Reductive activation and redox properties of hydrogenase from Desulfovibrio gigas

The oxidative inactivation of FeFe hydrogenase reveals the flexibility of the H-cluster

Nature is a valuable source of inspiration in the design of catalysts, and various approaches are used to elucidate the mechanism of hydrogenases, the enzymes that oxidize or produce H2. In FeFe hydrogenases, H2 oxidation occurs at the H-cluster, and catalysis involves H2 binding on the vacant coordination site of an iron centre. Here, we show that the reversible oxidative inactivation of this enzyme results from the binding of H2 to coordination positions that are normally blocked by intrinsic CO ligands. This flexibility of the coordination sphere around the reactive iron centre confers on the enzyme the ability to avoid harmful reactions under oxidizing conditions, including exposure to O2. The versatile chemistry of the diiron cluster in the natural system might inspire the design of novel synthetic catalysts for H2 oxidation.

Inhibition of respiration and nitrate assimilation enhances photohydrogen evolution under low oxygen concentrations in Synechocystis sp. PCC 6803

In cyanobacterial membranes photosynthetic light reaction and respiration are intertwined. It was shown that the single hydrogenase of Synechocystis sp. PCC 6803 is connected to the light reaction. We conducted measurements of hydrogenase activity, fermentative hydrogen evolution and photohydrogen production of deletion mutants of respiratory electron transport complexes. All single, double and triple mutants of the three terminal respiratory oxidases and the ndhB-mutant without a functional complex I were studied. After activating the hydrogenase by applying anaerobic conditions in the dark hydrogen production was measured at the onset of light. Under these conditions respiratory capacity and amount of photohydrogen produced were found to be inversely correlated. Especially the absence of the quinol oxidase induced an increased hydrogenase activity and an increased production of hydrogen in the light compared to wild type cells. Our results support that the hydrogenase as well as the quinol oxidase function as electron valves under low oxygen concentrations. When the activities of photosystem II and I (PSII and PSI) are not in equilibrium or in case that the light reaction is working at a higher pace than the dark reaction, the hydrogenase is necessary to prevent an acceptor side limitation of PSI, and the quinol oxidase to prevent an overreduction of the plastoquinone pool (acceptor side of PSII). Besides oxygen, nitrate assimilation was found to be an important electron sink. Inhibition of nitrate reductase resulted in an increased fermentative hydrogen production as well as higher amounts of photohydrogen.

Sustained photoevolution of molecular hydrogen in a mutant of Synechocystis sp. strain PCC 6803 deficient in the type I NADPH-dehydrogenase complex

The interaction between hydrogen metabolism, respiration, and photosynthesis was studied in vivo in whole cells of Synechocystis sp. strain PCC 6803 by continuously monitoring the changes in gas concentrations (H2, CO2, and O2) with an online mass spectrometer. The in vivo activity of the bidirectional [NiFe]hydrogenase [H2:NAD(P) oxidoreductase], encoded by the hoxEFUYH genes, was also measured independently by the proton-deuterium (H-D) exchange reaction in the presence of D2. This technique allowed us to demonstrate that the hydrogenase was insensitive to light, was reversibly inactivated by O2, and could be quickly reactivated by NADH or NADPH (+H2). H2 was evolved by cells incubated anaerobically in the dark, after an adaptation period. This dark H2 evolution was enhanced by exogenously added glucose and resulted from the oxidation of NAD(P)H produced by fermentation reactions. Upon illumination, a short (less than 30-s) burst of H2 output was observed, followed by rapid H2 uptake and a concomitant decrease in CO2 concentration in the cyanobacterial cell suspension. Uptake of both H2 and CO2 was linked to photosynthetic electron transport in the thylakoids. In the ndhB mutant M55, which is defective in the type I NADPH-dehydrogenase complex (NDH-1) and produces only low amounts of O2 in the light, H2 uptake was negligible during dark-to-light transitions, allowing several minutes of continuous H2 production. A sustained rate of photoevolution of H2 corresponding to 6 micro mol of H2 mg of chlorophyll(-1) h(-1) or 2 ml of H2 liter(-1) h(-1) was observed over a longer time period in the presence of glucose and was slightly enhanced by the addition of the O2 scavenger glucose oxidase. By the use of the inhibitors DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] and DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone), it was shown that two pathways of electron supply for H2 production operate in M55, namely photolysis of water at the level of photosystem II and carbohydrate-mediated reduction of the plastoquinone pool.

