The difference a Se makes? Oxygen-tolerant hydrogen production by the [NiFeSe]-hydrogenase from Desulfomicrobium baculatum

Replacing a Cysteine Ligand by Selenocysteine in a [NiFe]-Hydrogenase Unlocks Hydrogen Production Activity and Addresses the Role of Concerted Proton-Coupled Electron Transfer in Electrocatalytic Reversibility

Hydrogenases catalyze hydrogen/proton interconversion that is normally electrochemically reversible (having minimal overpotential requirement), a special property otherwise almost exclusive to platinum metals. The mechanism of [NiFe]-hydrogenases includes a long-range proton-coupled electron-transfer process involving a specific Ni-coordinated cysteine and the carboxylate of a nearby glutamate. A variant in which this cysteine has been exchanged for selenocysteine displays two distinct changes in electrocatalytic properties, as determined by protein film voltammetry. First, proton reduction, even in the presence of H2 (a strong product inhibitor), is greatly enhanced relative to H2 oxidation: this result parallels a characteristic of natural [NiFeSe]-hydrogenases which are superior H2 production catalysts. Second, an inflection (an S-shaped "twist" in the trace) appears around the formal potential, the small overpotentials introduced in each direction (oxidation and reduction) signaling a departure from electrocatalytic reversibility. Concerted proton-electron transfer offers a lower energy pathway compared to stepwise transfers. Given the much lower proton affinity of Se compared to that of S, the inflection provides compelling evidence that concerted proton-electron transfer is important in determining why [NiFe]-hydrogenases are reversible electrocatalysts.

Outer-sphere effects on the O2 sensitivity, catalytic bias and catalytic reversibility of hydrogenases

The comparison of homologous metalloenzymes, in which the same inorganic active site is surrounded by a variable protein matrix, has demonstrated that residues that are remote from the active site may have a great influence on catalytic properties. In this review, we summarise recent findings on the diverse molecular mechanisms by which the protein matrix may define the oxygen tolerance, catalytic directionality and catalytic reversibility of hydrogenases, enzymes that catalyse the oxidation and evolution of H2. These mechanisms involve residues in the second coordination sphere of the active site metal ion, more distant residues affecting protein flexibility through their side chains, residues lining the gas channel and even accessory subunits. Such long-distance effects, which contribute to making enzymes efficient, robust and different from one another, are a source of wonder for biochemists and a challenge for synthetic bioinorganic chemists.

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.

Promising approaches for the assembly of the catalytically active, recombinant Desulfomicrobium baculatum hydrogenase with substitutions at the active site

BACKGROUND

Hydrogenases (H2ases) are metalloenzymes capable of the reversible conversion of protons and electrons to molecular hydrogen. Exploiting the unique enzymatic activity of H2ases can lead to advancements in the process of biohydrogen evolution and green energy production.

RESULTS

Here we created of a functional, optimized operon for rapid and robust production of recombinant [NiFe] Desulfomicrobium baculatum hydrogenase (Dmb H2ase). The conversion of the [NiFeSe] Dmb H2ase to [NiFe] type was performed on genetic level by site-directed mutagenesis. The native dmb operon includes two structural H2ase genes, coding for large and small subunits, and an additional gene, encoding a specific maturase (protease) that is essential for the proper maturation of the enzyme. Dmb, like all H2ases, needs intricate bio-production machinery to incorporate its crucial inorganic ligands and cofactors. Strictly anaerobic, sulfate reducer D. baculatum bacteria are distinct, in terms of their biology, from E. coli. Thus, we introduced a series of alterations within the native dmb genes. As a result, more than 100 elements, further compiled into 32 operon variants, were constructed. The initial requirement for a specific maturase was omitted by the artificial truncation of the large Dmb subunit. The assembly of the produced H2ase subunit variants was investigated both, in vitro and in vivo. This approach resulted in 4 recombinant [NiFe] Dmb enzyme variants, capable of H2 evolution. The aim of this study was to overcome the gene expression, protein biosynthesis, maturation and ligand loading bottlenecks for the easy, fast, and cost-effective delivery of recombinant [NiFe] H2ase, using a commonly available E. coli strains.

CONCLUSION

The optimized genetic constructs together with the developed growth and purification procedures appear to be a promising platform for further studies toward fully-active and O2 tolerant, recombinant [NiFeSe] Dmb H2ase, resembling the native Dmb enzyme. It could likely be achieved by selective cysteine to selenocysteine substitution within the active site of the [NiFe] Dmb variant.

