Mössbauer characterization of the iron-sulfur clusters in Desulfovibrio vulgaris hydrogenase

Chloride- and Hydrosulfide-Bound 2Fe Complexes as Models of the Oxygen-Stable State of [FeFe] Hydrogenase

[FeFe] hydrogenases demonstrate remarkable catalytic efficiency in hydrogen evolution and oxidation processes. However, susceptibility of these enzymes to oxygen-induced degradation impedes their practical deployment in hydrogen-production devices and fuel cells. Recent investigations into the oxygen-stable (Hinact) state of the H-cluster revealed its inherent capacity to resist oxygen degradation. Herein, we present findings on Cl- and SH-bound [2Fe-2S] complexes, bearing relevance to the oxygen-stable state within a biological context. A characteristic attribute of these complexes is the terminal Cl-/SH- ligation to the iron center bearing the CO bridge. Structural analysis of the t-Cl demonstrates a striking resemblance to the Hinact state of DdHydAB and CbA5H. The t-Cl/t-SH exhibit reversible oxidation, with both redox species, electronically, being the first biomimetic analogs to the Htrans and Hinact states. These complexes exhibit notable resistance against oxygen-induced decomposition, supporting the potential oxygen-resistant nature of the Htrans and Hinact states. The swift reductive release of the Cl-/SH-group demonstrates its labile and kinetically controlled binding. The findings garnered from these investigations offer valuable insights into properties of the enzymatic O2-stable state, and key factors governing deactivation and reactivation conversion. This work contributes to the advancement of bio-inspired molecular catalysts and the integration of enzymes and artificial catalysts into H2-evolution devices and fuel-cell applications.

Oxygen sensitivity of [FeFe]-hydrogenase: a comparative study of active site mimics inside vs. outside the enzyme

[FeFe]-hydrogenase is nature's most efficient proton reducing and H2-oxidizing enzyme. However, biotechnological applications are hampered by the O2 sensitivity of this metalloenzyme, and the mechanism of aerobic deactivation is not well understood. Here, we explore the oxygen sensitivity of four mimics of the organometallic active site cofactor of [FeFe]-hydrogenase, [Fe2(adt)(CO)6-x(CN)x]x- and [Fe2(pdt)(CO)6-x(CN)x]x- (x = 1, 2) as well as the corresponding cofactor variants of the enzyme by means of infrared, Mössbauer, and NMR spectroscopy. Additionally, we describe a straightforward synthetic recipe for the active site precursor complex Fe2(adt)(CO)6. Our data indicate that the aminodithiolate (adt) complex, which is the synthetic precursor of the natural active site cofactor, is most oxygen sensitive. This observation highlights the significance of proton transfer in aerobic deactivation, and supported by DFT calculations facilitates an identification of the responsible reactive oxygen species (ROS). Moreover, we show that the ligand environment of the iron ions critically influences the reactivity with O2 and ROS like superoxide and H2O2 as the oxygen sensitivity increases with the exchange of ligands from CO to CN-. The trends in aerobic deactivation observed for the model complexes are in line with the respective enzyme variants. Based on experimental and computational data, a model for the initial reaction of [FeFe]-hydrogenase with O2 is developed. Our study underscores the relevance of model systems in understanding biocatalysis and validates their potential as important tools for elucidating the chemistry of oxygen-induced deactivation of [FeFe]-hydrogenase.

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.

A personal account on 25 years of scientific literature on [FeFe]-hydrogenase

[FeFe]-hydrogenases are gas-processing metalloenzymes that catalyze H₂ oxidation and proton reduction (H₂ release) in microorganisms. Their high turnover frequencies and lack of electrical overpotential in the hydrogen conversion reaction has inspired generations of biologists, chemists, and physicists to explore the inner workings of [FeFe]-hydrogenase. Here, we revisit 25 years of scientific literature on [FeFe]-hydrogenase and propose a personal account on 'must-read' research papers and review article that will allow interested scientists to follow the recent discussions on catalytic mechanism, O₂ sensitivity, and the in vivo synthesis of the active site cofactor with its biologically uncommon ligands carbon monoxide and cyanide. Focused on-but not restricted to-structural biology and molecular biophysics, we highlight future directions that may inspire young investigators to pursue a career in the exciting and competitive field of [FeFe]-hydrogenase research.

Second and Outer Coordination Sphere Effects in Nitrogenase, Hydrogenase, Formate Dehydrogenase, and CO Dehydrogenase

Gases like H2, N2, CO2, and CO are increasingly recognized as critical feedstock in "green" energy conversion and as sources of nitrogen and carbon for the agricultural and chemical sectors. However, the industrial transformation of N2, CO2, and CO and the production of H2 require significant energy input, which renders processes like steam reforming and the Haber-Bosch reaction economically and environmentally unviable. Nature, on the other hand, performs similar tasks efficiently at ambient temperature and pressure, exploiting gas-processing metalloenzymes (GPMs) that bind low-valent metal cofactors based on iron, nickel, molybdenum, tungsten, and sulfur. Such systems are studied to understand the biocatalytic principles of gas conversion including N2 fixation by nitrogenase and H2 production by hydrogenase as well as CO2 and CO conversion by formate dehydrogenase, carbon monoxide dehydrogenase, and nitrogenase. In this review, we emphasize the importance of the cofactor/protein interface, discussing how second and outer coordination sphere effects determine, modulate, and optimize the catalytic activity of GPMs. These may comprise ionic interactions in the second coordination sphere that shape the electron density distribution across the cofactor, hydrogen bonding changes, and allosteric effects. In the outer coordination sphere, proton transfer and electron transfer are discussed, alongside the role of hydrophobic substrate channels and protein structural changes. Combining the information gained from structural biology, enzyme kinetics, and various spectroscopic techniques, we aim toward a comprehensive understanding of catalysis beyond the first coordination sphere.

