Energy-Converting [NiFe] Hydrogenases from Archaea and Extremophiles: Ancestors of Complex I

[NiFe] hydrogenases are well-characterized enzymes that have a key function in the H2 metabolism of various microorganisms. In the recent years a subfamily of [NiFe] hydrogenases with unique properties has been identified. The members of this family form multisubunit membrane-bound enzyme complexes composed of at least four hydrophilic and two integral membrane proteins. These six conserved subunits, which built the core of these hydrogenases, have closely related counterparts in energy-conserving NADH:quinone oxidoreductases (complex I). However, the reaction catalyzed by these hydrogenases differs significantly from the reaction catalyzed by complex I. For some of these hydrogenases the physiological role is to catalyze the reduction of H+ with electrons derived from reduced ferredoxins or poly-ferredoxins. This exergonic reaction is coupled to energy conservation by means of electron-transport phosphorylation. Other members of this hydrogenase family mainly function to provide the cell with reduced ferredoxin with H2 as electron donor in a reaction driven by reverse electron transport. As complex I these hydrogenases function as ion pumps and have therefore been designated as energy-converting [NiFe] hydrogenases.     

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)     

Selenium is involved in regulation of periplasmic hydrogenase gene expression in Desulfovibrio vulgaris Hildenborough

Desulfovibrio vulgaris Hildenborough is a good model organism to study hydrogen metabolism in sulfate-reducing bacteria. Hydrogen is a key compound for these organisms, since it is one of their major energy sources in natural habitats and also an intermediate in the energy metabolism. The D. vulgaris Hildenborough genome codes for six different hydrogenases, but only three of them, the periplasmic-facing [FeFe], [FeNi]1, and [FeNiSe] hydrogenases, are usually detected. In this work, we studied the synthesis of each of these enzymes in response to different electron donors and acceptors for growth as well as in response to the availability of Ni and Se. The formation of the three hydrogenases was not very strongly affected by the electron donors or acceptors used, but the highest levels were observed after growth with hydrogen as electron donor and lowest with thiosulfate as electron acceptor. The major effect observed was with inclusion of Se in the growth medium, which led to a strong repression of the [FeFe] and [NiFe]1 hydrogenases and a strong increase in the [NiFeSe] hydrogenase that is not detected in the absence of Se. Ni also led to increased formation of the [NiFe]1 hydrogenase, except for growth with H2, where its synthesis is very high even without Ni added to the medium. Growth with H2 results in a strong increase in the soluble forms of the [NiFe]1 and [NiFeSe] hydrogenases. This study is an important contribution to understanding why D. vulgaris Hildenborough has three periplasmic hydrogenases. It supports their similar physiological role in H2 oxidation and reveals that element availability has a strong influence in their relative expression.     

The crystal structure of a reduced [NiFeSe] hydrogenase provides an image of the activated catalytic center.

BACKGROUND: [NiFeSe] hydrogenases are metalloenzymes that catalyze the reaction H2<-->2H+ + 2e-. They are generally heterodimeric, contain three iron-sulfur clusters in their small subunit and a nickel-iron-containing active site in their large subunit that includes a selenocysteine (SeCys) ligand. RESULTS: We report here the X-ray structure at 2.15 A resolution of the periplasmic [NiFeSe] hydrogenase from Desulfomicrobium baculatum in its reduced, active form. A comparison of active sites of the oxidized, as-prepared, Desulfovibrio gigas and the reduced D. baculatum hydrogenases shows that in the reduced enzyme the nickel-iron distance is 0.4 A shorter than in the oxidized enzyme. In addition, the putative oxo ligand, detected in the as-prepared D. gigas enzyme, is absent from the D. baculatum hydrogenase. We also observe higher-than-average temperature factors for both the active site nickel-selenocysteine ligand and the neighboring Glu18 residue, suggesting that both these moieties are involved in proton transfer between the active site and the molecular surface. Other differences between [NiFeSe] and [NiFe] hydrogenases are the presence of a third [4Fe4S] cluster replacing the [3Fe4S] cluster found in the D. gigas enzyme, and a putative iron center that substitutes the magnesium ion that has already been described at the C terminus of the large subunit of two [NiFe] hydrogenases. CONCLUSIONS: The heterolytic cleavage of molecular hydrogen seems to be mediated by the nickel center and the selenocysteine residue. Beside modifying the catalytic properties of the enzyme, the selenium ligand might protect the nickel atom from oxidation. We conclude that the putative oxo ligand is a signature of inactive 'unready' [NiFe] hydrogenases.     