Biological and Catalytic Properties of Selenoproteins

Selenocysteine is a catalytic residue at the active site of all selenoenzymes in bacteria and mammals, and it is incorporated into the polypeptide backbone by a co-translational process that relies on the recoding of a UGA termination codon into a serine/selenocysteine codon. The best-characterized selenoproteins from mammalian species and bacteria are discussed with emphasis on their biological function and catalytic mechanisms. A total of 25 genes coding for selenoproteins have been identified in the genome of mammals. Unlike the selenoenzymes of anaerobic bacteria, most mammalian selenoenzymes work as antioxidants and as redox regulators of cell metabolism and functions. Selenoprotein P contains several selenocysteine residues and serves as a selenocysteine reservoir for other selenoproteins in mammals. Although extensively studied, glutathione peroxidases are incompletely understood in terms of local and time-dependent distribution, and regulatory functions. Selenoenzymes take advantage of the nucleophilic reactivity of the selenolate form of selenocysteine. It is used with peroxides and their by-products such as disulfides and sulfoxides, but also with iodine in iodinated phenolic substrates. This results in the formation of Se-X bonds (X = O, S, N, or I) from which a selenenylsulfide intermediate is invariably produced. The initial selenolate group is then recycled by thiol addition. In bacterial glycine reductase and D-proline reductase, an unusual catalytic rupture of selenium-carbon bonds is observed. The exchange of selenium for sulfur in selenoproteins, and information obtained from model reactions, suggest that a generic advantage of selenium compared with sulfur relies on faster kinetics and better reversibility of its oxidation reactions.

Augmenting the Performance of Hydrogenase for Aerobic Photocatalytic Hydrogen Evolution via Solvent Tuning

This work showcases the performance of [NiFeSe] hydrogenase from Desulfomicrobium baculatum for solar-driven hydrogen generation in a variety of organic-based deep eutectic solvents. Despite its well-known sensitivity towards air and organic solvents, the hydrogenase shows remarkable performance under an aerobic atmosphere in these solvents when paired with a TiO2 photocatalyst. Tuning the water content further increases hydrogen evolution activity to a TOF of 60±3 s-1 and quantum yield to 2.3±0.4 % under aerobic conditions, compared to a TOF of 4 s-1 in a purely aqueous solvent. Contrary to common belief, this work therefore demonstrates that placing natural hydrogenases into non-natural environments can enhance their intrinsic activity beyond their natural performance, paving the way for full water splitting using hydrogenases.

Enhanced CO2 reduction and acetate synthesis in autotrophic biocathode by N-Hexanoyl-L-homoserine lactone (C6HSL)-based quorum-sensing regulation

The aim of this study was to investigate the ecological role of quorum-sensing signaling molecule on the autotrophic biocathode for CO₂ reduction and acetate synthesis. As a typical quorum-sensing signaling molecule, N-Hexanoyl-L-homoserine lactone (C6HSL) was used to regulate the construction of cathode biofilm. Results showed that the maximum acetate production from CO₂ reduction improved by 94.8%, and the maximum Faraday efficiency of the microbial electrosynthesis system enhanced by 71.7%, with the regulation of C6HSL. Electrochemical analyses indicated that higher electrochemical activity and lower charge resistance of biocathode were obtained with C6HSL than without C6HSL. Confocal laser scanning microscopy and electron inhibitor experiment suggested that exogenous C6HSL increased living biomass in the biofilm and facilitated the electron transfer pathway related to NADH dehydrogenase-CoQ and proton motive force. With the C6HSL regulation, the relative abundance of hydrogen producers (e.g., Desulfovibrio and Desulfomicrobium) increased, contributing to the improved performance of autotrophic biocathode.