Electronic and Structural Properties of the Double Cubane Iron-Sulfur Cluster

The double-cubane cluster (DCC) refers to an [Fe8S9] iron-sulfur complex that is otherwise only known to exist in nitrogenases. Containing a bridging µ2-S ligand, the DCC in the DCC-containing protein (DCCP) is covalently linked to the protein scaffold via six coordinating cysteine residues. In this study, the nature of spin coupling and the effect of spin states on the cluster’s geometry are investigated computationally. Using density functional theory (DFT) and a broken symmetry (BS) approach to study the electronic ground state of the system, we computed the exchange interaction between the spin-coupled spins of the four FeFe dimers contained in the DCC. This treatment yields results that are in excellent agreement with both computed and experimentally determined exchange parameters for analogously coupled di-iron complexes. Hybrid quantum mechanical (QM)/molecular mechanical (MM) geometry optimizations show that cubane cluster A closest to charged amino acid side chains (Arg312, Glu140, Lys146) is less compact than cluster B, indicating that electrons of the same spin in a charged environment seek maximum separation. Overall, this study provides the community with a fundamental reference for subsequent studies of DCCP, as well as for investigations of other [Fe8S9]-containing enzymes.

TiO2 protection layer and well-matched interfaces enhance the stability of Cu2ZnSnS4/CdS/TiO2 for visible light driven water splitting

Cu₂ZnSnS₄-Based heterostructure materials present broad spectral absorption and a high absorption coefficient in the visible region; however, their photocatalytic activity and stability are still impeded due to photochemical corrosion.

Caught in the Hinact : Crystal Structure and Spectroscopy Reveal a Sulfur Bound to the Active Site of an O2 -stable State of [FeFe] Hydrogenase

[FeFe] hydrogenases are the most active H2 converting catalysts in nature, but their extreme oxygen sensitivity limits their use in technological applications. The [FeFe] hydrogenases from sulfate reducing bacteria can be purified in an O2 -stable state called Hinact . To date, the structure and mechanism of formation of Hinact remain unknown. Our 1.65 Å crystal structure of this state reveals a sulfur ligand bound to the open coordination site. Furthermore, in-depth spectroscopic characterization by X-ray absorption spectroscopy (XAS), nuclear resonance vibrational spectroscopy (NRVS), resonance Raman (RR) spectroscopy and infrared (IR) spectroscopy, together with hybrid quantum mechanical and molecular mechanical (QM/MM) calculations, provide detailed chemical insight into the Hinact state and its mechanism of formation. This may facilitate the design of O2 -stable hydrogenases and molecular catalysts.

Mössbauer Spectroscopy and Theoretical Studies of Iron Bimetallic Complexes Showing Electrocatalytic Hydrogen Evolution

Mössbauer spectroscopy and density functional theory (DFT) calculations are reported for the mononuclear Fe-nitrosyl complex [Fe( N, N'-bis(2-mercaptoethyl)-1,4-diazacycloheptane)NO] {[Fe(bme-dach)(NO)] (1)} and the series of dithiolate-bridged dinuclear complexes M-Fe(CO)Cp [M = Fe(bme-dach)(NO) (1-A), Ni(bme-dach) (2-A), and Co(bme-dach)(NO) (3-A)], in which M is a metallo-ligand to Fe(CO)Cp⁺ (Fe'_Cp). The latter is an organometallic fragment in which Fe is coordinated by one CO and one cyclopentadienyl ligand. Complexes 1-A and 2-A were previously shown to have electrocatalytic hydrogen evolution activity. Mononuclear {Fe-NO}⁷ complex 1, with overall spin of ¹/₂, has an isomer shift of 0.23(2) mm/s [Δ E_Q = 1.37(2) mm/s] and magnetic hyperfine couplings of {-38 T, -26.8 T, 8.6 T}. In complexes 2-A and 3-A, Fe'(CO)Cp⁺ has a diamagnetic ground state and δ = 0.33(2) mm/s (Δ E_Q ≈ 1.78 mm/s), consistent with a low-spin Fe^II site. In contrast, in complex 1-A, M = Fe(bme-dach)(NO) (i.e., complex 1) the magnetic hyperfine interactions of both metallo-ligand, M, and low-spin Fe'_Cp are perturbed and Fe'_Cp exhibits small magnetic hyperfine interactions, although its isomer shift and quadrupole splittings are largely unaltered. The DFT calculations for 1-A are in agreement with the paramagnetism observed for the Fe'(CO)Cp⁺ iron site.

