The mechanisms of H2 activation and CO binding by hydrogenase I and hydrogenase II of Clostridium pasteurianum

Hydrogenase I (bidirectional) and hydrogenase II (uptake) of Clostridium pasteurianum have been investigated by electron paramagnetic resonance (EPR) spectroscopy, in the presence and absence of the inhibitor, CO. These hydrogenases contain both a novel type of iron-sulfur cluster (H), which is the proposed site of H2 catalysis, and ferredoxin-type [4Fe-4S] clusters (F). The results show that the H clusters of these two hydrogenases have very different properties. The H cluster of oxidized hydrogenase II (Hox-II) exhibits three distinct EPR signals, two of which are pH-dependent. Hox-II binds CO reversibly to give a single, pH-independent species with a novel, rhombic EPR spectrum. The H cluster of reduced hydrogenase II (Hred-II) does not react with CO. In contrast, the EPR spectrum of Hox-I appears homogeneous and independent of pH. Hox-I has a much lower affinity for CO than Hox-II, and binds CO irreversibly to give an axial EPR signal. Hred-I also binds CO irreversibly. The EPR spectra of Fred-I and Fred-II show little or no change after CO treatment. Prior exposure to CO does not affect the catalytic activity of the reduced or oxidized hydrogenases when assayed in the absence of CO, but both enzymes are irreversibly inactivated if CO is present during catalysis. Mechanisms for H2 activation by hydrogenase I and hydrogenase II are proposed from the determined midpoint potentials (Em, pH 8.0) of H-I and H-II (Em approximately -400 mV, -CO; approximately -360 mV, +CO), F-I (Em = -420 mV, +/- CO), and F-II (Em = -180 mV, +/- CO). These allow one to rationalize the different modes of CO binding to the two hydrogenases and suggest why hydrogenase II preferentially catalyzes H2 oxidation. The results are discussed in light of recent spectroscopic data on the structures of the two H clusters.     

Electron paramagnetic resonance studies of the low temperature photolytic behavior of oxidized hydrogenase I from Clostridium pasteurianum

The effects of CO and O2 on the EPR spectrum of oxidized Clostridium pasteurianum hydrogenase I have been investigated both before and after prolonged exposure to white light at 8 K and 30 K. Low concentrations of O2 were found to induce analogous changes in the EPR spectrum as CO, i.e. conversion of the rhombic signal with g approximately 2.10, 2.04, 2.00, a characteristic of the novel H2-activating center in oxidized Fe-hydrogenases, to an axial signal with g approximately 2.07, 2.01, 2.01. The results suggest a common binding site and mode of coordination for CO and O2 and permit rationalization of conflicting reports from different laboratories concerning the EPR properties of oxidized Fe-hydrogenases. The CO- and O2-induced axial EPR signals were found to be light-sensitive at low temperatures. Moreover, they exhibited indistinguishable and unusual photolysis behavior with the dominant photo-product being dependent on the temperature at which illumination was performed. At 8 K, photodissociation of CO or O2 occurs, resulting in an EPR signal identical with that of the oxidized enzyme in the absence of CO or O2. However, at 30 K, the dominant photoproduct is a rhombic EPR signal with g approximately 2.26, 2.12, 1.89. While the origin of this new EPR signal is uncertain, the g-value anisotropy and relaxation characteristics resemble those of a low spin Fe(III) center. These two photoproducts cannot be thermally or photolytically interconverted, but both are quantitatively reconverted to the original axial EPR signal on warming in the dark to 200 K. A tentative working hypothesis for the nature of the H2-activating center of Fe-hydrogenases is presented that is consistent with the available physiochemical data and permits rationalization of the novel photolysis behavior.     

An investigation of hydrogenase I and hydrogenase II from Clostridium pasteurianum by resonance Raman spectroscopy. Evidence for a [2Fe-2S] cluster in hydrogenase I

Resonance Raman spectra are reported for hydrogenase I and II from Clostridium pasteurianum. These spectra show overlapping bands with contributions from [4Fe-4S] clusters, known to be present in these enzymes, and from novel FeS centers of hitherto undefined structure. For hydrogenase I there are strong bands at 288 and 394 cm-1, which are seen in [2Fe-2S] proteins and in no other FeS species so far examined. In contrast these bands do not appear for hydrogenase II, whose resonance Raman spectrum is dominated by [4Fe-4S] cluster modes. These results provide the first structural information on the hydrogenase I FeS center involved in H2 activation and demonstrate structural differences between hydrogenase I and hydrogenase II.     