Enzymatic X-ray absorption spectroelectrochemistry

The ability to observe the changes that occur at an enzyme active site during electrocatalysis can provide very valuable information for understanding the mechanism and ultimately aid in catalyst design. Herein, we discuss the development of X-ray absorption spectroscopy (XAS) in combination with electrochemistry for operando studies of enzymatic systems. XAS has had a long history of enabling geometric and electronic structural insights into the catalytic active sites of enzymes, however, XAS combined with electrochemistry (XA-SEC) has been exceedingly rare in bioinorganic applications. Herein, we discuss the challenges and opportunities of applying operando XAS to enzymatic electrocatalysts. The challenges due to the low concentration of the photoabsorber and the instability of the protein in the X-ray beam are discussed. Methods for immobilizing enzymes on the electrodes, while maintaining full redox control are highlighted. A case study of combined XAS and electrochemistry applied to a [NiFe] hydrogenase is presented. By entrapping the [NiFe] hydrogenase in a redox polymer, relatively high protein concentrations can be achieved on the electrode surface, while maintaining redox control. Overall, it is demonstrated that the experiments are feasible, but require precise redox control over the majority of the absorber atoms and careful controls to discriminate between electrochemically-driven changes and beam damage. Opportunities for future applications are discussed.

Understanding the local chemical environment of bioelectrocatalysis

Bioelectrochemistry employs an array of high-surface-area meso- and macroporous electrode architectures to increase protein loading and the electrochemical current response. While the local chemical environment has been studied in small-molecule and heterogenous electrocatalysis, conditions in enzyme electrochemistry are still commonly established based on bulk solution assays, without appropriate consideration of the nonequilibrium conditions of the confined electrode space. Here, we apply electrochemical and computational techniques to explore the local environment of fuel-producing oxidoreductases within porous electrode architectures. This improved understanding of the local environment enabled simple manipulation of the electrolyte solution by adjusting the bulk pH and buffer pKa to achieve an optimum local pH for maximal activity of the immobilized enzyme. When applied to macroporous inverse opal electrodes, the benefits of higher loading and increased mass transport were employed, and, consequently, the electrolyte adjusted to reach −8.0 mA ⋅ cm−2 for the H2 evolution reaction and −3.6 mA ⋅ cm−2 for the CO2 reduction reaction (CO2RR), demonstrating an 18-fold improvement on previously reported enzymatic CO2RR systems. This research emphasizes the critical importance of understanding the confined enzymatic chemical environment, thus expanding the known capabilities of enzyme bioelectrocatalysis. These considerations and insights can be directly applied to both bio(photo)electrochemical fuel and chemical synthesis, as well as enzymatic fuel cells, to significantly improve the fundamental understanding of the enzyme–electrode interface as well as device performance.

Harnessing selenocysteine to enhance microbial cell factories for hydrogen production

Hydrogen is a clean, renewable energy source, that when combined with oxygen, produces heat and electricity with only water vapor as a biproduct. Furthermore, it has the highest energy content by weight of all known fuels. As a result, various strategies have engineered methods to produce hydrogen efficiently and in quantities that are of interest to the economy. To approach the notion of producing hydrogen from a biological perspective, we take our attention to hydrogenases which are naturally produced in microbes. These organisms have the machinery to produce hydrogen, which when cleverly engineered, could be useful in cell factories resulting in large production of hydrogen. Not all hydrogenases are efficient at hydrogen production, and those that are, tend to be oxygen sensitive. Therefore, we provide a new perspective on introducing selenocysteine, a highly reactive proteinogenic amino acid, as a strategy towards engineering hydrogenases with enhanced hydrogen production, or increased oxygen tolerance.

Photocatalyst-enzyme hybrid systems for light-driven biotransformation

Enzymes catalyse target reactions under mild conditions with high efficiency, as well as excellent regional-, stereo-, and enantiomeric selectivity. Photocatalysis utilises sustainable and environment-friendly light power to realise efficient chemical conversion. By combining the interdisciplinary advantages of photo- and enzymatic catalysis, the photocatalyst-enzyme hybrid systems have proceeded various light-driven biotransformation with high efficiency under environmentally benign conditions, thus, attracting unparalleled focus during the last decades. It has also been regarded as a promising pathway towards green chemistry utilising ubiquitous solar energy. This systematic review gives insight into this research field by classifying the existing photocatalyst-enzyme hybrid systems into three sections based on different hybridizing modes between photo- and enzymatic catalysis. Furthermore, existing challenges and proposed strategies are discussed within this context. The first system summarised is the cofactor-mediated hybrid system, in which natural/artificial cofactors act as reducing equivalents that connect photocatalysts with enzymes for light-driven enzymatic biotransformation. Second, the direct contact-based photocatalyst-enzyme hybrid systems are described, including two different kinds of electron exchange sites on the enzyme molecules. Third, some cases where photocatalysts and enzymes are integrated into a reaction cascade with specific intermediates will be discussed in the following chapter. Finally, we provide perspective concerning the future of this field.

