2 are present, their retention times are too close to each other for total separation and only one peak is observed with a thermal conductivity detector. In order to resolve the peaks from one another, a fraction of the gas released from the gas chromatograph column is admitted to the ion source of a mass spectrometer, where the gases are separated according to their respective masses. Because of a peak-jumping system, the different components involved in the exchange and in the conversion reactions can be scanned so that the spectra corresponding to mass m/e 2 (para and ortho H2), m/e 3 (HD), and m/e 4 (D2) can be obtained simultaneously. This technique has been employed to resolve a controversial problem concerning the occurrence or lack of any para-orthohydrogen conversion in heavy water. Actually both exchange and conversion were demonstrated to occur with a (NiFe) hydrogenase, whereas with a (NiFeSe) hydrogenase, which had an exchange activity equivalent to that of the former, practically no para-ortho conversion could be observed in D2O. These findings are related to the constitutional and catalytic properties of the hydrogenases belonging to the different classes.

The iron-sulfur centers of the soluble [NiFeSe] hydrogenase, from Desulfovibrio baculatus (DSM 1743). EPR and Mössbauer characterization

The soluble (cytoplasmic plus periplasmic) Ni/Fe-S/Se-containing hydrogenase from Desulfovibrio baculatus (DSM 1743) was purified from cells grown in an 57Fe-enriched medium, and its iron-sulfur centers were extensively characterized by Mössbauer and EPR spectroscopies. The data analysis excludes the presence of a [3Fe-4S] center, either in the native (as isolated) or in the hydrogen-reduced states. In the native state, the non-heme iron atoms are arranged as two diamagnetic [4Fe-4S]2+ centers. Upon reduction, these two centers exhibit distinct and unusual Mössbauer spectroscopic parameters. The centers were found to have similar mid-point potentials (approximately -315 mV) as determined by oxidation-reduction titratins followed by EPR.

Characterization of the nickel-iron periplasmic hydrogenase from Desulfovibrio fructosovorans

The periplasmic hydrogenase from Desulfovibrio fructosovorans grown on fructose/sulfate medium was purified to homogeneity. It exhibits a molecular mass of 88 kDa and is composed of two different subunits of 60 kDa and 28.5 kDa. The absorption spectrum of the enzyme is characteristic of an iron-sulfur protein and its absorption coefficients at 400 and 280 nm are 50 and 180 mM-1 cm-1, respectively. D. fructosovorans hydrogenase contains 11 +/- 1 iron atoms, 0.9 +/- 0.15 nickel atom and 12 +/- 1 acid-labile sulfur atoms/molecule but does not contain selenium. The amino acid composition of the protein and of its subunits, as well as the N-terminal sequences of the small and large subunits, have been determined. The cysteine residues of the protein are distributed between the large (9 residues) and the small subunits (11 residues). Electron spin resonance (ESR) properties of the enzyme are consistent with the presence of nickel(III), [3Fe-4S] and [4Fe-4S] clusters. The hydrogenase of D. fructosovorans isolated under aerobic conditions required an incubation with hydrogen or other reductants in order to express its full catalytic activity. H2 uptake and H2 evolution activities doubled after a 3-h incubation under reducing conditions. Comparison with the (NiFe) hydrogenase from D. gigas shows great structural similarities between the two proteins. However, there are significant differences between the catalytic properties of the two enzymes which can be related to the respective state of their nickel atom. ESR showed a higher proportion of the Ni-B species (g = 2.33, 2.16, 2.01) which can be related to a more facile conversion to the ready state. The periplasmic location of the enzyme and the presence of hydrogenase activity in other cellular compartments are discussed in relation to the ability of D. fructosovorans to participate actively in interspecies hydrogen transfer.

Reconstitution and properties of a coenzyme F420-mediated formate hydrogenlyase system in Methanobacterium formicicum

Formate hydrogenlyase activity in a cell extract of Methanobacterium formicicum was abolished by removal of coenzyme F420; addition of purified coenzyme F420 restored activity. Formate hydrogenlyase activity was reconstituted with three purified components from M. formicicum: coenzyme F420-reducing hydrogenase, coenzyme F420-reducing formate dehydrogenase, and coenzyme F420. The reconstituted system required added flavin adenine dinucleotide (FAD) for maximal activity. Without FAD, the formate dehydrogenase and hydrogenase rapidly lost coenzyme F420-dependent activity relative to methyl viologen-dependent activity. Immunoadsorption of formate dehydrogenase or coenzyme F420-reducing hydrogenase from the cell extract greatly reduced formate hydrogenlyase activity; addition of the purified enzymes restored activity. The formate hydrogenlyase activity was reversible, since both the cell extract and the reconstituted system produced formate from H2 plus CO2 and HCO3-.

