Reversible catalysis

Indications for a universal hydrogen catalysis mechanism in [FeFe]-hydrogenases of different phylogenetic groups

[FeFe]-hydrogenases are metalloenzymes catalysing bidirectional hydrogen (H2) turnover. These enzymes are generally considered to be extremely efficient and fast catalysts. However, [FeFe]-hydrogenases constitute a very diverse enzyme family that can be divided into several distinct phylogenetic groups, denoted as groups A-G. Very little is known about the properties of [FeFe]-hydrogenases outside of the intensively studied group A, but recent studies on putatively sensory group C and D enzymes have revealed distinct differences in reactivity. The variation in structure, reactivity and physiological function observed between phylogenetic groups raises the question if all [FeFe]-hydrogenases follow the same mechanism for H2 turnover. Here, we provide the first detailed spectroscopic investigation of a slow-acting putatively sensory group D [FeFe]-hydrogenase from Thermoanaerobacter mathranii (TamHydS). Photo-reduction enabled us to characterize redox states in group D [FeFe]-hydrogenase via infrared spectroscopy under catalytic conditions. The sequential population of redox states similar to group A [FeFe]-hydrogenases supports the notion that group A and D [FeFe]-hydrogenases follow a universal catalytic mechanism. However, clear differences between enzymes from different phylogenetic groups become evident when comparing the relative stability and protonation state of suggested key catalytic intermediates. Moreover, the spectroscopic data collected on TamHydS provides new insight into the structure of the reduced active site, lending further support for the notion of a retained bridging CO ligand throughout the entire catalytic cycle.

Flow of Energy and Information in Molecular Machines

Molecular machines transduce free energy between different forms throughout all living organisms. Unlike their macroscopic counterparts, molecular machines are characterized by stochastic fluctuations, overdamped dynamics, and soft components, and operate far from thermodynamic equilibrium. In addition, information is a relevant free energy resource for molecular machines, leading to new modes of operation for nanoscale engines. Toward the objective of engineering synthetic nanomachines, an important goal is to understand how molecular machines transduce free energy to perform their functions in biological systems. In this review, we discuss the nonequilibrium thermodynamics of free energy transduction within molecular machines, with a focus on quantifying energy and information flows between their components. We review results from theory, modeling, and inference from experiments that shed light on the internal thermodynamics of molecular machines, and ultimately explore what we can learn from considering these interactions.

Probing the Influence of the Protein Scaffold on H-Cluster Reactivity via Gain-of-Function Studies─Improved H2 Evolution and O2 Tolerance through Rational Design of [FeFe] Hydrogenase

[FeFe] hydrogenases make up a structurally diverse family of metalloenzymes that catalyze proton/dihydrogen interconversion. They can be classified into phylogenetically distinct groups denoted A-G, which differ in structure and reactivity. Prototypical Group A hydrogenases have high turnover rates and remarkable energy efficiency. As compared to Group A enzymes, the putatively sensory Group D hydrogenase from Thermoanaerobacter mathranii (TamHydS) has a thousand-fold lower H2 evolution rate and a high overpotential requirement to drive catalysis (irreversible) but shows increased inhibitor tolerance. This divergence in structure and activity between hydrogenases makes them ideal models for studying second (active-site environment) and outer (e.g., substrate transport) coordination sphere effects on metal cofactors. Herein, we generated three TamHydS-based variants, each mimicking proposed key structural features of Group A hydrogenase: the "active site" (AS), "proton-transfer pathway" (PTP), and "combined" (CM = AS + PTP) variant. A fourth single-point variant, A137C, which introduces a proposed critical cysteine in the active site, was characterized as a reference. No change in isolation resulted in Group A-like behavior; i.e., no positive impact on catalytic performance was observed. The CM variant, however, showed increased H2 evolution activity but retained the overpotential requirement. Additionally, the CM variant improved the already relatively high stability of TamHydS against O2 and CO inhibition. These findings show that activity rates, (ir)reversibility, and susceptibility to gaseous inhibitors are decoupled. Moreover, the results highlight the importance of exploring hydrogenase diversity as a path toward understanding the structural factors that enable the outstanding catalytic properties of [FeFe] hydrogenases.