The H(2) sensor of Ralstonia eutropha is a member of the subclass of regulatory [NiFe] hydrogenases

Two energy-generating hydrogenases enable the aerobic hydrogen bacterium Ralstonia eutropha (formerly Alcaligenes eutrophus) to use molecular hydrogen as the sole energy source. The complex synthesis of the nickel-iron-containing enzymes has to be efficiently regulated in response to H(2), which is available in low amounts in aerobic environments. H(2) sensing in R. eutropha is achieved by a hydrogenase-like protein which controls the hydrogenase gene expression in concert with a two-component regulatory system. In this study we show that the H(2) sensor of R. eutropha is a cytoplasmic protein. Although capable of H(2) oxidation with redox dyes as electron acceptors, the protein did not support lithoautotrophic growth in the absence of the energy-generating hydrogenases. A specifically designed overexpression system for R. eutropha provided the basis for identifying the H(2) sensor as a nickel-containing regulatory protein. The data support previous results which showed that the sensor has an active site similar to that of prototypic [NiFe] hydrogenases (A. J. Pierik, M. Schmelz, O. Lenz, B. Friedrich, and S. P. J. Albracht, FEBS Lett. 438:231-235, 1998). It is demonstrated that in addition to the enzymatic activity the regulatory function of the H(2) sensor is nickel dependent. The results suggest that H(2) sensing requires an active [NiFe] hydrogenase, leaving the question open whether only H(2) binding or subsequent H(2) oxidation and electron transfer processes are necessary for signaling. The regulatory role of the H(2)-sensing hydrogenase of R. eutropha, which has also been investigated in other hydrogen-oxidizing bacteria, is intimately correlated with a set of typical structural features. Thus, the family of H(2) sensors represents a novel subclass of [NiFe] hydrogenases denoted as the "regulatory hydrogenases."     

Electron transfer between hydrogenases and mono- and multiheme cytochromes in Desulfovibrio ssp

 A comparative study of electron transfer between the 16 heme high molecular mass cytochrome (Hmc) from Desulfovibrio vulgaris Hildenborough and the [Fe] and [NiFe] hydrogenases from the same organism was carried out, both in the presence and in the absence of catalytic amounts of cytochrome c ₃. For comparison, this study was repeated with the [NiFe] hydrogenase from D. gigas. Hmc is very slowly reduced by the [Fe] hydrogenase, but faster by either of the two [NiFe] hydrogenases. In the presence of cytochrome c ₃, in equimolar amounts to the hydrogenases, the rates of electron transfer are significantly increased and are similar for the three hydrogenases. The results obtained indicate that the reduction of Hmc by the [Fe] or [NiFe] hydrogenases is most likely mediated by cytochrome c ₃. A similar study with D. vulgaris Hildenborough cytochrome c ₅₅₃ shows that, in contrast, this cytochrome is reduced faster by the [Fe] hydrogenase than by the [NiFe] hydrogenases. However, although catalytic amounts of cytochrome c ₃ have no effect in the reduction by the [Fe] hydrogenase, it significantly increases the rate of reduction by the [NiFe] hydrogenases. Received: 14 April 1998 / Accepted: 25 June 1998     

How Salmonella oxidises H(2) under aerobic conditions

Salmonella enterica serovar Typhimurium is a Gram negative bacterial pathogen and a common cause of food-borne illness. Molecular hydrogen has been shown to be a key respiratory electron donor during infection and H(2) oxidation can be catalysed by three genetically-distinct [NiFe] hydrogenases. Of these, hydrogenases-1 (Hyd-1) and Hyd-2 have well-characterised homologues in Escherichia coli. The third, designated Hyd-5 here, is peculiar to Salmonella and is expressed under aerobic conditions. In this work, Salmonella was genetically modified to enable the isolation and characterisation of Hyd-5. Electrochemical analysis established that Hyd-5 is a H(2)-oxidising enzyme that functions in very low levels of H(2) and sustains this activity in high levels of O(2). In addition, electron paramagnetic resonance spectroscopy of the Hyd-5 isoenzyme reveals a complex paramagnetic FeS signal at high potentials which is comparable to that observed for other O(2)-tolerant respiratory [NiFe] hydrogenases. Taken altogether, Hyd-5 can be classified as an O(2)-tolerant hydrogenase that confers upon Salmonella the ability to use H(2) as an electron donor in aerobic respiration.     