Autocatalytic oscillations in the early phase of the photoreduced methyl viologen-initiated fast kinetic reaction of hydrogenase

The kinetic characteristics of the hydrogen uptake reaction of hydrogenase, obtained by conventional activity measurements, led to the proposal of an autocatalytic reaction step in the hydrogenase cycle or during the activation process. The autocatalytic behavior of an enzyme reaction may result in oscillating concentrations of enzyme intermediates and/or products contributing to the autocatalytic step. This behavior has been investigated in the early phase of the hydrogenase-methyl viologen reaction. To measure fast hydrogenase kinetics, flash-reduced methyl viologen has been used as a light-induced trigger in transient kinetic phenomena associated with intermolecular electron transfer to hydrogenase. Here we report fast kinetic measurements of the hydrogenase-methyl viologen reaction by use of the excimer laser flash-reduced redox dye. The results are evaluated on the assumption of an autocatalytic reaction in the hydrogenase kinetic cycle. The kinetic constants of the autocatalytic reaction, i.e. the methyl viologen binding to and release from hydrogenase, were determined, and limits of the kinetic constants relating to the intramolecular (intraenzyme) reactions were set.

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.

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.

Changes in the electronic structure around Ni in oxidized and reduced selenium-containing hydrogenases from Methanococcus voltae

The selenium-containing F420-reducing hydrogenase from Methanococcus voltae was anaerobically purified to a specific hydrogen-uptake activity of 350 U/mg protein as determined with the natural electron acceptor. The concentrated enzyme was used for EPR-spectroscopic investigations. As isolated, the enzyme showed an EPR spectrum with g(xyz) values of 2.21, 2.15 and 2.01. Illumination of such samples at low temperatures led to an EPR spectrum with g(xyz) values of 2.05, 2.11 and 2.29. These spectra are typical for [NiFe]hydrogenases in the active state. Spectra of samples enriched in 77Se showed a hyperfine interaction between the unpaired spin of the nickel ion and the nuclear spin of one 77Se atom before and after illumination. A 90 degree flip of the electronic z-axis is proposed to explain the hyperfine interaction in both states. This has been demonstrated previously only for the F420-non-reducing hydrogenase from M. voltae, where the selenium atom is present as a selenocysteine residue on an unusually small separate subunit [Sorgenfrei, O., Klein, A. & Albracht, S. P. J. (1993) FEBS Lett. 332, 291-297]. The results demonstrate that the three-dimensional structures of the active sites in the selenium-containing F420-reducing and F420-non-reducing hydrogenases from M. voltae are highly similar and hence are not influenced by the unusual subunit structure of the latter enzyme. Oxidized samples containing either natural selenium or 77Se were prepared from the F420-reducing and the selenium-containing F420-non-reducing hydrogenase. Both enzymes exhibited EPR spectra typical for [NiFe]hydrogenases in the inactive 'ready' state. In contrast to the reduced form, no splitting of the nickel-derived signal due to the nuclear spin of 77Se was observed in the oxidized state, indicating that the electronic z-axis is perpendicular to the Ni-Se direction.