Understanding Ligand Effects on Bielectronic Transitions: Chemo- and Electroreduction of Rhodium Bis(Diphosphine) Complexes to Low Oxidation States

Rhodium complexes in the -I and 0 oxidation states are of great potential interest in catalytic applications. In contrast to their rhodium +I congeners, however, the structural and electronic parameters governing their access and stability are far less understood. Herein, we investigate the two-electron reduction of a parameterized series of bis(diphosphine) Rh complexes [Rh(dxpy)2]NTf2 (x=P-substituent, y=alkanediyl bridging P atoms). Through (electro)reductions from the RhI parents, Rh-I d10-complexes were obtained and characterized spectroscopically, including 103Rh NMR data. The reductive steps convolute with structural rearrangements from square planar to tetrahedral coordination. We found that the extent of these reorganisations defines whether the first E0(RhI/0) and second E0(Rh0/-I) reduction potentials are normally ordered, leading to monoelectronic stepwise transitions, or inverted, giving bielectronic events. Reductionist approaches based on Hammett parameters or the P-Rh-P bite angles provide only partial correlations with the redox potentials. However, we identified the C-O stretch of analogue diphosphine complexes as an expedient computational parameter that enables these correlations through both electronic and geometric features, even in a predictive manner. Gaining control over two-electron reduction behaviors through rationalized ligand effects has potential impact beyond Rh complexes, for molecular and enzymatic metal sites commonly exhibiting bielectronic transitions.

Levodopa: From Biological Significance to Continuous Monitoring

A continuous levodopa sensor can improve the quality of life for patients suffering with Parkinson's disease by enhancing levodopa titration and treatment effectiveness; however, its development is currently hindered by the absence of a specific levodopa molecular recognition element and limited insights into how real-time monitoring might affect clinical outcomes. This gap in research contributes to clinician uncertainty regarding the practical value of continuous levodopa monitoring data. This paper examines the current state of levodopa sensing and the inherent limitations in today's methods. Further, these challenges are described, including aspects such as interference from the metabolic pathway and adjunct medications, temporal resolution, and clinical questions, with a specific focus on a comprehensive selection of molecules, such as adjunct medications and structural isomers, as an interferent panel designed to assess and validate future levodopa sensors. We review insights and lessons from previously reported levodopa sensors and present a comparative analysis of potential molecular recognition elements, discussing their advantages and drawbacks.

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.

Interactive biocatalysis achieved by driving enzyme cascades inside a porous conducting material

An emerging concept and platform, the electrochemical Leaf (e-Leaf), offers a radical change in the way tandem (multi-step) catalysis by enzyme cascades is studied and exploited. The various enzymes are loaded into an electronically conducting porous material composed of metallic oxide nanoparticles, where they achieve high concentration and crowding - in the latter respect the environment resembles that found in living cells. By exploiting efficient electron tunneling between the nanoparticles and one of the enzymes, the e-Leaf enables the user to interact directly with complex networks, rendering simultaneous the abilities to energise, control and observe catalysis. Because dispersion of intermediates is physically suppressed, the output of the cascade - the rate of flow of chemical steps and information - is delivered in real time as electrical current. Myriad enzymes of all major classes now become effectively electroactive in a technology that offers scalability between micro-(analytical, multiplex) and macro-(synthesis) levels. This Perspective describes how the e-Leaf was discovered, the steps in its development so far, and the outlook for future research and applications.