Hybrid Amyloid-Based Redox Hydrogel for Bioelectrocatalytic H2 Oxidation

An artificial amyloid-based redox hydrogel was designed for mediating electron transfer between a [NiFeSe] hydrogenase and an electrode. Starting from a mutated prion-forming domain of fungal protein HET-s, a hybrid redox protein containing a single benzyl methyl viologen moiety was synthesized. This protein was able to self-assemble into structurally homogenous nanofibrils. Molecular modeling confirmed that the redox groups are aligned along the fibril axis and are tethered to its core by a long, flexible polypeptide chain that allows close encounters between the fibril-bound oxidized or reduced redox groups. Redox hydrogel films capable of immobilizing the hydrogenase under mild conditions at the surface of carbon electrodes were obtained by a simple pH jump. In this way, bioelectrodes for the electrocatalytic oxidation of H₂ were fabricated that afforded catalytic current densities of up to 270 μA cm^-2 , with an overpotential of 0.33 V, under quiescent conditions at 45 °C.

The physiology and evolution of microbial selenium metabolism

Selenium is an essential trace element whose compounds are widely metabolized by organisms from all three domains of life. Moreover, phylogenetic evidence indicates that selenium species, along with iron, molybdenum, tungsten, and nickel, were metabolized by the last universal common ancestor of all cellular lineages, primarily for the synthesis of the 21st amino acid selenocysteine. Thus, selenium metabolism is both environmentally ubiquitous and a physiological adaptation of primordial life. Selenium metabolic reactions comprise reductive transformations both for assimilation into macromolecules and dissimilatory reduction of selenium oxyanions and elemental selenium during anaerobic respiration. This review offers a comprehensive overview of the physiology and evolution of both assimilatory and dissimilatory selenium metabolism in bacteria and archaea, highlighting mechanisms of selenium respiration. This includes a thorough discussion of our current knowledge of the physiology of selenocysteine synthesis and incorporation into proteins in bacteria obtained from structural biology. Additionally, this is the first comprehensive discussion in a review of the incorporation of selenium into the tRNA nucleoside 5-methylaminomethyl-2-selenouridine and as an inorganic cofactor in certain molybdenum hydroxylase enzymes. Throughout, conserved mechanisms and derived features of selenium metabolism in both domains are emphasized and discussed within the context of the global selenium biogeochemical cycle.

Selective cysteine-to-selenocysteine changes in a [NiFe]-hydrogenase confirm a special position for catalysis and oxygen tolerance

In [NiFe]-hydrogenases, the active-site Ni is coordinated by four cysteine-S ligands (Cys; C), two of which are bridging to the Fe(CO)(CN)2 fragment. Substitution of a single Cys residue by selenocysteine (Sec; U) occurs occasionally in nature. Using a recent method for site-specific Sec incorporation into proteins, each of the four Ni-coordinating cysteine residues in the oxygen-tolerant Escherichia coli [NiFe]-hydrogenase-1 (Hyd-1) has been replaced by U to identify its importance for enzyme function. Steady-state solution activity of each Sec-substituted enzyme (on a per-milligram basis) is lowered, although this may reflect the unquantified presence of recalcitrant inactive/immature/misfolded forms. Protein film electrochemistry, however, reveals detailed kinetic data that are independent of absolute activities. Like native Hyd-1, the variants have low apparent KMH2 values, do not produce H2 at pH 6, and display the same onset overpotential for H2 oxidation. Mechanistically important differences were identified for the C576U variant bearing the equivalent replacement found in native [NiFeSe]-hydrogenases, its extreme O2 tolerance (apparent KMH2 and Vmax [solution] values relative to native Hyd-1 of 0.13 and 0.04, respectively) implying the importance of a selenium atom in the position cis to the site where exogenous ligands (H-, H2, O2) bind. Observation of the same unusual electrocatalytic signature seen earlier for the proton transfer-defective E28Q variant highlights the direct role of the chalcogen atom (S/Se) at position 576 close to E28, with the caveat that Se is less effective than S in facilitating proton transfer away from the Ni during H2 oxidation by this enzyme.