Voltammetric studies of the catalytic electron-transfer process between the Desulfovibrio gigas hydrogenase and small proteins isolated from the same genus

The kinetics of electron transfer between the Desulfovibrio gigas hydrogenase and several electron-transfer proteins from Desulfovibrio species were investigated by cyclic voltammetry, square-wave voltammetry and chronoamperometry. The cytochrome c3 from Desulfovibrio vulgaris (Hildenborough), Desulfovibrio desulfuricans (Norway 4), Desulfovibrio desulfuricans (American Type Culture Collection 27774) and D. gigas (NCIB 9332) were used as redox carriers. They differ in their redox potentials and isoelectric point. Depending on the pH, all the reduced forms of these cytochromes were effective in electron exchange with hydrogenase. Other small electron-transfer proteins such as ferredoxin I, ferredoxin II and rubredoxin from D. gigas were tentatively used as redox carriers. Only ferredoxin II was effective in mediating electron exchange between hydrogenase and the working electrode. The second-order rate constants k for the reaction between reduced proteins and hydrogenase were calculated based on the theory of the simplest electrocatalytic mechanism [Moreno, C., Costa, C., Moura, I., Le Gall, J., Liu, M. Y., Payne, W. J., van Dijk, C. & Moura, J. J. G. (1993) Eur. J. Biochem. 212, 79-86] and the results obtained by cyclic voltammetry were compared with those obtained by chronoamperometry. Values for k of 10(5)-10(6) M-1 s-1 (cytochrome c3 as electron carrier) and 10(4) M-1 s-1 (ferredoxin II as the electron carrier) were determined. The rate-constant values are discussed in terms of the existence of an electrostatic interaction between the electrode surface and the redox carrier and between the redox carrier and a positively charged part of the enzyme.

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.

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.

Effect of redox potential on the catalytic properties of the NAD-dependent hydrogenase from Alcaligenes eutrophus Z1

The effect of redox potential on the catalytic activities of the soluble hydrogenase from the hydrogen bacterium Alcaligenes eutrophus Z1 was studied. Several transitions were observed on the enzyme catalytic activity vs potential profiles. The coenzyme-dependent activities of the hydrogenase, its diaphorase activity and activity toward NAD, are controlled by the Em -300 mV, while the process of hydrogen evolution from reduced methyl viologen is governed by the midpoint redox potential of -435 mV. This value of Em was independent of pH in the range 5 to 8. The redox potential of the medium appears to be one of the major factors determining the hydrogenase activation, inactivation, and catalytic properties. It is suggested that a change in the redox state of the enzyme electron transport chain is followed by structural rearrangements within the protein affecting both the hydrogenase catalytic activity and stability. The probable mechanism of enzyme activity regulation is discussed.

Effect of redox potential on the activation of the NAD-dependent hydrogenase from Alcaligenes eutrophus Z1

A formal kinetic treatment of the autocatalytic activation cycle of the NAD-dependent hydrogenase from Alcaligenes eutrophus Z1 is presented. The value for the enzyme first-order activation rate constant is estimated to be (2.0 +/- 0.6) s-1 (pH 7.8, 25 degrees C). The effect of the redox potential on the activation properties of the NAD-dependent hydrogenase is studied. Hydrogenase activation is controlled by a midpoint redox potential of approximately -100 mV (pH 7.8). Once activated the enzyme is not immediately transformed back into an inactive state on rapid reoxidation and is able to preserve its catalytic properties for at least 3-4 h of intense oxigenation. Several lines of evidence show that the reductive activation of the NAD-dependent hydrogenase is accompanied by a structural reorganization of the protein. A possible origin of the -100 mV transition is discussed. A model for the activation process of the NAD-dependent hydrogenase is suggested.

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)

Crystallization, preliminary X-ray study and crystal activity of the hydrogenase from Desulfovibrio gigas

Hydrogenase (EC 1.12) from Desulfovibrio gigas is a dimeric enzyme (26 and 62 (X 10(3) Mr) that catalyzes the reversible oxidation of molecular hydrogen. Single crystals of hydrogenase have been produced using the hanging drop method, with either PEG (polyethylene glycol) 6000 or ammonium sulfate as precipitants at pH 6.5. X-ray examination of the crystals indicates that those obtained with ammonium sulfate are suitable for structure determination to at least 3.0 A resolution when synchrotron radiation Sources are used (1 A = 0.1 nm). The crystals are monoclinic, with space group C2, and cell dimensions a = 257.0 A, b = 184.7 A, c = 148.3 A and beta = 101.3 degrees, and contain between four and ten molecules per asymmetric unit. The enzyme can be reactivated within the crystals under reducing conditions without crystal damage.