The three classes of hydrogenases from sulfate-reducing bacteria of the genus Desulfovibrio

Three types of hydrogenases have been isolated from the sulfate-reducing bacteria of the genus Desulfovibrio. They differ in their subunit and metal compositions, physico-chemical characteristics, amino acid sequences, immunological reactivities, gene structures and their catalytic properties. Broadly, the hydrogenases can be considered as 'iron only' hydrogenases and nickel-containing hydrogenases. The iron-sulfur-containing hydrogenase ([Fe] hydrogenase) contains two ferredoxin-type (4Fe-4S) clusters and an atypical iron-sulfur center believed to be involved in the activation of H2. The [Fe] hydrogenase has the highest specific activity in the evolution and consumption of hydrogen and in the proton-deuterium exchange reaction and this enzyme is the most sensitive to CO and NO2-. It is not present in all species of Desulfovibrio. The nickel-(iron-sulfur)-containing hydrogenases [( NiFe] hydrogenases) possess two (4Fe-4S) centers and one (3Fe-xS) cluster in addition to nickel and have been found in all species of Desulfovibrio so far investigated. The redox active nickel is ligated by at least two cysteinyl thiolate residues and the [NiFe] hydrogenases are particularly resistant to inhibitors such as CO and NO2-. The genes encoding the large and small subunits of a periplasmic and a membrane-bound species of the [NiFe] hydrogenase have been cloned in Escherichia (E.) coli and sequenced. Their derived amino acid sequences exhibit a high degree of homology (70%); however, they show no obvious metal-binding sites or homology with the derived amino acid sequence of the [Fe] hydrogenase. The third class is represented by the nickel-(iron-sulfur)-selenium-containing hydrogenases [( NiFe-Se] hydrogenases) which contain nickel and selenium in equimolecular amounts plus (4Fe-4S) centers and are only found in some species of Desulfovibrio. The genes encoding the large and small subunits of the periplasmic hydrogenase from Desulfovibrio (D.) baculatus (DSM 1743) have been cloned in E. coli and sequenced. The derived amino acid sequence exhibits homology (40%) with the sequence of the [NiFe] hydrogenase and the carboxy-terminus of the gene for the large subunit contains a codon (TGA) for selenocysteine in a position homologous to a codon (TGC) for cysteine in the large subunit of the [NiFe] hydrogenase. EXAFS and EPR studies with the 77Se-enriched D. baculatus hydrogenase indicate that selenium is a ligand to nickel and suggest that the redox active nickel is ligated by at least two cysteinyl thiolate and one selenocysteine selenolate residues.(ABSTRACT TRUNCATED AT 400 WORDS)

Electron transfer between the hydrogenase from Desulfovibrio vulgaris (Hildenborough) and viologens. 1. Investigations by cyclic voltammetry

The electron transfer kinetics between the hydrogenase from Desulvovibrio vulgaris (strain Hildenborough) and three different viologen mediators has been investigated by cyclic voltammetry. The mediators methyl viologen, di(n-aminopropyl) viologen and propyl viologen sulfonate differ in redox potential and in net charge. Dependent on the pH both the one- and two-electron-reduced forms or only the two-electron-reduced form of the viologens are effective in electron exchange with hydrogenase. Calculations of the second-order rate constant k for the reaction between reduced viologen and hydrogenase are based on the theory of the simplest electrocatalytic mechanism. Values for k are in the range of 10(6)-10(7) M-1 s-1 and increase in the direction propyl viologen sulfonate----methyl viologen----di(n-aminopropyl) viologen. An explanation is based on electrostatic interactions. It is proposed that the electron transfer reaction is the rate-determining step in the catalytic mechanism.

