The regulatory H2-sensing [NiFe] hydrogenase of the beta-proteobacterium Ralstonia eutropha displays an Ni-C "active" state after reduction with H2 that is very similar to the reduced Ni-C state of standard [NiFe] hydrogenases. Pulse electron nuclear double resonance (ENDOR) and four-pulse ESEEM (hyperfine sublevel correlation, HYSCORE) spectroscopy are applied to obtain structural information on this state via detection of the electron-nuclear hyperfine coupling constants. Two proton hyperfine couplings are determined by analysis of ENDOR spectra recorded over the full magnetic field range of the EPR spectrum. These are associated with nonexchangeable protons and belong to the beta-CH(2) protons of a bridging cysteine of the NiFe center. The signals of a third proton exhibit a large anisotropic coupling (Ax = 18.4 MHz, Ay = -10.8 MHz, Az = -18 MHz). They disappear from the 1H region of the ENDOR spectra after exchange of H2O with 2H2O and activation with 2H2 instead of H2 gas. They reappear in the 2H region of the ENDOR and HYSCORE spectra. Based on a comparison with the spectroscopically similar [NiFe] hydrogenase of Desulfovibrio vulgaris Miyazaki F, for which the g-tensor orientation of the Ni-C state with respect to the crystal structure is known (Foerster et al. J. Am. Chem. Soc. 2003, 125, 83-93), an assignment of the 1H hyperfine couplings is proposed. The exchangeable proton resides in a bridging position between the Ni and Fe and is assigned to a formal hydride ion. After illumination at low temperature (T = 10 K), the Ni-L state is formed. For the Ni-L state, the strong hyperfine coupling observed for the exchangeable hydrogen in Ni-C is lost, indicating a cleavage of the metal-hydride bond(s). These experiments give first direct information on the position of hydrogen binding in the active NiFe center of the regulatory hydrogenase. It is proposed that such a binding situation is also present in the active Ni-C state of standard hydrogenases.

Recent research on hydrogenases has been notably motivated by a desire to utilize these remarkable hydrogen oxidation catalysts in biotechnological applications. Progress in the development of such applications is substantially hindered by the oxygen sensitivity of the majority of hydrogenases. This problem tends to inspire the study of organisms such as Ralstonia eutropha H16 that produce oxygen-tolerant [NiFe]-hydrogenases. R. eutropha H16 serves as an excellent model system in that it produces three distinct [NiFe]-hydrogenases that each serve unique physiological roles: a membrane-bound hydrogenase (MBH) coupled to the respiratory chain, a cytoplasmic, soluble hydrogenase (SH) able to generate reducing equivalents by reducing NAD⁺ at the expense of hydrogen, and a regulatory hydrogenase (RH) which acts in a signal transduction cascade to control hydrogenase gene transcription. This review will present recent results regarding the biosynthesis, regulation, structure, activity, and spectroscopy of these enzymes. This information will be discussed in light of the question how do organisms adapt the prototypical [NiFe]-hydrogenase system to function in the presence of oxygen.

Hydrogenases, abundant proteins in the microbial world, catalyze cleavage of H 2 into protons and electrons or the evolution of H 2 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 H 2 -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.Hydrogen metabolism, catalyzed by hydrogenase, is widespread in the microbial world. According to the composition of the hydrogen-activating site, hydrogenases are classified as [NiFe], [FeFe], and [Fe] enzymes (1, 2).[NiFe] hydrogenases predominantly oxidize hydrogen to obtain reducing equivalents, whereas [FeFe] hydrogenases are mostly involved in the reduction of protons to dispose of reducing power. Crystal structures, available for both types of hydrogenase, uncovered a complex architecture of the hydrogen-activating site, showing in addition to the cysteine-bound metals diatomic ligands such as CO and CN Ϫ (3). Hydrogenases are usually sensitive to inhibition by oxygen. In particular [FeFe] hydrogenases are irreversibly destroyed by oxygen, whereas oxygen does not affect the structural integrity of [NiFe] hydrogenases but reversibly inactivates their catalytic function. It was shown by various spectroscopic techniques that an oxygen species is bound between nickel and iron (4). This bridging ligand occupies the position that is required for binding of a formal hydride under turnover conditions (5). The bridging oxygen ligand is removed reductively, hence giving hydrogen access to the catalytic site.Some microorganisms that thrive in oxic environments harbor oxygen-tolerant [NiFe] hydrogenases capable of metabolizing hydrogen under aerobic conditions. Oxygen-insensitive hydrogenases are of increasing biotechnological interest, e.g. as catalysts in fuel cells or in biological hydrogen production (6, 7). The ␤-proteobacterium Ralstonia eutropha hosts three different oxygen-tolerant [NiFe] hydrogenases, which enable the organism to use hydrogen as the sole energy source in the presence of oxygen. The periplasmically oriented membranebound hydrogenase (MBH) 1 is connected to the respiratory chain via a b-type cytochr...