Putative Extracellular Electron Transfer in Methanogenic Archaea

It has been suggested that a few methanogens are capable of extracellular electron transfers. For instance, Methanosarcina barkeri can directly capture electrons from the coexisting microbial cells of other species. Methanothrix harundinacea and Methanosarcina horonobensis retrieve electrons from Geobacter metallireducens via direct interspecies electron transfer (DIET). Recently, Methanobacterium, designated strain YSL, has been found to grow via DIET in the co-culture with Geobacter metallireducens. Methanosarcina acetivorans can perform anaerobic methane oxidation and respiratory growth relying on Fe(III) reduction through the extracellular electron transfer. Methanosarcina mazei is capable of electromethanogenesis under the conditions where electron-transfer mediators like H₂ or formate are limited. The membrane-bound multiheme c-type cytochromes (MHC) and electrically-conductive cellular appendages have been assumed to mediate the extracellular electron transfer in bacteria like Geobacter and Shewanella species. These molecules or structures are rare but have been recently identified in a few methanogens. Here, we review the current state of knowledge for the putative extracellular electron transfers in methanogens and highlight the opportunities and challenges for future research.

Exploring the gas access routes in a [NiFeSe] hydrogenase using crystals pressurized with krypton and oxygen

Hydrogenases are metalloenzymes that catalyse both H₂ evolution and uptake. They are gas-processing enzymes with deeply buried active sites, so the gases diffuse through channels that connect the active site to the protein surface. The [NiFeSe] hydrogenases are a special class of hydrogenases containing a selenocysteine as a nickel ligand; they are more catalytically active and less O₂-sensitive than standard [NiFe] hydrogenases. Characterisation of the channel system of hydrogenases is important to understand how the inhibitor oxygen reaches the active site to cause oxidative damage. To this end, crystals of Desulfovibrio vulgaris Hildenborough [NiFeSe] hydrogenase were pressurized with krypton and oxygen, and a method for tracking labile O₂ molecules was developed, for mapping a hydrophobic channel system similar to that of the [NiFe] enzymes as the major route for gas diffusion.

The roles of chalcogenides in O2 protection of H2ase active sites

At some point, all HER (Hydrogen Evolution Reaction) catalysts, important in sustainable H2O splitting technology, will encounter O2 and O2-damage. The [NiFeSe]-H2ases and some of the [NiFeS]-H2ases, biocatalysts for reversible H2 production from protons and electrons, are exemplars of oxygen tolerant HER catalysts in nature. In the hydrogenase active sites oxygen damage may be extensive (irreversible) as it is for the [FeFe]-H2ase or moderate (reversible) for the [NiFe]-H2ases. The affinity of oxygen for sulfur, in [NiFeS]-H2ase, and selenium, in [NiFeSe]-H2ase, yielding oxygenated chalcogens results in maintenance of the core NiFe unit, and myriad observable but inactive states, which can be reductively repaired. In contrast, the [FeFe]-H2ase active site has less possibilities for chalcogen-oxygen uptake and a greater chance for O2-attack on iron. Exposure to O2 typically leads to irreversible damage. Despite the evidence of S/Se-oxygenation in the active sites of hydrogenases, there are limited reported synthetic models. This perspective will give an overview of the studies of O2 reactions with the hydrogenases and biomimetics with focus on our recent studies that compare sulfur and selenium containing synthetic analogues of the [NiFe]-H2ase active sites.

Redox-Polymer-Wired [NiFeSe] Hydrogenase Variants with Enhanced O2 Stability for Triple-Protected High-Current-Density H2 -Oxidation Bioanodes

Variants of the highly active [NiFeSe] hydrogenase from D. vulgaris Hildenborough that exhibit enhanced O2 tolerance were used as H2 -oxidation catalysts in H2 /O2 biofuel cells. Two [NiFeSe] variants were electrically wired by means of low-potential viologen-modified redox polymers and evaluated with respect to H2 -oxidation and stability against O2 in the immobilized state. The two variants showed maximum current densities of (450±84) μA cm-2 for G491A and (476±172) μA cm-2 for variant G941S on glassy carbon electrodes and a higher O2 tolerance than the wild type. In addition, the polymer protected the enzyme from O2 damage and high-potential inactivation, establishing a triple protection for the bioanode. The use of gas-diffusion bioanodes provided current densities for H2 -oxidation of up to 6.3 mA cm-2 . Combination of the gas-diffusion bioanode with a bilirubin oxidase-based gas-diffusion O2 -reducing biocathode in a membrane-free biofuel cell under anode-limiting conditions showed unprecedented benchmark power densities of 4.4 mW cm-2 at 0.7 V and an open-circuit voltage of 1.14 V even at moderate catalyst loadings, outperforming the previously reported system obtained with the [NiFeSe] wild type and the [NiFe] hydrogenase from D. vulgaris Miyazaki F.