NAD-dependent hydrogenase from Alcaligenes eutrophus Z1: does it have a regulatory centre?

Evidence is presented for the existence of a relatively high-potential regulatory centre in the NAD-dependent hydrogenase from the hydrogen oxidizing bacterium Alcaligenes eutrophus Z1. Reduction of the hydrogenase to the redox potentials lower than -100 mV converts the enzyme into a catalytically active state that is remarkably stable to oxidants. Once activated, the enzyme does not loose its activity on intensive oxygenation for at least 3 hours. A novel hydrogenase ESR signal with a wide temperature optimum and a approximately -100 mV midpoint redox potential was detected. We suggest that the reduction of this redox centre trigger conformational changes in the inactive oxidized enzyme molecule, thus reorganizing the latter into the active one.

Electron transfer between reduced methyl viologen and oxidized glutathione: a new assay of Saccharomyces cerevisiae glutathione reductase

Pure glutathione reductase from Saccharomyces cerevisiae catalyzed under anaerobic conditions the enzymatic reduction of GSSG using electrochemically reduced methyl viologen as electron donor. The new assay was completely dependent on the amount of active enzyme present, and involved the formation of 1 mol GSH per mole of reduced methyl viologen consumed. The enzyme followed a standard Michaelis-Menten kinetics; a Km = 230 microM for reduced methyl viologen and a turnover number of 969 mumol GSSG reduced per minute per micromole enzyme were determined. The enzymatic activity seemed to depend on the redox potential, showing half-maximal activity at -0.407 V. The enzyme was quite specific: the activity using reduced benzyl viologen as electron donor was just 1.5% of that obtained with reduced methyl viologen at the same concentration and potential. Glutathione reductase was totally inactivated after a brief anaerobic exposure with reduced methyl viologen in the absence of GSSG; a partial reactivation was observed following addition of glutathione disulfide. No inhibition of the methyl viologen-dependent activity was observed in the presence of 2',5'-ADP or 2'-P-5'-ADP-ribose, two NADP(H) analogs, at concentrations which drastically inhibited the NADPH-dependent activity, thus suggesting that the reduced viologen does not interact with the pyridine nucleotide-binding site.

EPR of a novel high-spin component in activated hydrogenase from Desulfovibrio vulgaris (Hildenborough)

The EPR of reoxidized hydrogenase from Desulfovibrio vulgaris (H.) has been reinvestigated. In contrast to other workers [(1984) Proc. Natl. Acad. Sci. USA 81, 3728-3732] we find the axial signal with g = 2.06; 2.01 to be only a minor component of concentration 0.03 spin/mol. In the spectrum of fully active reoxidized enzyme this signal is overshadowed by a rhombic signal (0.1 spin/mol) with g = 2.11; 2.05; 2.00 reminiscent of the only signal found for other oxidized bidirectional hydrogenases. In addition, a novel signal has been detected with geff = 5.0 which, under the assumptions that S = 2 and [delta ms] = 2, quantitates to roughly one spin/mol. Ethylene glycol affects the relative intensity of the different signals. It is suggested that O2 sensitization parallels a spin-state transition of an iron-sulfur cluster.

Activation and active sites of nickel-containing hydrogenases

Hydrogenases that contain nickel and iron-sulphur clusters also have a regulatory mechanism, by which exposure to oxidants such as oxygen prevents their reaction with hydrogen. Treatment with reducing agents then causes reactivation. In some hydrogenases from Desulfovibrio species, there is evidence that there are at least two different deactivated states, which differ in their rates of reductive reactivation. The membrane-bound hydrogenase of D. desulfuricans, Norway strain, the periplasmic hydrogenase of D. gigas and the membrane-bound hydrogenase of Alcaligenes eutrophus can be isolated in a state (termed "Unready") which requires up to several hours for full activation by hydrogen. By contrast the soluble hydrogenases of D. desulfuricans and A. eutrophus can be reactivated relatively rapidly. In all of these enzymes, with the exception of the latter one, the existence of the activated and deactivated states can be correlated with different ESR-detectable forms of nickel. The possible functions of nickel and [Fe-4S] clusters in catalysis are discussed.

