Hydrogen cycling by enzymes: electrocatalysis and implications for future energy technology

Hydrogenases

Hydrogenases catalyze the simple yet important interconversion between H₂ and protons and electrons. Found throughout prokaryotes, lower eukaryotes, and archaea, hydrogenases are used for a variety of redox and signaling purposes and are found in many different forms. This diverse group of metalloenzymes is divided into [NiFe], [FeFe], and [Fe] variants, based on the transition metal contents of their active sites. A wide array of biochemical and spectroscopic methods has been used to elucidate hydrogenases, and this along with a general description of the main enzyme types and catalytic mechanisms is discussed in this chapter.

Implementation of photobiological H2 production: the O 2 sensitivity of hydrogenases

The search for the ultimate carbon-free fuel has intensified in recent years, with a major focus on photoproduction of H2. Biological sources of H2 include oxygenic photosynthetic green algae and cyanobacteria, both of which contain hydrogenase enzymes. Although algal and cyanobacterial hydrogenases perform the same enzymatic reaction through metallo-clusters, their hydrogenases have evolved separately, are expressed differently (transcription of algal hydrogenases is anaerobically induced, while bacterial hydrogenases are constitutively expressed), and display different sensitivity to O2 inactivation. Among various physiological factors, the sensitivity of hydrogenases to O2 has been one of the major factors preventing implementation of biological systems for commercial production of renewable H2. This review addresses recent strategies aimed at engineering increased O2 tolerance into hydrogenases (as of now mainly unsuccessful), as well as towards the development of methods to bypass the O2 sensitivity of hydrogenases (successful but still yielding low solar conversion efficiencies). The author concludes with a description of current approaches from various laboratories to incorporate multiple genetic traits into either algae or cyanobacteria to jointly address limiting factors other than the hydrogenase O2 sensitivity and achieve more sustained H2 photoproduction activity.

Metagenomic Sequencing Unravels Gene Fragments with Phylogenetic Signatures of O2-Tolerant NiFe Membrane-Bound Hydrogenases in Lacustrine Sediment

Many promising hydrogen technologies utilising hydrogenase enzymes have been slowed by the fact that most hydrogenases are extremely sensitive to O₂. Within the group 1 membrane-bound NiFe hydrogenase, naturally occurring tolerant enzymes do exist, and O₂ tolerance has been largely attributed to changes in iron–sulphur clusters coordinated by different numbers of cysteine residues in the enzyme’s small subunit. Indeed, previous work has provided a robust phylogenetic signature of O₂ tolerance [1], which when combined with new sequencing technologies makes bio prospecting in nature a far more viable endeavour. However, making sense of such a vast diversity is still challenging and could be simplified if known species with O₂-tolerant enzymes were annotated with information on metabolism and natural environments. Here, we utilised a bioinformatics approach to compare O₂-tolerant and sensitive membrane-bound NiFe hydrogenases from 177 bacterial species with fully sequenced genomes for differences in their taxonomy, O₂ requirements, and natural environment. Following this, we interrogated a metagenome from lacustrine surface sediment for novel hydrogenases via high-throughput shotgun DNA sequencing using the Illumina™ MiSeq platform. We found 44 new NiFe group 1 membrane-bound hydrogenase sequence fragments, five of which segregated with the tolerant group on the phylogenetic tree of the enzyme’s small subunit, and four with the large subunit, indicating de novo O₂-tolerant protein sequences that could help engineer more efficient hydrogenases.

Covalent attachment of FeFe hydrogenases to carbon electrodes for direct electron transfer

Direct electron transfer between enzymes and electrodes is now commonly achieved, but obtaining protein films that are very stable may be challenging. This is particularly crucial in the case of hydrogenases, the enzymes that catalyze the biological conversion between dihydrogen and protons, because the instability of the hydrogenase films may prevent the use of these enzymes as electrocatalysts of H(2) oxidation and production in biofuel cells and photoelectrochemical cells. Here we show that two different FeFe hydrogenases (from Chamydomonas reinhardtii and Clostridium acetobutylicum) can be covalently attached to functionalized pyrolytic graphite electrodes using peptidic coupling. In both cases, a surface patch of lysine residues makes it possible to favor an orientation that is efficient for fast, direct electron transfer. High hydrogen-oxidation current densities are maintained for up to one week, the only limitation being the intrinsic stability of the enzyme. We also show that covalent attachment has no effect on the catalytic properties of the enzyme, which means that this strategy can also used be for electrochemical studies of the catalytic mechanism.