Studying O2 pathways in [NiFe]- and [NiFeSe]-hydrogenases

Hydrogenases are efficient biocatalysts for H₂ production and oxidation with various potential biotechnological applications.[NiFe]-class hydrogenases are highly active in both production and oxidation processes—albeit primarily biased to the latter—but suffer from being sensitive to O₂.[NiFeSe] hydrogenases are a subclass of [NiFe] hydrogenases with, usually, an increased insensitivity to aerobic environments. In this study we aim to understand the structural causes of the low sensitivity of a [NiFeSe]-hydrogenase, when compared with a [NiFe] class enzyme, by studying the diffusion of O₂. To unravel the differences between the two enzymes, we used computational methods comprising Molecular Dynamics simulations with explicit O₂ and Implicit Ligand Sampling methodologies. With the latter, we were able to map the free energy landscapes for O₂ permeation in both enzymes. We derived pathways from these energy landscapes and selected the kinetically more relevant ones with reactive flux analysis using transition path theory. These studies evidence the existence of quite different pathways in both enzymes and predict a lower permeation efficiency for O₂ in the case of the [NiFeSe]-hydrogenase when compared with the [NiFe] enzyme. These differences can explain the experimentally observed lower inhibition by O₂ on [NiFeSe]-hydrogenases, when compared with [NiFe]-hydrogenases. A comprehensive map of the residues lining the most important O₂ pathways in both enzymes is also presented.

Integration of a Hydrogenase in a Lead Halide Perovskite Photoelectrode for Tandem Solar Water Splitting

Lead halide perovskite solar cells are notoriously moisture-sensitive, but recent encapsulation strategies have demonstrated their potential application as photoelectrodes in aqueous solution. However, perovskite photoelectrodes rely on precious metal co-catalysts, and their combination with biological materials remains elusive in integrated devices. Here, we interface [NiFeSe] hydrogenase from Desulfovibrio vulgaris Hildenborough, a highly active enzyme for H₂ generation, with a triple cation mixed halide perovskite. The perovskite-hydrogenase photoelectrode produces a photocurrent of -5 mA cm^-2 at 0 V vs RHE during AM1.5G irradiation, is stable for 12 h and the hydrogenase exhibits a turnover number of 1.9 × 10⁶. The positive onset potential of +0.8 V vs RHE allows its combination with a BiVO₄ water oxidation photoanode to give a self-sustaining, bias-free photoelectrochemical tandem system for overall water splitting (solar-to-hydrogen efficiency of 1.1%). This work demonstrates the compatibility of immersed perovskite elements with biological catalysts to produce hybrid photoelectrodes with benchmark performance, which establishes their utility in semiartificial photosynthesis.

Semi-biological approaches to solar-to-chemical conversion

This review provides an overview of the cross-disciplinary field of semi-artificial photosynthesis, which combines strengths of biocatalysis and artificial photosynthesis to develop new concepts and approaches for solar-to-chemical conversion.

Characterization of the [NiFeSe] hydrogenase from Desulfovibrio vulgaris Hildenborough

The [NiFeSe] hydrogenases are a subgroup of the well-characterized family of [NiFe] hydrogenases, in which a selenocysteine is a ligand to the nickel atom in the binuclear NiFe active site instead of cysteine. These enzymes display very interesting catalytic properties for biological hydrogen production and bioelectrochemical applications: high H₂ production activity, bias for H₂ evolution, low H₂ inhibition, and some degree of O₂ tolerance. Here we describe the methodologies employed to study the [NiFeSe] hydrogenase isolated from the sulfate-reducing bacteria D. vulgaris Hildenborough and the creation of a homologous expression system for production of variant forms of the enzyme.