The purification of hydrogenase II (uptake hydrogenase) from the anaerobic N2-fixing bacterium Clostridium pasteurianum

The H₂ uptake activity (units/mg protein) of Clostridium pasteurianum cells with methylene blue as the electron acceptor increases with cell density independent of the growth conditions. The H₂ evolution activity (units/mg protein) of the same cells with reduced methyl viologen as the electron donor remains fairly constant under all growth conditions tested. Cells grown under N₂-fixing conditions have the highest H₂ uptake activity and were used for the purification of hydrogenase II (uptake hydrogenase). Attempts to separate hydrogenase II from hydrogenase I (bidirectional hydrogenase) by a previously published method were unreliable. We report here a new large-scale purification procedure which employs a rapid membrane filtration system to fractionate cell-free extracts. Hydrogenases I and II were easily filtered into the low-molecular-weight fraction (M_r less than 100 000), and from this, hydrogenase II was further purified to a homogeneous state. Hydrogenase II is a monomeric iron-sulfur protein of molecular weight 53 000 containing eight iron atoms and eight acid-labile sulfur atoms per molecule. Hydrogenase II catalyzes both H₂ oxidation and H₂ evolution at rates of 3000 and 5.9 μmol H₂ consumed or evolved/min per mg protein, respectively. The purification procedure for hydrogenase II using the filtration system described greatly facilitates the large-scale purification of hydrogenase I and other enzymes from cell-free extracts of C. pasteurianum.     

Solution structure of reduced Clostridium pasteurianum rubredoxin

The solution structure of reduced Clostridium pasteurianum rubredoxin (MW 6100) is reported here. The protein is highly paramagnetic, with iron(II) being in the S=2 spin state. The Hβ protons of the ligating cysteines are barely observed, and not specifically assigned. Seventy-six percent of the protons have been assigned and 1267 NOESY peaks (of which 1037 are meaningful) have been observed. Nonselective T ₁ measurements have been measured by recording four nonselective 180°-τ-NOESY at different τ values, and fitting the intensity recoveries to an exponential recovery. Thirty-six metal-proton upper and lower distance constraints have been obtained from the above measurements. The use of such constraints is assessed with respect to spin delocalization on the sulfur donor atoms. The solution structure obtained with the program DYANA has been refined through restrained energy minimization. A final family of 20 conformers is obtained with no distance violations larger than 0.24?Å, and RMSD values to the mean structure of 0.58 and 1.03?Å for backbone and all heavy atoms, respectively (measured on residues 3–53). The structure is compared to the X-ray structure of the oxidized and of the zinc substituted protein, and to the available structures of other rubredoxins. In particular, the comparison with the crystal structure and the solution structure of the Zn derivative of the highly thermostable Pyrococcus furiosus rubredoxin suggested that the relatively low thermal stability of the clostridial rubredoxin may be tentatively ascribed to the loosening of its secondary structure elements. This research is a further achievement at the frontier of solution structure determinations of paramagnetic proteins.     

Structural similarities between the N-terminal domain of Clostridium pasteurianum hydrogenase and plant-type ferredoxins

An N-terminal domain of Clostridium pasteurianum hydrogenase I, encompassing 76 residues out of the 574 composing the full-size enzyme, had previously been overproduced in Escherichia coli and shown to form a stable fold around a [2Fe-2S] cluster. This domain displays only marginal sequence similarity with [2Fe-2S] proteins of known structure, and therefore, two-dimensional 1H NMR has been implemented to elucidate features of the polypeptide fold. Despite the perturbing presence of the paramagnetic [2Fe-2S] cluster, 57 spin systems were detected in the TOCSY spectra, 52 of which were sequentially assigned through NOE connectivities. Several secondary structure elements were identified. The N terminus of the protein consists of two antiparallel beta strands followed by an alpha helix contacting both strands. Two additional antiparallel beta strands, one of them at the C terminus of the sequence, form a four-stranded beta sheet together with the two N-terminal strands. The proton resonances that can be attributed to this beta2alphabeta2 structural motif undergo no paramagnetic perturbations, suggesting that it is distant from the [2Fe-2S] cluster. In plant- and mammalian-type ferredoxins, a very similar structural pattern is found in the part of the protein farthest from the [2Fe-2S] cluster. This indicates that the N-terminal domain of C. pasteurianum hydrogenase folds in a manner very similar to those of plant- and mammalian-type ferredoxins over a significant part (ca. 50%) of its structure. Even in the vicinity of the metal site, where 1H NMR data are blurred by paramagnetic interactions, the N-terminal domains of hydrogenase and mammalian- and plant-type ferredoxins most likely display significant structural similarity, as inferred from local sequence alignments and from previously reported circular dichroism and resonance Raman spectra. These data afford structural information on a kind of [2Fe-2S] cluster-containing domain that occurs in a number of redox enzymes and complexes. In addition, together with previously published sequence alignments, they highlight the widespread distribution of the plant-type ferredoxin fold in bioenergetic systems encompassing anaerobic metabolism, photosynthesis, and aerobic respiratory chains.     