Selective catalysts for the hydrogen oxidation and oxygen reduction reactions by patterning of platinum with calix[4]arene molecules

The design of new catalysts for polymer electrolyte membrane fuel cells must be guided by two equally important fundamental principles: optimization of their catalytic behaviour as well as the long-term stability of the metal catalysts and supports in hostile electrochemical environments. The methods used to improve catalytic activity are diverse, ranging from the alloying and de-alloying of platinum to the synthesis of platinum core-shell catalysts. However, methods to improve the stability of the carbon supports and catalyst nanoparticles are limited, especially during shutdown (when hydrogen is purged from the anode by air) and startup (when air is purged from the anode by hydrogen) conditions when the cathode potential can be pushed up to 1.5 V (ref. 11). Under the latter conditions, stability of the cathode materials is strongly affected (carbon oxidation reaction) by the undesired oxygen reduction reaction (ORR) on the anode side. This emphasizes the importance of designing selective anode catalysts that can efficiently suppress the ORR while fully preserving the Pt-like activity for the hydrogen oxidation reaction. Here, we demonstrate that chemically modified platinum with a self-assembled monolayer of calix[4]arene molecules meets this challenging requirement.

Intermediates in the catalytic cycle of [NiFe] hydrogenase: functional spectroscopy of the active site

The [NiFe] hydrogenase from the anaerobic sulphate reducing bacterium Desulfovibrio vulgaris Miyazaki F is an excellent model for constructing a mechanism for the function of the so-called 'oxygen-sensitive' hydrogenases. The present review focuses on spectroscopic investigations of the active site intermediates playing a role in the activation/deactivation and catalytic cycle of this enzyme as well as in the inhibition by carbon monoxide or molecular oxygen and the light-sensitivity of the hydrogenase. The methods employed include magnetic resonance and vibrational (FTIR) techniques combined with electrochemistry that deliver information about details of the geometrical and electronic structure of the intermediates and their redox behaviour. Based on these data a mechanistic scheme is developed.

Vibrational analysis of the model complex (mu-edt)[Fe(CO)(3)](2) and comparison to iron-only hydrogenase: the activation scale of hydrogenase model systems

Research on simple [FeFe] hydrogenase model systems of type (mu-S(2)R)[Fe(CO)(3)](2) (R = C(2)H(4) (edt), C(3)H(6) (pdt)) which have been shown to function as robust electrocatalysts for proton reduction, provides a reference to understand the electronic and vibrational properties of the active site of [FeFe] hydrogenases and of more sophisticated model systems. In this study, the solution and solid state Raman spectra of (mu-edt)[Fe(CO)(3)](2) and of the corresponding (13)CO-labeled complex are presented and analyzed in detail, with focus on the nu(C=O) and nu(Fe-CO)/delta(Fe-C=O) vibrational regions. These regions are specifically important as vibrations involving CO ligands serve as probes for the "electron richness" of low-valent transition metal centers and the geometric structures of the complexes. The obtained vibrational spectra have been completely assigned in terms of the nu(C=O), nu(Fe-CO), and delta(Fe-C=O) modes, and the force constants of the important C=O and Fe-CO bonds have been determined using our Quantum Chemistry Centered Normal Coordinate Analysis (QCC-NCA). In the 400-650 cm(-1) region, fifteen mixed nu(Fe-CO)/delta(Fe-C=O) modes have been identified. The most prominent Raman peaks at 454, 456, and 483 cm(-1) correspond to a combination of nu(Fe-CO) stretching and delta(Fe-C=O) linear bending modes. The less intense peaks at 416 cm(-1) and 419 cm(-1) correspond to pure delta(Fe-C=O) linear bends. In the nu(C=O) region, the nu(C=O) normal modes at lower energy (1968 and 1964 cm(-1)) are almost pure equatorial (eq) nu(C=O)(eq) stretching vibrations, whereas the remaining four nu(C=O) normal modes show dominant (C=O)(eq) (2070 and 1961 cm(-1)) and (C=O)(ax) (2005 and 1979 cm(-1); ax = axial) contributions. Importantly, an inverse correlation between the f(C=O)(ax/eq) and f(Fe-CO)(ax/eq) force constants is obtained, in agreement with the idea that the Fe(I)-CO bond in these types of complexes is dominated by pi-backdonation. Compared to the reduced form of [FeFe] hydrogenase (H(red)), the nu(C=O) vibrational frequencies of (mu-edt)[Fe(CO)(3)](2) are higher in energy, indicating that the dinuclear iron core in (mu-edt)[Fe(CO)(3)](2) is less electron rich compared to H(red) in the actual enzyme. Finally, quantum yields for the photodecomposition of (mu-edt)[Fe(CO)(3)](2) have been determined.