Differential Protonation at the Catalytic Six-Iron Cofactor of [FeFe]-Hydrogenases Revealed by 57Fe Nuclear Resonance X-ray Scattering and Quantum Mechanics/Molecular Mechanics Analyses

[FeFe]-hydrogenases are efficient biological hydrogen conversion catalysts and blueprints for technological fuel production. The relations between substrate interactions and electron/proton transfer events at their unique six-iron cofactor (H-cluster) need to be elucidated. The H-cluster comprises a four-iron cluster, [4Fe4S], linked to a diiron complex, [FeFe]. We combined ⁵⁷Fe-specific X-ray nuclear resonance scattering experiments (NFS, nuclear forward scattering; NRVS, nuclear resonance vibrational spectroscopy) with quantum-mechanics/molecular-mechanics computations to study the [FeFe]-hydrogenase HYDA1 from a green alga. Selective ⁵⁷Fe labeling at only [4Fe4S] or [FeFe], or at both subcomplexes was achieved by protein expression with a ⁵⁷Fe salt and in vitro maturation with a synthetic diiron site precursor containing ⁵⁷Fe. H-cluster states were populated under infrared spectroscopy control. NRVS spectral analyses facilitated assignment of the vibrational modes of the cofactor species. This approach revealed the H-cluster structure of the oxidized state (Hox) with a bridging carbon monoxide at [FeFe] and ligand rearrangement in the CO-inhibited state (Hox-CO). Protonation at a cysteine ligand of [4Fe4S] in the oxidized state occurring at low pH (HoxH) was indicated, in contrast to bridging hydride binding at [FeFe] in a one-electron reduced state (Hred). These findings are direct evidence for differential protonation either at the four-iron or diiron subcomplex of the H-cluster. NFS time-traces provided Mössbauer parameters such as the quadrupole splitting energy, which differ among cofactor states, thereby supporting selective protonation at either subcomplex. In combination with data for reduced states showing similar [4Fe4S] protonation as HoxH without (Hred') or with (Hhyd) a terminal hydride at [FeFe], our results imply that coordination geometry dynamics at the diiron site and proton-coupled electron transfer to either the four-iron or the diiron subcomplex discriminate catalytic and regulatory functions of [FeFe]-hydrogenases. We support a reaction cycle avoiding diiron site geometry changes during rapid H₂ turnover.

Sulfide Protects [FeFe] Hydrogenases From O2

[FeFe] hydrogenases catalyze proton reduction and hydrogen oxidation with high rates and efficiency under physiological conditions, but are highly oxygen sensitive. The [FeFe] hydrogenase from Desulfovibrio desulfuricans ( DdHydAB) can be purified under air in an oxygen stable inactive state H_ox^air. The formation of the H_ox^air state in vitro allows the handling of hydrogenases in air, making their implementation in biotechnological applications more feasible. Here, we report a simple and robust protocol for the formation of the H_ox^air state in DdHydAB and the [FeFe] hydrogenase from Chlamydomonas reinhardtii, which is based on high potential inactivation in the presence of sulfide.

From protein engineering to artificial enzymes - biological and biomimetic approaches towards sustainable hydrogen production

Hydrogen gas is used extensively in industry today and is often put forward as a suitable energy carrier due its high energy density. Currently, the main source of molecular hydrogen is fossil fuels via steam reforming. Consequently, novel production methods are required to improve the sustainability of hydrogen gas for industrial processes, as well as paving the way for its implementation as a future solar fuel. Nature has already developed an elaborate hydrogen economy, where the production and consumption of hydrogen gas is catalysed by hydrogenase enzymes. In this review we summarize efforts on engineering and optimizing these enzymes for biological hydrogen gas production, with an emphasis on their inorganic cofactors. Moreover, we will describe how our understanding of these enzymes has been applied for the preparation of bio-inspired/-mimetic systems for efficient and sustainable hydrogen production.

[FeFe]-Hydrogenases: recent developments and future perspectives

[FeFe]-Hydrogenases are the most efficient enzymes for catalytic hydrogen turnover.

Direct Observation of an Iron-Bound Terminal Hydride in [FeFe]-Hydrogenase by Nuclear Resonance Vibrational Spectroscopy

[FeFe]-hydrogenases catalyze the reversible reduction of protons to molecular hydrogen with extremely high efficiency. The active site ("H-cluster") consists of a [4Fe-4S]H cluster linked through a bridging cysteine to a [2Fe]H subsite coordinated by CN- and CO ligands featuring a dithiol-amine moiety that serves as proton shuttle between the protein proton channel and the catalytic distal iron site (Fed). Although there is broad consensus that an iron-bound terminal hydride species must occur in the catalytic mechanism, such a species has never been directly observed experimentally. Here, we present FTIR and nuclear resonance vibrational spectroscopy (NRVS) experiments in conjunction with density functional theory (DFT) calculations on an [FeFe]-hydrogenase variant lacking the amine proton shuttle which is stabilizing a putative hydride state. The NRVS spectra unequivocally show the bending modes of the terminal Fe-H species fully consistent with widely accepted models of the catalytic cycle.