Selenium versus sulfur: Reversibility of chemical reactions and resistance to permanent oxidation in proteins and nucleic acids

This review highlights the contributions of Jean Chaudière to the field of selenium biochemistry. Chaudière was the first to recognize that one of the main reasons that selenium in the form of selenocysteine is used in proteins is due to the fact that it strongly resists permanent oxidation. The foundations for this important concept was laid down by Al Tappel in the 1960's and even before by others. The concept of oxygen tolerance first recognized in the study of glutathione peroxidase was further advanced and refined by those studying [NiFeSe]-hydrogenases, selenosubtilisin, and thioredoxin reductase. After 200 years of selenium research, work by Marcus Conrad and coworkers studying glutathione peroxidase-4 has provided definitive evidence for Chaudière's original hypothesis (Ingold et al., 2018) [36]. While the reaction of selenium with oxygen is readily reversible, there are many other examples of this phenomenon of reversibility. Many reactions of selenium can be described as "easy in - easy out". This is due to the strong nucleophilic character of selenium to attack electrophiles, but then this reaction can be reversed due to the strong electrophilic character of selenium and the weakness of the selenium-carbon bond. Several examples of this are described.

Solar Water Splitting with a Hydrogenase Integrated in Photoelectrochemical Tandem Cells

Hydrogenases (H₂ases) are benchmark electrocatalysts for H₂ production, both in biology and (photo)catalysis in vitro. We report the tailoring of a p‐type Si photocathode for optimal loading and wiring of H₂ase through the introduction of a hierarchical inverse opal (IO) TiO₂ interlayer. This proton‐reducing Si|IO‐TiO₂|H₂ase photocathode is capable of driving overall water splitting in combination with a photoanode. We demonstrate unassisted (bias‐free) water splitting by wiring Si|IO‐TiO₂|H₂ase to a modified BiVO₄ photoanode in a photoelectrochemical (PEC) cell during several hours of irradiation. Connecting the Si|IO‐TiO₂|H₂ase to a photosystem II (PSII) photoanode provides proof of concept for an engineered Z‐scheme that replaces the non‐complementary, natural light absorber photosystem I with a complementary abiotic silicon photocathode.

Photobiocatalysis: Activating Redox Enzymes by Direct or Indirect Transfer of Photoinduced Electrons

Biocatalytic transformation has received increasing attention in the green synthesis of chemicals because of the diversity of enzymes, their high catalytic activities and specificities, and mild reaction conditions. The idea of solar energy utilization in chemical synthesis through the combination of photocatalysis and biocatalysis provides an opportunity to make the "green" process greener. Oxidoreductases catalyze redox transformation of substrates by exchanging electrons at the enzyme's active site, often with the aid of electron mediator(s) as a counterpart. Recent progress indicates that photoinduced electron transfer using organic (or inorganic) photosensitizers can activate a wide spectrum of redox enzymes to catalyze fuel-forming reactions (e.g., H₂ evolution, CO₂ reduction) and synthetically useful reductions (e.g., asymmetric reduction, oxygenation, hydroxylation, epoxidation, Baeyer-Villiger oxidation). This Review provides an overview of recent advances in light-driven activation of redox enzymes through direct or indirect transfer of photoinduced electrons.

Development of air-stable hydrogen evolution catalysts

Obtaining abundant pure hydrogen by reduction of water has an important implication in the development of clean and renewable energy. Hence research focused on the development of non-noble metal based facile and energy efficient catalysts for proton reduction is on the rise. However, for practical utilization, it is necessary that these complexes function unabated in the presence of atmospheric oxygen and other common contaminants in abundant water sources. There has been very little activity towards the development of oxygen-tolerant hydrogen producing catalysts. This article aims to draw attention to this issue of oxygen sensitivity in the HER and highlights the development of a few air-stable HER catalysts (enzymatic as well as artificial) elaborating the challenges involved and the techniques discovered to overcome this significant deterrent to large-scale hydrogen production by electrolysis from abundant water sources.

Selective cathodic microbial biofilm retention allows a high current-to-sulfide efficiency in sulfate-reducing microbial electrolysis cells

Selective microbial retention is of paramount importance for the long-term performance of cathodic sulfate reduction in microbial electrolysis cells (MECs) due to the slow growth rate of autotrophic sulfate-reducing bacteria. In this work, we investigate the biofilm retention and current-to-sulfide conversion efficiency using carbon granules (CG) or multi-wall carbon nanotubes deposited on reticulated vitreous carbon (MWCNT-RVC) as electrode materials. For ~2months, the MECs were operated at sulfate loading rates of 21 to 309gSO₄ -S/m²/d. Although MWCNT-RVC achieved a current density of 57±11A/m², greater than the 32±9A/m² observed using CG, both materials exhibited similar sulfate reduction rates (SRR), with MWCNT-RVC reaching 104±16gSO₄ -S/m²/d while 110±13gSO₄ -S/m²/d were achieved with CG. Pyrosequencing analysis of the 16S rRNA at the end of experimentation revealed a core community dominated by Desulfovibrio (28%), Methanobacterium (19%) and Desulfomicrobium (14%), on the MWCNT-RVC electrodes. While a similar Desulfovibrio relative abundance of 29% was found in CG-biofilms, Desulfomicrobium was found to be significantly less abundant (4%) and Methanobacterium practically absent (0.2%) on CG electrodes. Surprisingly, our results show that CG can achieve higher current-to-sulfide efficiencies at lower power consumption than the nano-modified three-dimensional MWCNT-RVC.