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. 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."Molecular hydrogen is frequently used as an energy source by diverse prokaryotic organisms. Many of these bacterial and archaeal species harbor multiple hydrogenases which mediate heterolytic cleavage of H 2 into 2 H ϩ and 2 e Ϫ .[NiFe] hydrogenases are the most dominant enzymes, representing a fairly conserved family of proteins, composed of at least a large active site-containing subunit and a small electron-transferring subunit which bears one to three FeS clusters (1, 2, 18).The facultative chemolithoautotrophic proteobacterium Ralstonia eutropha H16 (formerly Alcaligenes eutrophus [7]) harbors two energy-generating [NiFe] hydrogenases, a membrane-bound enzyme (MBH) and a cytoplasmic enzyme (SH). The MBH is primarily involved in electron transport-coupled phosphorylation, whereas the SH is able to reduce NAD and thus provides the cell with reducing equivalents (38, 40). The composition of the MBH resembles the prototype of [NiFe] hydrogenases whose atomic structure has been resolved by X-ray analysis (50). The two subunits of the R. eutropha MBH, encoded by hoxK and hoxG, are anchored to the outer face of the cytoplasmic membrane via a b-type cytochrome (4). The SH, encoded by hoxF, hoxU, hoxY, and hoxH, contains an FeS-flavoprotein in addition to the hydrogenase moiety (30). Mutants disrupted in either one of the two hydrogenases maintain their ability to grow on H 2 , which indicates that the two enzymes can replace each other physiologically (23).The hydrogenase-related genes of R. eutropha are organized in the MBH and the SH operons, which are regulated coordinately (42). The MBH operon comprises 10 MBH-specific genes in addition to a set of accessory genes whose products are involved in the complex posttranslational maturation of the hydrogenases and the regulation of both the MBH and the SH operon (5,11,24,41). The SH operon harbors the structural genes of the NAD-reducing hydrogenase together with a set of accessory genes which code for maturation proteins (45,47,52).Hydrogenase gene expression is controlled by the m...

Previous genetic studies have revealed a multicomponent signal transduction chain, consisting of an H 2 sensor, a histidine protein kinase, and a response regulator, which controls hydrogenase gene transcription in the proteobacterium Ralstonia eutropha. In this study, we isolated the H 2 sensor and demonstrated that the purified protein forms a complex with the histidine protein kinase. Biochemical and spectroscopic analysis revealed that the H 2 sensor is a cytoplasmic [NiFe]-hydrogenase with unique features. The H 2 -oxidizing activity was 2 orders of magnitude lower than that of standard hydrogenases and insensitive to oxygen, carbon monoxide, and acetylene. Interestingly, only H 2 production but no HD formation was detected in the D 2 /H ؉ exchange assay. Fourier transform infrared data showed an active site similar to that of standard [NiFe]-hydrogenases. It is suggested that the protein environment accounts for a restricted gas diffusion and for the typical kinetic parameters of the H 2 sensor. EPR analysis demonstrated that the [4Fe-4S] clusters within the small subunit were not reduced under hydrogen even in the presence of dithionite. Optical spectra revealed the presence of a novel, redox-active, n ‫؍‬ 2 chromophore that is reduced by H 2 . The possible involvement of this chromophore in signal transduction is discussed.

Infrared (IR) spectra in combination with chemical analyses have recently shown that the active Ni-Fe site of the soluble NAD(+)-reducing [NiFe]-hydrogenase from Ralstonia eutropha contains four cyanide groups and one carbon monoxide as ligands. Experiments presented here confirm this result, but show that a variable percentage of enzyme molecules loses one or two of the cyanide ligands from the active site during routine purification. For this reason the redox conditions during the purification have been optimized yielding hexameric enzyme preparations (HoxFUYHI(2)) with aerobic specific H(2)-NAD(+) activities of 150-185 mumol/min/mg of protein (up to 200% of the highest activity previously reported in the literature). The preparations were highly homogeneous in terms of the active site composition and showed superior IR spectra. IR spectro-electrochemical studies were consistent with the hypothesis that only reoxidation of the reduced enzyme with dioxygen leads to the inactive state, where it is believed that a peroxide group is bound to nickel. Electron paramagnetic resonance experiments showed that the radical signal from the NADH-reduced enzyme derives from the semiquinone form of the flavin (FMN-a) in the hydrogenase module (HoxYH dimer), but not of the flavin (FMN-b) in the NADH-dehydrogenase module (HoxFU dimer). It is further demonstrated that the hexameric enzyme remains active in the presence of NADPH and air, whereas NADH and air lead to rapid destruction of enzyme activity. It is proposed that the presence of NADPH in cells keeps the enzyme in the active state.

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