Proton Coupled Electronic Rearrangement within the H-Cluster as an Essential Step in the Catalytic Cycle of [FeFe] Hydrogenases

The active site of [FeFe] hydrogenases, the H-cluster, consists of a [4Fe-4S] cluster connected via a bridging cysteine to a [2Fe] complex carrying CO and CN^- ligands as well as a bridging aza-dithiolate ligand (ADT) of which the amine moiety serves as a proton shuttle between the protein and the H-cluster. During the catalytic cycle, the two subclusters change oxidation states: [4Fe-4S]_H²⁺ ⇔ [4Fe-4S]_H⁺ and [Fe(I)Fe(II)]_H ⇔ [Fe(I)Fe(I)]_H thereby enabling the storage of the two electrons needed for the catalyzed reaction 2H⁺ + 2e^- ⇄ H₂. Using FTIR spectro-electrochemistry on the [FeFe] hydrogenase from Chlamydomonas reinhardtii (CrHydA1) at different pH values, we resolve the redox and protonation events in the catalytic cycle and determine their intrinsic thermodynamic parameters. We show that the singly reduced state H_red of the H-cluster actually consists of two species: H_red = [4Fe-4S]_H⁺ - [Fe(I)Fe(II)]_H and H_redH⁺ = [4Fe-4S]_H²⁺ - [Fe(I)Fe(I)]_H (H⁺) related by proton coupled electronic rearrangement. The two redox events in the catalytic cycle occur on the [4Fe-4S]_H subcluster at similar midpoint-potentials (-375 vs -418 mV); the protonation event (H_red/H_redH⁺) has a pK_a ≈ 7.2.

Identification of a Catalytic Iron-Hydride at the H-Cluster of [FeFe]-Hydrogenase

Hydrogenases couple electrochemical potential to the reversible chemical transformation of H₂ and protons, yet the reaction mechanism and composition of intermediates are not fully understood. In this Communication we describe the biophysical properties of a hydride-bound state (H_hyd) of the [FeFe]-hydrogenase from Chlamydomonas reinhardtii. The catalytic H-cluster of [FeFe]-hydrogenase consists of a [4Fe-4S] subcluster ([4Fe-4S]_H) linked by a cysteine thiol to an azadithiolate-bridged 2Fe subcluster ([2Fe]_H) with CO and CN^- ligands. Mössbauer analysis and density functional theory (DFT) calculations show that H_hyd consists of a reduced [4Fe-4S]_H⁺ coupled to a diferrous [2Fe]_H with a terminally bound Fe-hydride. The existence of the Fe-hydride in H_hyd was demonstrated by an unusually low Mössbauer isomer shift of the distal Fe of the [2Fe]_H subcluster. A DFT model of H_hyd shows that the Fe-hydride is part of a H-bonding network with the nearby bridging azadithiolate to facilitate fast proton exchange and catalytic turnover.

Vibrational spectroscopy reveals the initial steps of biological hydrogen evolution

[FeFe] hydrogenases are biocatalytic model systems for the exploitation and investigation of catalytic hydrogen evolution. Here, we used vibrational spectroscopic techniques to characterize, in detail, redox transformations of the [FeFe] and [4Fe4S] sub-sites of the catalytic centre (H-cluster) in a monomeric [FeFe] hydrogenase. Through the application of low-temperature resonance Raman spectroscopy, we discovered a novel metastable intermediate that is characterized by an oxidized [FeIFeII] centre and a reduced [4Fe4S]1+ cluster. Based on this unusual configuration, this species is assigned to the first, deprotonated H-cluster intermediate of the [FeFe] hydrogenase catalytic cycle. Providing insights into the sequence of initial reaction steps, the identification of this species represents a key finding towards the mechanistic understanding of biological hydrogen evolution.

Models of the Ni-L and Ni-SIa States of the [NiFe]-Hydrogenase Active Site

A new class of synthetic models for the active site of [NiFe]-hydrogenases are described. The Ni(I/II)(SCys)2 and Fe(II)(CN)2CO sites are represented with (RC5H4)Ni(I/II) and Fe(II)(diphos)(CO) modules, where diphos = 1,2-C2H4(PPh2)2(dppe) or cis-1,2-C2H2(PPh2)2(dppv). The two bridging thiolate ligands are represented by CH2(CH2S)2(2-) (pdt(2-)), Me2C(CH2S)2(2-) (Me2pdt(2-)), and (C6H5S)2(2-). The reaction of Fe(pdt)(CO)2(dppe) and [(C5H5)3Ni2]BF4 affords [(C5H5)Ni(pdt)Fe(dppe)(CO)]BF4 ([1a]BF4). Monocarbonyl [1a]BF4 features an S = 0 Ni(II)Fe(II) center with five-coordinated iron, as proposed for the Ni-SIa state of the enzyme. One-electron reduction of [1a](+) affords the S = 1/2 derivative [1a](0), which, according to density functional theory (DFT) calculations and electron paramagnetic resonance and Mössbauer spectroscopies, is best described as a Ni(I)Fe(II) compound. The Ni(I)Fe(II) assignment matches that for the Ni-L state in [NiFe]-hydrogenase, unlike recently reported Ni(II)Fe(I)-based models. Compound [1a](0) reacts with strong acids to liberate 0.5 equiv of H2 and regenerate [1a](+), indicating that H2 evolution is catalyzed by [1a](0). DFT calculations were used to investigate the pathway for H2 evolution and revealed that the mechanism can proceed through two isomers of [1a](0) that differ in the stereochemistry of the Fe(dppe)CO center. Calculations suggest that protonation of [1a](0) (both isomers) affords Ni(III)-H-Fe(II) intermediates, which represent mimics of the Ni-C state of the enzyme.