Protection and Reactivation of the [NiFeSe] Hydrogenase from Desulfovibrio vulgaris Hildenborough under Oxidative Conditions

We report on the fabrication of bioanodes for H₂ oxidation based on [NiFeSe] hydrogenase. The enzyme was electrically wired by means of a specifically designed low-potential viologen-modified polymer, which delivers benchmark H₂ oxidizing currents even under deactivating conditions owing to efficient protection against O₂ combined with a viologen-induced reactivation of the O₂ inhibited enzyme. Moreover, the viologen-modified polymer allows for electrochemical co-deposition of polymer and biocatalyst and, by this, for control of the film thickness. Protection and reactivation of the enzyme was demonstrated in thick and thin reaction layers.

Photoelectrocatalytic H2 evolution in water with molecular catalysts immobilised on p-Si via a stabilising mesoporous TiO2 interlayer

A versatile platform for the immobilisation of molecular catalysts on a readily-prepared Si photocathode with a mesoporous TiO₂ layer is reported.

The development of photoelectrodes capable of light-driven hydrogen evolution from water is an important approach for the storage of solar energy in the form of a chemical energy carrier. However, molecular catalyst-based photocathodes remain scarcely reported and typically suffer from low efficiencies and/or stabilities due to inadequate strategies for interfacing the molecular component with the light-harvesting material. In this study, we report the straightforward preparation of a p-silicon|mesoporous titania|molecular catalyst photocathode assembly that is active towards proton reduction in aqueous media with an onset potential of +0.4 V vs. RHE. The mesoporous TiO₂ scaffold acts as an electron shuttle between the silicon and the catalyst, while also stabilising the silicon from passivation and enabling a high loading of molecular catalysts (>30 nmol (geometrical cm)^–2). When a Ni bis(diphosphine)-based catalyst is anchored on the surface of the electrode, a high turnover number of ∼1 × 10³ was obtained from photoelectrolysis under UV-filtered simulated solar irradiation at 1 Sun after 24 h at pH 4.5. Notwithstanding its aptitude for molecular catalyst immobilisation, the p-Si|TiO₂ photoelectrode showed great versatility towards different catalysts and pH conditions, with photoelectrocatalytic H₂ generation also being achieved with platinum and a hydrogenase as catalyst, highlighting the flexible platform it represents for many potential reductive catalysis transformations.

Proton Transfer in the Catalytic Cycle of [NiFe] Hydrogenases: Insight from Vibrational Spectroscopy

Catalysis of H₂ production and oxidation reactions is critical in renewable energy systems based around H₂ as a clean fuel, but the present reliance on platinum-based catalysts is not sustainable. In nature, H₂ is oxidized at minimal overpotential and high turnover frequencies at [NiFe] catalytic sites in hydrogenase enzymes. Although an outline mechanism has been established for the [NiFe] hydrogenases involving heterolytic cleavage of H₂ followed by a first and then second transfer of a proton and electron away from the active site, details remain vague concerning how the proton transfers are facilitated by the protein environment close to the active site. Furthermore, although [NiFe] hydrogenases from different organisms or cellular environments share a common active site, they exhibit a broad range of catalytic characteristics indicating the importance of subtle changes in the surrounding protein in controlling their behavior. Here we review recent time-resolved infrared (IR) spectroscopic studies and IR spectroelectrochemical studies carried out in situ during electrocatalytic turnover. Additionally, we re-evaluate the significant body of IR spectroscopic data on hydrogenase active site states determined through more conventional solution studies, in order to highlight mechanistic steps that seem to apply generally across the [NiFe] hydrogenases, as well as steps which so far seem limited to specific groups of these enzymes. This analysis is intended to help focus attention on the key open questions where further work is needed to assess important aspects of proton and electron transfer in the mechanism of [NiFe] hydrogenases.