Properties of the iron-sulfur cluster electron transfer relay in an [FeFe]-hydrogenase that is tuned for H2 oxidation catalysis

[FeFe]-hydrogenases catalyze the reversible oxidation of H2 from electrons and protons at an organometallic active site cofactor named the H-cluster. In addition to the H-cluster, most [FeFe]-hydrogenases possess accessory FeS cluster (F-cluster) relays that function in mediating electron transfer with catalysis. There is significant variation in the structural properties of F-cluster relays among the [FeFe]-hydrogenases; however, it is unknown how this variation relates to the electronic and thermodynamic properties, and thus the electron transfer properties, of enzymes. Clostridium pasteurianum [FeFe]-hydrogenase II (CpII) exhibits a large catalytic bias for H2 oxidation (compared to H2 production), making it a notable system for examining if F-cluster properties contribute to the overall function and efficiency of the enzyme. By applying a combination of multifrequency and potentiometric electron paramagnetic resonance, we resolved two electron paramagnetic resonance signals with distinct power- and temperature-dependent properties at g = 2.058 1.931 1.891 (F2.058) and g = 2.061 1.920 1.887 (F2.061), with assigned midpoint potentials of -140 ± 18 mV and -406 ± 12 mV versus normal hydrogen electrode, respectively. Spectral analysis revealed features consistent with spin-spin coupling between the two [4Fe-4S] F-clusters, and possible functional models are discussed that account for the contribution of coupling to the electron transfer landscape. The results signify the interplay of electronic coupling and free energy properties and parameters of the FeS clusters to the electron transfer mechanism through the relay and provide new insight as to how relays functionally complement the catalytic directionality of active sites to achieve highly efficient catalysis.

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.

Kinetic Asymmetry and Directionality of Nonequilibrium Molecular Systems

Scientists have long been fascinated by the biomolecular machines in living systems that process energy and information to sustain life. The first synthetic molecular rotor capable of performing repeated 360° rotations due to a combination of photo- and thermally activated processes was reported in 1999. The progress in designing different molecular machines in the intervening years has been remarkable, with several outstanding examples appearing in the last few years. Despite the synthetic accomplishments, there remains confusion regarding the fundamental design principles by which the motions of molecules can be controlled, with significant intellectual tension between mechanical and chemical ways of thinking about and describing molecular machines. A thermodynamically consistent analysis of the kinetics of several molecular rotors and pumps shows that while light driven rotors operate by a power-stroke mechanism, kinetic asymmetry-the relative heights of energy barriers-is the sole determinant of the directionality of catalysis driven machines. Power-strokes-the relative depths of energy wells-play no role whatsoever in determining the sign of the directionality. These results, elaborated using trajectory thermodynamics and the nonequilibrium pump equality, show that kinetic asymmetry governs the response of many non-equilibrium chemical phenomena.

Cable Bacteria Skeletons as Catalytically Active Electrodes

Cable bacteria are multicellular, filamentous bacteria that use internal conductive fibers to transfer electrons over centimeter distances from donors within anoxic sediment layers to oxygen at the surface. We extracted the fibers and used them as free-standing bio-based electrodes to investigate their electrocatalytic behavior. The fibers catalyzed the reversible interconversion of oxygen and water, and an electric current was running through the fibers even when the potential difference was generated solely by a gradient of oxygen concentration. Oxygen reduction as well as oxygen evolution were confirmed by optical measurements. Within living cable bacteria, oxygen reduction by direct electrocatalysis on the fibers and not by membrane-bound proteins readily explains exceptionally high cell-specific oxygen consumption rates observed in the oxic zone, while electrocatalytic water oxidation may provide oxygen to cells in the anoxic zone.

Kinetic Modeling of the Reversible or Irreversible Electrochemical Responses of FeFe-Hydrogenases