Classification and phylogeny of hydrogenases

Hydrogenases (H2ases) catalyze the reversible oxidation of molecular hydrogen and play a central role in microbial energy metabolism. Most of these enzymes are found in Archaea and Bacteria, but a few are present in Eucarya as well. They can be distributed into three classes: the [Fe]-H2ases, the [NiFe]-H2ases, and the metal-free H2ases. The vast majority of known H2ases belong to the first two classes, and over 100 of these enzymes have been characterized genetically and/or biochemically. Compelling evidence from sequences and structures indicates that the [NiFe]- and [Fe]-H2ases are phylogenetically distinct classes of proteins. The catalytic core of the [NiFe]-H2ases is a heterodimeric protein, although additional subunits are present in many of these enzymes. Functional classes of [NiFe]-H2ases have been defined, and they are consistent with categories defined by sequence similarity of the catalytic subunits. The catalytic core of the [Fe]-H2ases is a ca. 350-residue domain that accommodates the active site (H-cluster). A few monomeric [Fe]-H2ases are barely larger than the H-cluster domain. Many others are monomeric as well, but possess additional domains that contain redox centers, mostly iron-sulfur. Some [Fe]-H2ases are oligomeric. The modular structure of H2ases is strikingly illustrated in recently unveiled sequences and structures. It is also remarkable that most of the accessory domains and subunits of H2ases have counterparts in other redox complexes, in particular NADH-ubiquinone oxidoreductase (Complex I) of respiratory chains. Microbial genome sequences are bringing forth a significant body of additional H2ase sequence data and contribute to the understanding of H2ase distribution and evolution. Altogether, the available data suggest that [Fe]-H2ases are restricted to Bacteria and Eucarya, while [NiFe]-H2ases, with one possible exception, seem to be present only in Archaea and Bacteria. H2ase processing and maturation involve the products of several genes which have been identified and are currently being characterized in the case of the [NiFe]-H2ases. In contrast, near to nothing is known regarding the maturation of the [Fe]-H2ases. Inspection of the currently available genome sequences suggests that the [NiFe]-H2ase maturation proteins have no similar counterparts in the genomes of organisms possessing [Fe]-H2ases only. This observation, if confirmed, would be consistent with the phylogenetic distinctiveness of the two classes of H2ases. Sequence alignments of catalytic subunits of H2ases have been implemented to construct phylogenetic trees that were found to be consistent, in the main, with trees derived from other data. On the basis of the comparisons performed and discussed here, proposals are made to simplify and rationalize the nomenclature of H2ase-encoding genes.     

Analyses of the large subunit histidine-rich motif expose an alternative proton transfer pathway in [NiFe] hydrogenases

A highly conserved histidine-rich region with unknown function was recognized in the large subunit of [NiFe] hydrogenases. The HxHxxHxxHxH sequence occurs in most membrane-bound hydrogenases, but only two of these histidines are present in the cytoplasmic ones. Site-directed mutagenesis of the His-rich region of the T. roseopersicina membrane-attached Hyn hydrogenase disclosed that the enzyme activity was significantly affected only by the replacement of the His104 residue. Computational analysis of the hydrogen bond network in the large subunits indicated that the second histidine of this motif might be a component of a proton transfer pathway including Arg487, Asp103, His104 and Glu436. Substitutions of the conserved amino acids of the presumed transfer route impaired the activity of the Hyn hydrogenase. Western hybridization was applied to demonstrate that the cellular level of the mutant hydrogenases was similar to that of the wild type. Mostly based on theoretical modeling, few proton transfer pathways have already been suggested for [NiFe] hydrogenases. Our results propose an alternative route for proton transfer between the [NiFe] active center and the surface of the protein. A novel feature of this model is that this proton pathway is located on the opposite side of the large subunit relative to the position of the small subunit. This is the first study presenting a systematic analysis of an in silico predicted proton translocation pathway in [NiFe] hydrogenases by site-directed mutagenesis.     