Tropospheric H(2) budget and the response of its soil uptake under the changing environment

Molecular hydrogen (H(2)) is an indirect greenhouse gas present at the trace level in the atmosphere. So far, the sum of its sources and sinks is close to equilibrium, but its large-scale utilization as an alternative energy carrier would alter its atmospheric burden. The magnitude of the emissions associated with a future H(2)-based economy is difficult to predict and remains a matter of debate. Previous attempts to predict the impact that a future H(2)-based economy would exert on tropospheric chemistry were realized by considering a steady rate of microbial-mediated soil uptake, which is currently responsible of ~80% of the tropospheric H(2) losses. Although soil uptake, also known as dry deposition is the most important sink for tropospheric H(2), microorganisms involved in the activity remain elusive. Given that microbial-mediated H(2) soil uptake is influenced by several environmental factors, global change should exert a significant effect on the activity and then, assuming a steady H(2) soil uptake rate for the future may be mistaken. Here, we present an overview of tropospheric H(2) sources and sinks with an emphasis on microbial-mediated soil uptake process. Future researches are proposed to investigate the influence that global change would exert on H(2) dry deposition and to identify microorganisms involved H(2) soil uptake activity.

Hydrogen production under aerobic conditions by membrane-bound hydrogenases from Ralstonia species

Studies have been carried out to establish the ability of O2-tolerant membrane-bound [NiFe] hydrogenases (MBH) from Ralstonia sp. to catalyze H2 production in addition to H2 oxidation. These hydrogenases are not noted for H2-evolution activity, and this is partly due to strong product inhibition. However, when adsorbed on a rotating disk graphite electrode the enzymes produce H2 efficiently, provided the H2 product is continuously removed by rapidly rotating the electrode and flowing N2 through the gastight electrochemical cell. Electrocatalytic H2 production proceeds with minimal overpotentiala significant observation because lowering the overpotential (the electrochemically responsive activation barrier) is seen as crucial in developing small-molecule catalysts for H2 production. A mutant having a high KM for H2 oxidation did not prove to be a better H2 producer relative to the wild type, thus suggesting that weak binding of H2 does not itself confer a tendency to be a H2 producer. Inhibition by H2 is much stronger than inhibition by CO and, most significantly, even O2. Consequently, H2 can be produced sustainably in the presence of O2 as long as the H2 is removed continuously, thereby proving the feasibility for biological H2 production in air.

Biocatalysts for fuel cells: efficient hydrogenase orientation for H2 oxidation at electrodes modified with carbon nanotubes

We report the modification of gold and graphite electrodes with commercially available carbon nanotubes for immobilization of Desulfovibrio fructosovorans [NiFe] hydrogenase, for hydrogen evolution or consumption. Multiwalled carbon nanotubes, single-walled carbon nanotubes (SWCNs), and amine-modified and carboxyl-functionalized SWCNs were used and compared throughout. Two separate methods were performed: covalent attachment of oriented hydrogenase by controlled architecture of carbon nanotubes at gold electrodes, and adsorption of hydrogenase at carbon-nanotube-coated pyrolytic graphite electrodes. In the case of self-assembled carbon nanotubes at gold electrodes, hydrogenase orientation based on electrostatic interaction with the electrode surface was found to control the electrocatalytic process for H(2) oxidation. In the case of carbon nanotube coatings on pyrolytic graphite electrodes, catalysis was controlled more by the geometry of the nanotubes than by the orientation of the enzyme. Noticeably, shortened SWCNs were demonstrated to allow direct electron transfer and generate high and quite stable current densities for H(2) oxidation via adsorbed hydrogenase, despite having many carboxylic surface functions that could yield unfavorable hydrogenase orientation for direct electron transfer. This result is attributable to the high degree of oxygenated surface functions in addition to the length of shortened SWCNs that yields highly divided materials.