The enzyme FeFe-hydrogenase catalyzes H2 evolution and oxidation at an active site that consists of a [4Fe-4S] cluster bridged to a [Fe2(CO)3(CN)2(azadithiolate)] subsite. Previous investigations of its mechanism were mostly conducted on a few "prototypical" FeFe-hydrogenases, such as that from Chlamydomonas reinhardtii(Cr HydA1), but atypical hydrogenases have recently been characterized in an effort to explore the diversity of this class of enzymes. We aim at understanding why prototypical hydrogenases are active in either direction of the reaction in response to a small deviation from equilibrium, whereas the homologous enzyme from Thermoanaerobacter mathranii (Tam HydS) shows activity only under conditions of very high driving force, a behavior that was referred to as "irreversible catalysis". We follow up on previous spectroscopic studies and recent developments in the kinetic modeling of bidirectional reactions to investigate and compare the catalytic cycles of Cr HydA1 and Tam HydS under conditions of direct electron transfer with an electrode. We compare the hypothetical catalytic cycles described in the literature, and we show that the observed changes in catalytic activity as a function of potential, pH, and H2 concentration can be explained with the assumption that the same catalytic mechanism applies. This helps us identify which variations in properties of the catalytic intermediates give rise to the distinct "reversible" or "irreversible" catalytic behaviors.

Electrocatalytic Interconversions of CO2 and Formate on a Versatile Iron-Thiolate Platform

Exploring bidirectional CO2/HCO2- catalysis holds significant potential in constructing integrated (photo)electrochemical formate fuel cells for energy storage and applications. Herein, we report selective CO2/HCO2- electrochemical interconversion by exploiting the flexible coordination modes and rich redox properties of a versatile iron-thiolate platform, Cp*Fe(II)L (L = 1,2-Ph2PC6H4S-). Upon oxidation, this iron complex undergoes formate binding to generate a diferric formate complex, [(L-)2Fe(III)(μ-HCO2)Fe(III)]+, which exhibits remarkable electrocatalytic performance for the HCO2--to-CO2 transformation with a maximum turnover frequency (TOFmax) ∼103 s-1 and a Faraday efficiency (FE) ∼92(±4)%. Conversely, this iron system also allows for reduction at -1.85 V (vs Fc+/0) and exhibits an impressive FE ∼93 (±3)% for the CO2-to-HCO2- conversion. Mechanism studies revealed that the HCO2--to-CO2 electrocatalysis passes through dicationic [(L2)-•Fe(III)(μ-HCO2)Fe(III)]2+ generated by unconventional oxidation of the diferric formate species taking place at ligand L, while the CO2-to-HCO2- reduction involves a critical intermediate of [Fe(II)-H]- that was independently synthesized and structurally characterized.

Energy-efficient CO2/CO interconversion by homogeneous copper-based molecular catalysts

Facile conversion of CO2 to commercially viable carbon feedstocks offer a unique way to adopt a net-zero carbon scenario. Synthetic CO2-reducing catalysts have rarely exhibited energy-efficient and selective CO2 conversion. Here, the carbon monoxide dehydrogenase (CODH) enzyme blueprint is imitated by a molecular copper complex coordinated by redox-active ligands. This strategy has unveiled one of the rarest examples of synthetic molecular complex-driven reversible CO2 reduction/CO oxidation catalysis under regulated conditions, a hallmark of natural enzymes. The inclusion of a proton-exchanging amine groups in the periphery of the copper complex provides the leeway to modulate the biases of catalysts toward CO2 reduction and CO oxidation in organic and aqueous media. The detailed spectroelectrochemical analysis confirms the synchronous participation of copper and redox-active ligands along with the peripheral amines during this energy-efficient CO2 reduction/CO oxidation. This finding can be vital in abating the carbon footprint-free in multiple industrial processes.

A Chimeric NiFe Hydrogenase Heterodimer to Assess the Role of the Electron Transfer Chain in Tuning the Enzyme's Catalytic Bias and Oxygen Tolerance