Structural features of [NiFeSe] and [NiFe] hydrogenases determining their different properties: a computational approach

Hydrogenases are metalloenzymes that catalyze the reversible reaction \textH₂ \leftrightarrows 2\textH ⁺ + 2\texte ^- {\text{H}}_{2} \leftrightarrows 2{\text{H}}^{ + } + 2{\text{e}}^{ - } , being potentially useful in H₂ production or oxidation. [NiFeSe] hydrogenases are a particularly interesting subgroup of the [NiFe] class that exhibit tolerance to O₂ inhibition and produce more H₂ than standard [NiFe] hydrogenases. However, the molecular determinants responsible for these properties remain unknown. Hydrophobic pathways for H₂ diffusion have been identified in [NiFe] hydrogenases, as have proton transfer pathways, but they have never been studied in [NiFeSe] hydrogenases. Our aim was, for the first time, to characterize the H₂ and proton pathways in a [NiFeSe] hydrogenase and compare them with those in a standard [NiFe] hydrogenase. We performed molecular dynamics simulations of H₂ diffusion in the [NiFeSe] hydrogenase from Desulfomicrobium baculatum and extended previous simulations of the [NiFe] hydrogenase from Desulfovibrio gigas (Teixeira et al. in Biophys J 91:2035–2045, 2006). The comparison showed that H₂ density near the active site is much higher in [NiFeSe] hydrogenase, which appears to have an alternative route for the access of H₂ to the active site. We have also determined a possible proton transfer pathway in the [NiFeSe] hydrogenase from D. baculatum using continuum electrostatics and Monte Carlo simulation and compared it with the proton pathway we found in the [NiFe] hydrogenase from D. gigas (Teixeira et al. in Proteins 70:1010–1022, 2008). The residues constituting both proton transfer pathways are considerably different, although in the same region of the protein. These results support the hypothesis that some of the special properties of [NiFeSe] hydrogenases could be related to differences in the H₂ and proton pathways.     

Involvement of hyp Gene Products in Maturation of the H2-Sensing [NiFe] Hydrogenase of Ralstonia eutropha

The biosynthesis of [NiFe] hydrogenases is a complex process that requires the function of the Hyp proteins HypA, HypB, HypC, HypD, HypE, HypF, and HypX for assembly of the H(2)-activating [NiFe] site. In this study we examined the maturation of the regulatory hydrogenase (RH) of Ralstonia eutropha. The RH is a H(2)-sensing [NiFe] hydrogenase and is required as a constituent of a signal transduction chain for the expression of two energy-linked [NiFe] hydrogenases. Here we demonstrate that the RH regulatory activity was barely affected by mutations in hypA, hypB, hypC, and hypX and was not substantially diminished in hypD- and hypE-deficient strains. The lack of HypF, however, resulted in a 90% decrease of the RH regulatory activity. Fourier transform infrared spectroscopy and the incorporation of (63)Ni into the RH from overproducing cells revealed that the assembly of the [NiFe] active site is dependent on all Hyp functions, with the exception of HypX. We conclude that the entire Hyp apparatus (HypA, HypB, HypC, HypD, HypE, and HypF) is involved in an efficient incorporation of the [NiFe] center into the RH.     

Oxygen tolerance of the H2-sensing [NiFe] hydrogenase from Ralstonia eutropha H16 is based on limited access of oxygen to the active site

Hydrogenases, abundant proteins in the microbial world, catalyze cleavage of H2 into protons and electrons or the evolution of H2 by proton reduction. Hydrogen metabolism predominantly occurs in anoxic environments mediated by hydrogenases, which are sensitive to inhibition by oxygen. Those microorganisms, which thrive in oxic habitats, contain hydrogenases that operate in the presence of oxygen. We have selected the H2-sensing regulatory [NiFe] hydrogenase of Ralstonia eutropha H16 to investigate the molecular background of its oxygen tolerance. Evidence is presented that the shape and size of the intramolecular hydrophobic cavities leading to the [NiFe] active site of the regulatory hydrogenase are crucial for oxygen insensitivity. Expansion of the putative gas channel by site-directed mutagenesis yielded mutant derivatives that are sensitive to inhibition by oxygen, presumably because the active site has become accessible for oxygen. The mutant proteins revealed characteristics typical of standard [NiFe] hydrogenases as described for Desulfovibrio gigas and Allochromatium vinosum. The data offer a new strategy how to engineer oxygen-tolerant hydrogenases for biotechnological application.     