Infrared studies of the CO-inhibited form of the Fe-only hydrogenase from Clostridium pasteurianum I: examination of its light sensitivity at cryogenic temperatures

Infrared spectroscopy has been used to examine the oxidized and CO-inhibited forms of Fe-only hydrogenase I from Clostridium pasteurianum. For the oxidized enzyme, five bands are detected in the infrared spectral region between 2100 and 1800 cm(-1). The pattern of infrared bands is consistent with the presence of two terminally coordinated carbon monoxide molecules, two terminally coordinated cyanide molecules, and one bridging carbon monoxide molecule, ligated to the Fe atoms of the active site [2Fe] subcluster. Infrared spectra of the carbon monoxide-inhibited state, prepared using both natural abundance CO and 13CO, indicate that the two terminally coordinated CO ligands that are intrinsic to the enzyme are coordinated to different Fe atoms of the active site [2Fe] subcluster. Irradiation of the CO-inhibited state at cryogenic temperatures gives rise to two species with dramatically different infrared spectra. The first species has an infrared spectrum identical to the spectrum of the oxidized enzyme, and can be assigned as arising from the photolysis of the exogenous CO from the active site. This species, which has been observed in X-ray crystallographic measurements [Lemon, B. J., and Peters, J. W. (2000) J. Am. Chem. Soc. 122, 3793], decays above 150 K. The second light-induced species decays above 80 K and is characterized by loss of the infrared band associated with the Fe bridging CO at 1809 cm(-1). Potential models for the second photolysis event are discussed.     

An EPR and electron nuclear double resonance investigation of carbon monoxide binding to hydrogenase I (bidirectional) from Clostridium pasteurianum W5

Previous M?ssbauer and electron nuclear double resonance (ENDOR) studies of oxidized hydrogenase I (bidirectional) from Clostridium pasteurianum W5 demonstrated that this enzyme contains two diamagnetic [4Fe-4S]2+ clusters and an iron-sulfur center of unknown structure and composition that is characterized by its novel M?ssbauer and ENDOR properties. In the present study we combine ENDOR and EPR measurements to show that the novel cluster contains 3-4 iron atoms. In addition, we have used EPR and ENDOR spectroscopies to investigate the effect of binding the competitive inhibitor carbon monoxide to oxidized hydrogenase I, using 13C-labeled CO and enzyme isotopically enriched in 57Fe. Treatment of oxidized enzyme with CO causes the g-tensor of the paramagnetic center to change from rhombic to axial symmetry. The observation of a 13C signal by ENDOR spectroscopy and analysis of the EPR broadening show that a single CO covalently binds to the paramagnetic center. The 13C hyperfine coupling constant (Ac approximately equal to 21 MHz) is within the range observed for inorganic iron-carbonyl clusters. The observation of 57Fe ENDOR signals from two types of iron site ([A1c] approximately 30-34 MHz; [A2c] approximately 6 MHz) and resolved 57Fe hyperfine interactions in the EPR spectrum from two nuclei characterized by [A1c] confirm that the iron-sulfur cluster remains intact upon CO coordination, but show that CO binding greatly changes the 57Fe hyperfine coupling constants.     

M?ssbauer study of Clostridium pasteurianum hydrogenase II. Evidence for a novel three-iron cluster

Hydrogenase II contains two iron-sulfur clusters, one of the [4Fe-4S] type and one of unknown structure with unusual spectral properties (H-cluster). Using M?ssbauer spectroscopy we have studied the H-cluster under a variety of conditions. In the reduced state the cluster exhibits, in zero magnetic field, spectra with the typical 2:1 quadrupole pattern of reduced [3Fe-4S] clusters. However, whereas the latter are paramagnetic (S = 2) the H-cluster is diamagnetic (S = 0). Upon oxidation and exposure to CO the H-cluster exhibits an S = 1/2 EPR spectrum with g values at 2.03, 2.02, and 2.00. In this state, the M?ssbauer spectra reveal two cluster subsites with magnetic hyperfine coupling constants AI = +26.5 MHz and AII = -30 MHz. ENDOR data obtained by Hoffman and co-workers (Telser, J., Benecky, M. J., Adams, M. W. W., Mortenson, L. E., and Hoffman, B. M. (1986) J. Biol. Chem. 261, 13536-13541) show a 57Fe resonance at AIII approximately equal to 9.5 MHz. Analysis of the M?ssbauer spectra shows that this resonance represents one iron site. Our studies of the reduced and CO-bound oxidized states of hydrogenase II suggest that the H-cluster contains three iron atoms. The data obtained for the oxidized H-cluster suggest a novel type of 3-Fe cluster and bear little resemblance to those reported for oxidized [3Fe-4S] clusters with g = 2.01 EPR signals. In the reduced sample the [4Fe-4S]1+ cluster appears to occur in a mixture of two distinct electronic states.