Structural and electronic properties of the [FeFe] hydrogenase H-cluster in different redox and protonation states. A DFT investigation

The molecular and electronic structure of the Fe 6S 6 H-cluster of [FeFe] hydrogenase in relevant redox and protonation states have been investigated by DFT. The calculations have been carried out according to the broken symmetry approach and considering different environmental conditions. The large negative charge of the H-cluster leads, in a vacuum, to structures different from those observed experimentally in the protein. A better agreement with experimental data is observed for solvated complexes, suggesting that the protein environment could buffer the large negative charge of the H-cluster. The comparison of Fe 6S 6 and Fe 2S 2 DFT models shows that the presence of the Fe 4S 4 moiety does not affect appreciably the geometry of the [2Fe] H cluster. In particular, the Fe 4S 4 cluster alone cannot be invoked to explain the stabilization of the mu-CO forms observed in the enzyme (relative to all-terminal CO species). As for protonation of the hydrogen cluster, it turned out that mu-H species are always more stable than terminal hydride isomers, leading to the conclusion that specific interactions of the H-cluster with the environment, not considered in our calculations, would be necessary to reverse the stability order of mu-H and terminal hydrides. Otherwise, protonation of the metal center and H 2 evolution in the enzyme are predicted to be kinetically controlled processes. Finally, subtle modifications in the H-cluster environment can change the relative stability of key frontier orbitals, triggering electron transfer between the Fe 4S 4 and the Fe 2S 2 moieties forming the H-cluster.

Direct electrochemical study of the multiple redox centers of hydrogenase from Desulfovibrio gigas

Direct electrochemical response was first time observed for the redox centers of Desulfovibrio gigas [NiFe]-Hase, in non-turnover conditions, by cyclic voltammetry, in solution at glassy carbon electrode. The activation of the enzyme was achieved by reduction with H(2) and by electrochemical control and electrocatalytic activity was observed. The inactivation of the [NiFe]-Hase was also attained through potential control. All electrochemical data was obtained in the absence of enzyme inhibitors. The results are discussed in the context of the proposed mechanism currently accepted for activation/inactivation of [NiFe]-Hases.

Isolation and characterization of a new facultatively autotrophic hydrogen-oxidizing Betaproteobacterium, Hydrogenophaga sp. AH-24

A hydrogen-oxidizing bacterium strain AH-24 was isolated, which was classified in the genus Hydrogenophaga, based on the 16S rRNA gene sequence. The isolate possessed a typical yellow pigment of Hydrogenophaga species. Its closest relative was Hydrogenophaga pseudoflava, but the assimilation profile of sugar compounds resembled that of no species of Hydrogenophaga. The optimum temperature and pH for autotrophic growth were, respectively, 33-35 degrees C and 7.0. Most hydrogenase activity (benzyl viologen reducing activity) was localized in the membrane fraction (MF), but NAD(P)-reducing hydrogenase activity was detected in neither the membrane nor the soluble fractions. Cytochromes b561 and c551 were present in MF; both were reduced when hydrogen was supplied to the oxidized MF, suggesting involvement in respiratory H2 oxidation as electron carriers. Cytochrome b561 was inferred to function as the redox partner of the membrane-bound hydrogenase.

A stable electrode for high-potential, electrocatalytic O(2) reduction based on rational attachment of a blue copper oxidase to a graphite surface

Attachment of substrate-like anthracene based units to the surface of pyrolytic graphite greatly enhances the adsorption of high-potential fungal laccases, 'blue' Cu enzymes that catalyse the four-electron reduction of O(2), providing a stable cathode for enzymatic biological fuel cells and electrochemical studies.

Electricity from low-level H2 in still air--an ultimate test for an oxygen tolerant hydrogenase

We demonstrate an extreme test of O(2) tolerance for a biological hydrogen-cycling catalyst: the generation of electricity from just 3% H(2) released into still, ambient air using an open fuel cell comprising an anode modified with the unusual hydrogenase from Ralstonia metallidurans CH34, that oxidizes trace H(2) in atmospheric O(2), connected via a film of electrolyte to a cathode modified with the fungal O(2) reductase, laccase.