An iron-iron hydrogenase mimic with appended electron reservoir for efficient proton reduction in aqueous media

The transition from a fossil-based economy to a hydrogen-based economy requires cheap and abundant, yet stable and efficient, hydrogen production catalysts. Nature shows the potential of iron-based catalysts such as the iron-iron hydrogenase (H2ase) enzyme, which catalyzes hydrogen evolution at rates similar to platinum with low overpotential. However, existing synthetic H2ase mimics generally suffer from low efficiency and oxygen sensitivity and generally operate in organic solvents. We report on a synthetic H2ase mimic that contains a redox-active phosphole ligand as an electron reservoir, a feature that is also crucial for the working of the natural enzyme. Using a combination of (spectro)electrochemistry and time-resolved infrared spectroscopy, we elucidate the unique redox behavior of the catalyst. We find that the electron reservoir actively partakes in the reduction of protons and that its electron-rich redox states are stabilized through ligand protonation. In dilute sulfuric acid, the catalyst has a turnover frequency of 7.0 × 10(4) s(-1) at an overpotential of 0.66 V. This catalyst is tolerant to the presence of oxygen, thereby paving the way for a new generation of synthetic H2ase mimics that combine the benefits of the enzyme with synthetic versatility and improved stability.

Artificially maturated [FeFe] hydrogenase from Chlamydomonas reinhardtii: a HYSCORE and ENDOR study of a non-natural H-cluster

Hydrogenases are enzymes that catalyze the oxidation of H2 as well as the reduction of protons to form H2. The active site of [FeFe] hydrogenase is referred to as the "H-cluster" and consists of a "classical" [4Fe-4S] cluster connected via a bridging cysteine thiol group to a unique [2Fe]H sub-cluster, containing CN(-) and CO ligands as well as a bidentate azadithiolate ligand. It has been recently shown that the biomimetic [Fe2(adt)(CO)4(CN)2](2-) (adt(2-) = azadithiolate) complex resembling the diiron sub-cluster can be inserted in vitro into the apo-protein of [FeFe] hydrogenase, which contains only the [4Fe-4S] part of the H-cluster, resulting in a fully active enzyme. This synthetic tool allows convenient incorporation of a variety of diiron mimics, thus generating hydrogenases with artificial active sites. [FeFe] hydrogenase from Chlamydomonas reinhardtii maturated with the biomimetic complex [Fe2(pdt)(CO)4(CN)2](2-) (pdt(2-) = propanedithiolate), in which the bridging adt(2-) ligand is replaced by pdt(2-), can be stabilized in a state strongly resembling the active oxidized (Hox) state of the native protein. This state is EPR active and the signal originates from the mixed valence Fe(I)Fe(II) state of the diiron sub-cluster. Taking advantage of the variant with (15)N and (13)C isotope labeled CN(-) ligands we performed HYSCORE and ENDOR studies on this hybrid protein. The (13)C hyperfine couplings originating from both CN(-) ligands were determined and assigned. Only the (15)N coupling from the CN(-) ligand bound to the terminal iron was observed. Detailed orientation selective ENDOR and HYSCORE experiments at multiple field positions enabled the extraction of accurate data for the relative orientations of the nitrogen and carbon hyperfine tensors. These data are consistent with the crystal structure assuming a g-tensor orientation following the local symmetry of the binuclear sub-cluster.

EPR Spectroscopic Studies of [FeFe]-Hydrogenase Maturation

Proton reduction and H2 oxidation are key elementary reactions for solar fuel production. Hydrogenases interconvert H+ and H2 with remarkable efficiency and have therefore received much attention in this context. For [FeFe]-hydrogenases, catalysis occurs at a unique cofactor called the H-cluster. In this article, we discuss ways in which EPR spectroscopy has elucidated aspects of the bioassembly of the H-cluster, with a focus on four case studies: EPR spectroscopic identification of a radical en route to the CO and CN- ligands of the H-cluster, tracing 57Fe from the maturase HydG into the H-cluster, characterization of the auxiliary Fe-S cluster in HydG, and isotopic labeling of the CN- ligands of HydA for electronic structure studies of its Hox state. Advances in cell-free maturation protocols have enabled several of these mechanistic studies, and understanding H-cluster maturation may in turn provide insights leading to improvements in hydrogenase production for biotechnological applications.

Spectroscopic Investigations of [FeFe] Hydrogenase Maturated with [(57)Fe2(adt)(CN)2(CO)4](2-)

The preparation and spectroscopic characterization of a CO-inhibited [FeFe] hydrogenase with a selectively (57)Fe-labeled binuclear subsite is described. The precursor [(57)Fe2(adt)(CN)2(CO)4](2-) was synthesized from the (57)Fe metal, S8, CO, (NEt4)CN, NH4Cl, and CH2O. (Et4N)2[(57)Fe2(adt)(CN)2(CO)4] was then used for the maturation of the [FeFe] hydrogenase HydA1 from Chlamydomonas reinhardtii, to yield the enzyme selectively labeled at the [2Fe]H subcluster. Complementary (57)Fe enrichment of the [4Fe-4S]H cluster was realized by reconstitution with (57)FeCl3 and Na2S. The Hox-CO state of [2(57)Fe]H and [4(57)Fe-4S]H HydA1 was characterized by Mössbauer, HYSCORE, ENDOR, and nuclear resonance vibrational spectroscopy.