The observation that some homologous enzymes have the same active site but very different catalytic properties demonstrates the importance of long-range effects in enzyme catalysis, but these effects are often difficult to rationalize. The NiFe hydrogenases 1 and 2 (Hyd 1 and Hyd 2) from E. coli both consist of a large catalytic subunit that embeds the same dinuclear active site and a small electron-transfer subunit with a chain of three FeS clusters. Hyd 1 is mostly active in H2 oxidation and resistant to inhibitors, whereas Hyd 2 also catalyzes H2 production and is strongly inhibited by O2 and CO. Based on structural and site-directed mutagenesis data, it is currently believed that the catalytic bias and tolerance to O2 of Hyd 1 are defined by the distal and proximal FeS clusters, respectively. To test these hypotheses, we produced and characterized a hybrid enzyme made of the catalytic subunit of Hyd 1 and the electron transfer subunit of Hyd 2. We conclude that catalytic bias and sensitivity to CO are set by the catalytic subunit rather than by the electron transfer chain. We confirm the importance of the proximal cluster in making the enzyme Hyd 1 resist long-term exposure to O2, but we show that other structural determinants, in both subunits, contribute to O2 tolerance. A similar strategy based on the design of chimeric heterodimers could be used in the future to elucidate various structure-function relationships in hydrogenases and other multimeric metalloenzymes and to engineer useful hydrogenases that combine the desirable properties of distinct, homologous enzymes.

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.

Assessing the Extent of Potential Inversion by Cyclic Voltammetry: Theory, Pitfalls, and Application to a Nickel Complex with Redox-Active Iminosemiquinone Ligands

Potential inversion refers to the situation where a protein cofactor or a synthetic molecule can be oxidized or reduced twice in a cooperative manner; that is, the second electron transfer is easier than the first. This property is very important regarding the catalytic mechanism of enzymes that bifurcate electrons and the properties of bidirectional redox molecular catalysts that function in either direction of the reaction with no overpotential. Cyclic voltammetry is the most common technique for characterizing the thermodynamics and kinetics of electron transfer to or from these molecules. However, a gap in the literature is the absence of analytical predictions to help interpret the values of the voltammetric peak potentials when potential inversion occurs; the cyclic voltammograms are therefore often analyzed by simulating the data, with no discussion of the possibility of overfitting and often no estimation of the error on the determined parameters. Here we formulate the theory for the voltammetry of freely diffusing or surface-confined two-electron redox species in the experimentally relevant irreversible limit where the peak separation depends on the scan rate. We explain why the model is intrinsically underdetermined, and we illustrate this conclusion by analysis of the voltammetry of a nickel complex with redox-active iminosemiquinone ligands. Being able to characterize the thermodynamics of two-electron electron-transfer reactions will be crucial for designing more efficient catalysts.

Electrochemical Kinetics Support a Second Coordination Sphere Mechanism in Metal-Based Formate Dehydrogenase

Metal-based formate dehydrogenases are molybdenum or tungsten-dependent enzymes that catalyze the interconversion between formate and CO2 . According to the current consensus, the metal ion of the catalytic center in its active form is coordinated by 6 S (or 5 S and 1 Se) atoms, leaving no free coordination sites to which formate could bind to the metal. Some authors have proposed that one of the active site ligands decoordinates during turnover to allow formate binding. Another proposal is that the oxidation of formate takes place in the second coordination sphere of the metal. Here, we have used electrochemical steady-state kinetics to elucidate the order of the steps in the catalytic cycle of two formate dehydrogenases. Our results strongly support the "second coordination sphere" hypothesis.

Facile Functionalization of Carbon Electrodes for Efficient Electroenzymatic Hydrogen Production

Enzymatic electrocatalysis holds promise for new biotechnological approaches to produce chemical commodities such as molecular hydrogen (H2). However, typical inhibitory limitations include low stability and/or low electrocatalytic currents (low product yields). Here we report a facile single-step electrode preparation procedure using indium-tin oxide nanoparticles on carbon electrodes. The subsequent immobilization of a model [FeFe]-hydrogenase from Clostridium pasteurianum ("CpI") on the functionalized carbon electrode permits comparatively large quantities of H2 to be produced in a stable manner. Specifically, we observe current densities of >8 mA/cm2 at -0.8 V vs the standard hydrogen electrode (SHE) by direct electron transfer (DET) from cyclic voltammetry, with an onset potential for H2 production close to its standard potential at pH 7 (approximately -0.4 V vs. SHE). Importantly, hydrogenase-modified electrodes show high stability retaining ∼92% of their electrocatalytic current after 120 h of continuous potentiostatic H2 production at -0.6 V vs. SHE; gas chromatography confirmed ∼100% Faradaic efficiency. As the bioelectrode preparation method balances simplicity, performance, and stability, it paves the way for DET on other electroenzymatic reactions as well as semiartificial photosynthesis.