Identification of three classes of hydrogenase in the genus, Desulfovibrio

A comparison of amino-terminal amino acid sequences from the large and small subunits of hydrogenases from Desulfovibrio reveals significant differences. These results, in conjunction with antibody analyses, clearly indicate that the iron, iron + nickel, and iron + nickel + selenium containing hydrogenases represent three distinct classes of hydrogenase in Desulfovibrio.

Effect of pH on tritium exchange and hydrogen production and uptake in free-living cells and in bacteroids of Bradyrhizobium japonicum

Soybean nodule bacteroids and Bradyrhizobium japonicum free-living cells induced for H2-uptake hydrogenase, actively catalyze the evolution of H2 in a reaction highly dependent on the pH. The optimal pHs for the evolution and uptake reactions were 4.0 and 7.5-8.0, respectively. No differences were found between free-living cells and bacteroids with respect to hydrogen acceptor specificity, although absolute rates of H2 uptake were higher for free-living cells. Both types of cells were able to evolve hydrogen from reduced methyl viologen at low pH. These intact cells also catalyzed the exchange reaction between tritium and water in the absence of oxygen. The pH profile of the exchange activity showed two peaks at values near the optimal pHs for the evolution and uptake reactions.

Cloning and sequencing of the genes encoding the large and small subunits of the periplasmic (NiFeSe) hydrogenase of Desulfovibrio baculatus

The genes coding for the large and small subunits of the periplasmic hydrogenase from Desulfovibrio baculatus have been cloned and sequenced. The genes are arranged in an operon with the small subunit gene preceding the large subunit gene. The small subunit gene codes for a 32 amino acid leader sequence supporting the periplasmic localization of the protein, however no ferredoxin-like or other characteristic iron-sulfur coordination sites were observed. The periplasmic hydrogenases from D. baculatus (an NiFeSe protein) and D. vulgaris (an Fe protein) exhibit no homology suggesting that they are structurally different, unrelated entities.

Nickel-[iron-sulfur]-selenium-containing hydrogenases from Desulfovibrio baculatus (DSM 1743). Redox centers and catalytic properties

The hydrogenase from Desulfovibrio baculatus (DSM 1743) was purified from each of three different fractions: soluble periplasmic (wash), soluble cytoplasmic (cell disruption) and membrane-bound (detergent solubilization). Plasma-emission metal analysis detected in all three fractions the presence of iron plus nickel and selenium in equimolecular amounts. These hydrogenases were shown to be composed of two non-identical subunits and were distinct with respect to their spectroscopic properties. The EPR spectra of the native (as isolated) enzymes showed very weak isotropic signals centered around g approximately 2.0 when observed at low temperature (below 20 K). The periplasmic and membrane-bound enzymes also presented additional EPR signals, observable up to 77 K, with g greater than 2.0 and assigned to nickel(III). The periplasmic hydrogenase exhibited EPR features at 2.20, 2.06 and 2.0. The signals observed in the membrane-bound preparations could be decomposed into two sets with g at 2.34, 2.16 and approximately 2.0 (component I) and at 2.33, 2.24, and approximately 2.0 (component II). In the reduced state, after exposure to an H2 atmosphere, all the hydrogenase fractions gave identical EPR spectra. EPR studies, performed at different temperatures and microwave powers, and in samples partially and fully reduced (under hydrogen or dithionite), allowed the identification of two different iron-sulfur centers: center I (2.03, 1.89 and 1.86) detectable below 10 K, and center II (2.06, 1.95 and 1.88) which was easily saturated at low temperatures. Additional EPR signals due to transient nickel species were detected with g greater than 2.0, and a rhombic EPR signal at 77 K developed at g 2.20, 2.16 and 2.0. This EPR signal is reminiscent of the Ni-signal C (g at 2.19, 2.14 and 2.02) observed in intermediate redox states of the well characterized Desulfovibrio gigas hydrogenase (Teixeira et al. (1985) J. Biol. Chem. 260, 8942]. During the course of a redox titration at pH 7.6 using H2 gas as reductant, this signal attained a maximal intensity around -320 mV. Low-temperature studies of samples at redox states where this rhombic signal develops (10 K or lower) revealed the presence of a fast-relaxing complex EPR signal with g at 2.25, 2.22, 2.15, 2.12, 2.10 and broad components at higher field. The soluble hydrogenase fractions did not show a time-dependent activation but the membrane-bound form required such a step in order to express full activity.(ABSTRACT TRUNCATED AT 400 WORDS)

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.