Cofactor composition and function of a H2-sensing regulatory hydrogenase as revealed by Mössbauer and EPR spectroscopy

The regulatory hydrogenase (RH) from Ralstonia eutropha H16 acts as a sensor for the detection of environmental H₂ and regulates gene expression related to hydrogenase-mediated cellular metabolism. In marked contrast to prototypical energy-converting [NiFe] hydrogenases, the RH is apparently insensitive to inhibition by O₂ and CO. While the physiological function of regulatory hydrogenases is well established, little is known about the redox cycling of the [NiFe] center and the nature of the iron-sulfur (FeS) clusters acting as electron relay. The absence of any FeS cluster signals in EPR had been attributed to their particular nature, whereas the observation of essentially only two active site redox states, namely Ni-SI and Ni-C, invoked a different operant mechanism. In the present work, we employ a combination of Mössbauer, FTIR and EPR spectroscopic techniques to study the RH, and the results are consistent with the presence of three [4Fe-4S] centers in the small subunit. In the as-isolated, oxidized RH all FeS clusters reside in the EPR-silent 2+ state. Incubation with H₂ leads to reduction of two of the [4Fe-4S] clusters, whereas only strongly reducing agents lead to reduction of the third cluster, which is ascribed to be the [4Fe-4S] center in 'proximal' position to the [NiFe] center. In the two different active site redox states, the low-spin Fe^II exhibits distinct Mössbauer features attributed to changes in the electronic and geometric structure of the catalytic center. The results are discussed with regard to the spectral characteristics and physiological function of H₂-sensing regulatory hydrogenases.

[FeFe]- and [NiFe]-hydrogenase diversity, mechanism, and maturation

The [FeFe]- and [NiFe]-hydrogenases catalyze the formal interconversion between hydrogen and protons and electrons, possess characteristic non-protein ligands at their catalytic sites and thus share common mechanistic features. Despite the similarities between these two types of hydrogenases, they clearly have distinct evolutionary origins and likely emerged from different selective pressures. [FeFe]-hydrogenases are widely distributed in fermentative anaerobic microorganisms and likely evolved under selective pressure to couple hydrogen production to the recycling of electron carriers that accumulate during anaerobic metabolism. In contrast, many [NiFe]-hydrogenases catalyze hydrogen oxidation as part of energy metabolism and were likely key enzymes in early life and arguably represent the predecessors of modern respiratory metabolism. Although the reversible combination of protons and electrons to generate hydrogen gas is the simplest of chemical reactions, the [FeFe]- and [NiFe]-hydrogenases have distinct mechanisms and differ in the fundamental chemistry associated with proton transfer and control of electron flow that also help to define catalytic bias. A unifying feature of these enzymes is that hydrogen activation itself has been restricted to one solution involving diatomic ligands (carbon monoxide and cyanide) bound to an Fe ion. On the other hand, and quite remarkably, the biosynthetic mechanisms to produce these ligands are exclusive to each type of enzyme. Furthermore, these mechanisms represent two independent solutions to the formation of complex bioinorganic active sites for catalyzing the simplest of chemical reactions, reversible hydrogen oxidation. As such, the [FeFe]- and [NiFe]-hydrogenases are arguably the most profound case of convergent evolution. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Investigations on the role of proton-coupled electron transfer in hydrogen activation by [FeFe]-hydrogenase

Proton-coupled electron transfer (PCET) is a fundamental process at the core of oxidation-reduction reactions for energy conversion. The [FeFe]-hydrogenases catalyze the reversible activation of molecular H2 through a unique metallocofactor, the H-cluster, which is finely tuned by the surrounding protein environment to undergo fast PCET transitions. The correlation of electronic and structural transitions at the H-cluster with proton-transfer (PT) steps has not been well-resolved experimentally. Here, we explore how modification of the conserved PT network via a Cys → Ser substitution at position 169 proximal to the H-cluster of Chlamydomonas reinhardtii [FeFe]-hydrogenase (CrHydA1) affects the H-cluster using electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectroscopy. Despite a substantial decrease in catalytic activity, the EPR and FTIR spectra reveal different H-cluster catalytic states under reducing and oxidizing conditions. Under H2 or sodium dithionite reductive treatments, the EPR spectra show signals that are consistent with a reduced [4Fe-4S]H(+) subcluster. The FTIR spectra showed upshifts of νCO modes to energies that are consistent with an increase in oxidation state of the [2Fe]H subcluster, which was corroborated by DFT analysis. In contrast to the case for wild-type CrHydA1, spectra associated with Hred and Hsred states are less populated in the Cys → Ser variant, demonstrating that the exchange of -SH with -OH alters how the H-cluster equilibrates among different reduced states of the catalytic cycle under steady-state conditions.