The H2 sensor of Ralstonia eutropha: biochemical and spectroscopic analysis of mutant proteins modified at a conserved glutamine residue close to the [NiFe] active site

[NiFe] hydrogenases contain a highly conserved histidine residue close to the [NiFe] active site which is altered by a glutamine residue in the H(2)-sensing [NiFe] hydrogenases. In this study, we exchanged the respective glutamine residue of the H(2) sensor (RH) of Ralstonia eutropha, Q67 of the RH large subunit HoxC, by histidine, asparagine and glutamate. The replacement by histidine and asparagine resulted in slightly unstable RH proteins which were hardly affected in their regulatory and enzymatic properties. The exchange to glutamate led to a completely unstable RH protein. The purified wild-type RH and the mutant protein with the Gln/His exchange were analysed by continuous-wave and pulsed electron paramagnetic resonance (EPR) techniques. We observed a coupling of a nitrogen nucleus with the [NiFe] active site for the mutant protein which was absent in the spectrum of the wild-type RH. A combination of theoretical calculations with the experimental data provided an explanation for the observed coupling. It is shown that the coupling is due to the formation of a weak hydrogen bond between the protonated N(epsilon) nucleus of the histidine with the sulfur of a conserved cysteine residue which coordinates the metal atoms of the [NiFe] active site as a bridging ligand. The effect of this hydrogen bond on the local structure of the [NiFe] active site is discussed.     

The Origin of Cluster N2 of the Energy-Transducing NADH–Quinone Oxidoreductase: Comparisons of Phylogenetically Related Enzymes

NADH–quinone (Q) oxidoreductase is a large and complex redox proton pump, which utilizes the free energy derived from oxidation of NADH with lipophilic electron/proton carrier Q to translocate protons across the membrane to generate an electrochemical proton gradient ( ). Although its molecular mechanism is largely unknown, recent biochemical, biophysical, and molecular biological studies have revealed that particular subunits and cofactors play an essential role in the energy-coupling reaction. Based on these latest experimental data, we exhaustively analyzed the sequence information available from evolutionarily related enzymes such as [NiFe] hydrogenases. We found significant and conserved sequence differences in the PSST/Nqo6/NuoB, 49kDa/Nqo4/NuoD, and ND1/Nqo8/NuoH subunit homologs between complex I/NDH-1 and [NiFe] hydrogenases. The alterations, especially in the postulated ligand motif for cluster N2 in the PSST/Nqo6/NuoB subunits, appear to be evolutionarily important in determining the physiological function of complex I/NDH-1. These observations led us to propose a hypothetical evolutionary scheme: during the course of evolution, drastic changes have occurred in the putative cluster N2 binding site in the PSST/Nqo6/NuoB subunit and the progenitors of complex I/NDH-1 have concurrently become to utilize a lipophilic electron/proton carrier such as Q as its physiological substrate. This scheme provides new insights into the structure and function relationship of complex I/NDH-1 and may help us understand its energy-coupling mechanism.     

Characterization of a cyanobacterial-like uptake [NiFe] hydrogenase: EPR and FTIR spectroscopic studies of the enzyme from Acidithiobacillus ferrooxidans

Electron paramagnetic resonance (EPR) and Fourier transform IR studies on the soluble hydrogenase from Acidithiobacillus ferrooxidans are presented. In addition, detailed sequence analyses of the two subunits of the enzyme have been performed. They show that the enzyme belongs to a group of uptake [NiFe] hydrogenases typical for Cyanobacteria. The sequences have also a close relationship to those of the H₂-sensor proteins, but clearly differ from those of standard [NiFe] hydrogenases. It is concluded that the structure of the catalytic centre is similar, but not identical, to that of known [NiFe] hydrogenases. The active site in the majority of oxidized enzyme molecules, 97% in cells and more than 50% in the purified enzyme, is EPR-silent. Upon contact with H₂ these sites remain EPR-silent and show only a limited IR response. Oxidized enzyme molecules with an EPR-detectable active site show a Ni_r*-like EPR signal which is light-sensitive at cryogenic temperatures. This is a novelty in the field of [NiFe] hydrogenases. Reaction with H₂ converts these active sites to the well-known Ni_a-C* state. Illumination below 160 K transforms this state into the Ni_a-L* state. The reversal, in the dark at 200 K, proceeds via an intermediate Ni EPR signal only observed with the H₂-sensor protein from Ralstonia eutropha. The EPR-silent active sites in as-isolated and H₂-treated enzyme are also light-sensitive as observed by IR spectra at cryogenic temperatures. The possible origin of the light sensitivity is discussed. This study represents the first spectral characterization of an enzyme of the group of cyanobacterial uptake hydrogenases. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Oxygen-tolerant H2 oxidation by membrane-bound [NiFe] hydrogenases of ralstonia species. Coping with low level H2 in air