Differential inhibition of catalytic sites in Desulfovibrio gigas hydrogenase

The hydrogenase of Desulfovibrio gigas has been shown to contain one nickel atom, a cluster with three irons and two clusters of the [4Fe-4S] type in an 89 kDa molecule. Though evidence that the nickel ion is involved in the site of hydrogen activation has been presented for this and other hydrogenases, the role of nickel and of the other redox centres in the protein remains to be firmly identified. We have examined the effects of inhibitors of hydrogenase activity in an attempt to identify the functions of the prosthetic redox centres. We have shown carbon monoxide to inhibit at the site of hydrogen activation. The dye, procion red, was found to compete with electron acceptors at a different site, and partial denaturation with the detergent lithium dodecyl sulphate resulted in the differential inhibition of hydrogen activation and substrate reduction. These results imply the presence of distinct domains within the protein with different catalytic activities.

Redox properties and active center of phototrophic bacteria hydrogenases

It is shown that the activity of phototrophic bacteria hydrogenases depends on the redox potential (Eh) of the medium. Hydrogenase from the purple sulfur bacterium Thiocapsa roseopersicina strain BBS reversibly activates H2 at Eh less than -290 mV (pH 7.0). When Eh is increased from -290 to -170 mV, the enzyme is converted into an inactive form which is accompanied by one-electron oxidation of its Fe-S cluster. In contrast, the hydrogenases of the purple nonsulfur bacterium Rhodobacter capsulatus B10 and the green sulfur bacterium Chlorobium limicola forma thiosulfatophilum exhibit maximum activity at Eh greater than -300 mV, favourable only for H2 uptake. When Eh decreases the activities of these enzymes drop dramatically; this accounts for their unidirectional effect directed mainly towards H2 uptake. Such dependence on Eh of activity of hydrogenases from these bacteria correlates with their physiological function in the metabolism of phototrophic bacteria, i.e. with the catalysis of the H2 uptake reaction. Hydrogenases from purple bacteria contain nickel and a single Fe-S cluster. Metal chelators do not affect the activity of these enzymes, which indicates that iron and nickel are tightly bound to the apoprotein. Sulfhydryl compounds irreversibly inactivate T. roseopersicina hydrogenase by 30-40% in the presence of sulfide. Acetylene and carbon monoxide are reversible inhibitors of the enzyme. EPR and inhibitory analysis indicate a direct interaction of H2 with the nickel ion in the active center of the T. roseopersicina hydrogenase.

Activation and deactivation of the membrane-bound hydrogenase from Desulfovibrio desulfuricans, Norway strain

The hydrogenase from D. desulfuricans, when isolated in air, had a low activity in the hydrogen-methyl viologen reductase assay, and no activity in the hydrogen-methylene blue reductase assay. The activity increased markedly during incubation under hydrogen. This process is interpreted in terms of conversion of the enzyme from a relatively inactive Unready state to the Active state. Oxidation by dichloro-indophenol caused conversion to a state in which the hydrogen-uptake activity to methyl viologen was preserved, but hydrogen-methylene blue activity was not. This form is termed the Ready state. This behaviour resembles that of the hydrogenase of Desulfovibrio gigas and thus may be a widespread property of this class of hydrogenases. The electron-spin-resonance spectra of the D. desulfuricans enzyme showed the presence of [3Fe-xS] and [4Fe-4S] clusters. Spectra were also observed in the various states of activation of the enzyme. In these respects, the hydrogenase of D. desulfuricans resembles that from D. gigas, although the latter may have an additional iron-sulphur cluster.