Aerobic damage to [FeFe]-hydrogenases: activation barriers for the chemical attachment of O2

[FeFe]-hydrogenases are the best natural hydrogen-producing enzymes but their biotechnological exploitation is hampered by their extreme oxygen sensitivity. The free energy profile for the chemical attachment of O2 to the enzyme active site was investigated by using a range-separated density functional re-parametrized to reproduce high-level ab initio data. An activation free-energy barrier of 13 kcal mol(-1) was obtained for chemical bond formation between the di-iron active site and O2, a value in good agreement with experimental inactivation rates. The oxygen binding can be viewed as an inner-sphere electron-transfer process that is strongly influenced by Coulombic interactions with the proximal cubane cluster and the protein environment. The implications of these results for future mutation studies with the aim of increasing the oxygen tolerance of this enzyme are discussed.

The HydG enzyme generates an Fe(CO)2(CN) synthon in assembly of the FeFe hydrogenase H-cluster

Three iron-sulfur proteins--HydE, HydF, and HydG--play a key role in the synthesis of the [2Fe](H) component of the catalytic H-cluster of FeFe hydrogenase. The radical S-adenosyl-L-methionine enzyme HydG lyses free tyrosine to produce p-cresol and the CO and CN(-) ligands of the [2Fe](H) cluster. Here, we applied stopped-flow Fourier transform infrared and electron-nuclear double resonance spectroscopies to probe the formation of HydG-bound Fe-containing species bearing CO and CN(-) ligands with spectroscopic signatures that evolve on the 1- to 1000-second time scale. Through study of the (13)C, (15)N, and (57)Fe isotopologs of these intermediates and products, we identify the final HydG-bound species as an organometallic Fe(CO)2(CN) synthon that is ultimately transferred to apohydrogenase to form the [2Fe](H) component of the H-cluster.

Electronic and molecular structures of the active-site H-cluster in [FeFe]-hydrogenase determined by site-selective X-ray spectroscopy and quantum chemical calculations

Site-selective X-ray spectroscopy discriminated the cubane and diiron units in the H-cluster of [FeFe]-hydrogenase revealing its electronic and structural configurations.

Quantum mechanical methods for the investigation of metalloproteins and related bioinorganic compounds

It is well known that transition metal ions are often bound to proteins, conveying very specific functional properties. In fact, metalloproteins play crucial biological roles in the transport and activation of small molecules such as H2, O2, and N2, as well as in several other biochemical processes. However, even if the presence of transition metals in the active site of proteins allows a very rich biochemistry, the experimental disclosure of structure-activity relationships in metalloproteins is generally difficult exactly because of the presence of transition metals, which are intrinsically characterized by a very versatile and often elusive chemistry. For this reason, computational methods are becoming very popular tools in the characterization of metalloproteins. In particular, since computing power is becoming less and less expensive, due to the continuous technological development of CPUs, the computational tools suited to investigate metalloproteins are becoming more accessible and therefore more commonly used also in molecular biology and biochemistry laboratories. Here, we present the main procedures and computational methods based on quantum mechanics, which are commonly used to study the structural, electronic, and reactivity properties of metalloproteins and related bioinspired compounds, with a specific focus on the practical and technical aspects that must be generally tackled to properly study such biomolecular systems.

Molecular basis of [FeFe]-hydrogenase function: an insight into the complex interplay between protein and catalytic cofactor

The precise electrochemical features of metal cofactors that convey the functions of redox enzymes are essentially determined by the specific interaction pattern between cofactor and enclosing protein environment. However, while biophysical techniques allow a detailed understanding of the features characterizing the cofactor itself, knowledge about the contribution of the protein part is much harder to obtain. [FeFe]-hydrogenases are an interesting class of enzymes that catalyze both, H2 oxidation and the reduction of protons to molecular hydrogen with significant efficiency. The active site of these proteins consists of an unusual prosthetic group (H-cluster) with six iron and six sulfur atoms. While H-cluster architecture and catalytic states during the different steps of H2 turnover have been thoroughly investigated during the last 20 years, possible functional contributions from the polypeptide framework were only assumed according to the level of conservancy and X-ray structure analyses. Due to the recent development of simpler and more efficient expression systems the role of single amino acids can now be experimentally investigated. This article summarizes, compares and categorizes the results of recent investigations based on site directed and random mutagenesis according to their informative value about structure function relationships in [FeFe]-hydrogenases. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

EPR/ENDOR, Mössbauer, and quantum-chemical investigations of diiron complexes mimicking the active oxidized state of [FeFe]hydrogenase