Knallgas bacteria such as certain Ralstonia spp. are able to obtain metabolic energy by oxidizing trace levels of H2 using O2 as the terminal electron acceptor. The [NiFe] hydrogenases produced by these organisms are unusual in their ability to oxidize H2 in the presence of O2, which is a potent inactivator of most hydrogenases through attack at the active site. To probe the origin of this unusual O2 tolerance, we conducted a study on the membrane-bound hydrogenase from Ralstonia eutropha H16 and that of the closely related organism Ralstonia metallidurans CH34, which was purified using a new heterologous overproduction system. Direct electrochemical methods were used to determine apparent inhibition constants for O2 inhibition of H2 oxidation (K I(app)O2) for each enzyme. These values were at least 2 orders of magnitude higher than those of "standard" [NiFe] hydrogenases. Amino acids close to the active site were exchanged in the membrane-bound hydrogenase of R. eutropha H16 for those from standard hydrogenases to probe the role of individual residues in conferring O2 sensitivity. Michaelis constants for H2 (K M H2) were determined, and for some mutants these were increased more than 20-fold relative to the wild type. Mutations resulting in membrane-bound hydrogenase enzymes with increased K M H2 or decreased K I(app)O2 values were associated with impaired lithoautotrophic growth in the presence of high O2 concentrations.     

Purification and catalytic properties of a CO-oxidizing:H2-evolving enzyme complex from Carboxydothermus hydrogenoformans.

From the membrane fraction of the Gram-positive bacterium Carboxydothermus hydrogenoformans, an enzyme complex catalyzing the conversion of CO to CO2 and H2 was purified. The enzyme complex showed maximal CO-oxidizing:H2-evolving enzyme activity with 5% CO in the headspace (450 U per mg protein). Higher CO concentrations inhibited the hydrogenase present in the enzyme complex. For maximal activity, the enzyme complex had to be activated by either CO or strong reductants. The enzyme complex also catalyzed the CO- or H2-dependent reduction of methylviologen at 5900 and 180 U per mg protein, respectively. The complex was found to be composed of six hydrophilic and two hydrophobic polypeptides. The amino-terminal sequences of the six hydrophilic subunits were determined allowing the identification of the encoding genes in the preliminary genome sequence of C. hydrogenoformans. From the sequence analysis it was deduced that the enzyme complex is formed by a Ni-containing carbon monoxide dehydrogenase (CooS), an electron transfer protein containing four [4Fe-4S] clusters (CooF) and a membrane bound [NiFe] hydrogenase composed of four hydrophilic subunits and two membrane integral subunits. The hydrogenase part of the complex shows high sequence similarity to members of a small group of [NiFe] hydrogenases with sequence similarity to energy conserving NADH:quinone oxidoreductases. The data support a model in which the enzyme complex is composed of two catalytic sites, a CO-oxidizing site and a H2-forming site, which are connected via a different iron-sulfur cluster containing electron transfer subunits. The exergonic redox reaction catalyzed by the enzyme complex in vivo has to be coupled to energy conservation, most likely via the generation of a proton motive force.     

Exploring the Catalytic Core of Complex I by Yarrowia lipolytica Yeast Genetics

We have developed Yarrowia lipolytica as a model system to study mitochondrial complex I that combines the application of fast and convenient yeast genetics with efficient structural and functional analysis of its very stable complex I isolated by his–tag affinity purification with high yield. Guided by a structural model based on homologies between complex I and [NiFe] hydrogenases mutational analysis revealed that the 49 kDa subunit plays a central functional role in complex I. We propose that critical parts of the catalytic core of complex I have evolved from the hydrogen reactive site of [NiFe] hydrogenases and that iron–sulfur cluster N2 resides at the interface between the 49 kDa and PSST subunits. These findings are in full agreement with the semiquinone switch mechanism according to which coupling of electron and proton transfer in complex I is achieved by a single integrated pump comprising cluster N2, the binding site for substrate ubiquinone, and a tightly bound quinone or quinoid group.