Understanding the catalytic process of the heterolytic splitting and formation of molecular hydrogen is one of the key topics for the development of a future hydrogen economy. With an interest in elucidating the enzymatic mechanism of the [Fe(2)(S(2)C(2)H(4)NH)(CN)(2)(CO)(2)(μ-CO)] active center uniquely found in [FeFe]hydrogenases, we present a detailed spectroscopic and theoretical analysis of its inorganic model [Fe(2)(S(2)X)(CO)(3)(dppv)(PMe(3))](+) [dppv = cis-1,2-bis(diphenylphosphino)ethylene] in two forms with S(2)X = ethanedithiolate (1edt) and azadithiolate (1adt). These complexes represent models for the oxidized mixed-valent Fe(I)Fe(II) state analogous to the active oxidized "H(ox)" state of the native H-cluster. For both complexes, the (31)P hyperfine interactions were determined by pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) methods. For 1edt, the (57)Fe parameters were measured by electron spin-echo envelope modulation and Mössbauer spectroscopy, while for 1adt, (14)N and selected (1)H couplings could be obtained by ENDOR and hyperfine sublevel correlation spectroscopy. The spin density was found to be predominantly localized on the Fe(dppv) site. This spin distribution is different from that of the H-cluster, where both the spin and charge densities are delocalized over the two Fe centers. This difference is attributed to the influence of the "native" cubane subcluster that is lacking in the inorganic models. The degree and character of the unpaired spin delocalization was found to vary from 1edt, with an abiological dithiolate, to 1adt, which features the authentic cofactor. For 1adt, we find two (14)N signals, which are indicative for two possible isomers of the azadithiolate, demonstrating its high flexibility. All interaction parameters were also evaluated through density functional theory calculations at various levels.

Inhibition of biocatalysis in [Fe-Fe] hydrogenase by oxygen: molecular dynamics and density functional theory calculations

Designing O(2)-tolerant hydrogenases is a major challenge in applying [Fe-Fe]H(2)ases for H(2) production. The inhibition involves transport of oxygen through the enzyme to the H-cluster, followed by binding and subsequent deactivation of the active site. To explore the nature of the oxygen diffusion channel for the hydrogenases from Desulfovibrio desulfuricans (Dd) and Clostridium pasteurianum (Cp), empirical molecular dynamics simulations were performed. The dynamic nature of the oxygen pathways in Dd and Cp was elucidated, and insight is provided, in part, into the experimental observation on the difference of oxygen inhibition in Dd and the hydrogenase from Clostridium acetobutylicum (Ca, assumed homologous to Cp). Further, to gain an understanding of the mechanism of oxygen inhibition of the [Fe-Fe]H(2)ase, density functional theory calculations of model compounds composed of the H-cluster and proximate amino acids are reported. Confirmation of the experimentally based suppositions on inactivation by oxygen at the [2Fe](H) domain is provided, validating the model compounds used and oxidation state assumptions, further explaining the mode of damage. This unified approach provides insight into oxygen diffusion in the enzyme, followed by deactivation at the H-cluster.

Mixed-valence nickel-iron dithiolate models of the [NiFe]-hydrogenase active site

A series of mixed-valence nickel-iron dithiolates is described. Oxidation of (diphosphine)Ni(dithiolate)Fe(CO)(3) complexes 1, 2, and 3 with ferrocenium salts affords the corresponding tricarbonyl cations [(dppe)Ni(pdt)Fe(CO)(3)](+) ([1](+)), [(dppe)Ni(edt)Fe(CO)(3)](+) ([2](+)) and [(dcpe)Ni(pdt)Fe(CO)(3)](+) ([3](+)), respectively, where dppe = Ph(2)PCH(2)CH(2)PPh(2), dcpe = Cy(2)PCH(2)CH(2)PCy(2), (Cy = cyclohexyl), pdtH(2) = HSCH(2)CH(2)CH(2)SH, and edtH(2) = HSCH(2)CH(2)SH. The cation [2](+) proved unstable, but the propanedithiolates are robust. IR and EPR spectroscopic measurements indicate that these species exist as C(s)-symmetric species. Crystallographic characterization of [3]BF(4) shows that Ni is square planar. Interaction of [1]BF(4) with P-donor ligands (L) afforded a series of substituted derivatives of type [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = P(OPh)(3) ([4a]BF(4)), P(p-C(6)H(4)Cl)(3) ([4b]BF(4)), PPh(2)(2-py) ([4c]BF(4)), PPh(2)(OEt) ([4d]BF(4)), PPh(3) ([4e]BF(4)), PPh(2)(o-C(6)H(4)OMe) ([4f]BF(4)), PPh(2)(o-C(6)H(4)OCH(2)OMe) ([4g]BF(4)), P(p-tol)(3) ([4h]BF(4)), P(p-C(6)H(4)OMe)(3) ([4i]BF(4)), and PMePh(2) ([4j]BF(4)). EPR analysis indicates that ethanedithiolate [2](+) exists as a single species at 110 K, whereas the propanedithiolate cations exist as a mixture of two conformers, which are proposed to be related through a flip of the chelate ring. Mössbauer spectra of 1 and oxidized S = 1/2 [4e]BF(4) are both consistent with a low-spin Fe(I) state. The hyperfine coupling tensor of [4e]BF(4) has a small isotropic component and significant anisotropy. DFT calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the SOMOs in complexes of the present type are localized in an Fe(I)-centered d(z(2)) orbital. The DFT calculations allow an assignment of oxidation states of the metals and rationalization of the conformers detected by EPR spectroscopy. Treatment of [1](+) with CN(-) and compact basic phosphines results in complex reactions. With dppe, [1](+) undergoes quasi-disproportionation to give 1 and the diamagnetic complex [(dppe)Ni(pdt)Fe(CO)(2)(dppe)](2+) ([5](2+)), which features square-planar Ni linked to an octahedral Fe center.