High Titer of (S)-Equol Synthesis from Daidzein in Escherichia coli

(S)-Equol is the terminal metabolite of daidzein and plays important roles in human health. However, due to anaerobic inefficiency, limited productivity in (S)-equol-producing strains often hinders (S)-equol mass production. Here, a multi-enzyme cascade system was designed to generate a higher (S)-equol titer. First, full reversibility of the (S)-equol synthesis pathway was found and a blocking reverse conversion strategy was established. As biosynthetic genes are present in the microbial genome, an effective daidzein reductase was chosen using evolutionary principles. And our analyses showed that NADPH was crucial for the pathway. In response to this, a novel NADPH pool was redesigned after analyzing a cofactor metabolism model. By adjusting synthesis pathway genes at the right expression level, the entire synthesis pathway can take place smoothly. Thus, the cascade system was optimized by regulating the gene expression intensity. Finally, after optimizing fermentation conditions, a 5 L bioreactor was used to generate a high (S)-equol production titer (3418.5 mg/L), with a conversion rate of approximately 85.9%. This study shows a feasible green process route for the production of (S)-equol.

The Contribution of Proton-Donor pKa on Reactivity Profiles of [FeFe]-hydrogenases

The [FeFe]-hydrogenases are enzymes that catalyze the reversible activation of H2 coupled to the reduction-oxidation of electron carriers. Members of the different taxonomic groups of [FeFe]-hydrogenases display a wide range of preference, or bias, for H2 oxidation or H2 production reactions, despite sharing a common catalytic cofactor, or H-cluster. Identifying the properties that control reactivity remains an active area of investigation, and models have emerged that include diversity in the catalytic site coordination environments and compositions of electron transfer chains. The kinetics of proton-coupled electron transfer at the H-cluster might be expected to be a point of control of reactivity. To test this hypothesis, systematic changes were made to the conserved cysteine residue that functions in proton exchange with the H-cluster in the three model enzymes: CaI, CpII, and CrHydA1. CaI and CpII both employ electron transfer accessory clusters but differ in bias, whereas CrHydA1 lacks accessory clusters having only the H-cluster. Changing from cysteine to either serine (more basic) or aspartate (more acidic) modifies the sidechain pKa and thus the barrier for the proton exchange step. The reaction rates for H2 oxidation or H2 evolution were surveyed and measured for model [FeFe]-hydrogenases, and the results show that the initial proton-transfer step in [FeFe]-hydrogenase is tightly coupled to the control of reactivity; a change from cysteine to more basic serine favored H2 oxidation in all enzymes, whereas a change to more acidic aspartate caused a shift in preference toward H2 evolution. Overall, the changes in reactivity profiles were profound, spanning 105 in ratio of the H2 oxidation-to-H2 evolution rates. The fact that the change in reactivity follows a common trend implies that the effect of changing the proton-transfer residue pKa may also be framed as an effect on the scaling relationship between the H-cluster di(thiolmethyl)amine (DTMA) ligand pKa and E m values of the H-cluster. Experimental observations that support this relationship, and how it relates to catalytic function in [FeFe]-hydrogenases, are discussed.

Investigating the role of the strong field ligands in [FeFe] hydrogenase: spectroscopic and functional characterization of a semi-synthetic mono-cyanide active site

Artificial maturation of hydrogenases provides a path towards generating new semi-synthetic enzymes with novel catalytic properties. Here enzymes featuring a synthetic asymmetric mono-cyanide cofactor have been prepared using two different hydrogenase scaffolds. Their structure and reactivity was investigated in order to elucidate the design rationale behind the native di-cyanide cofactor, and by extension the second coordination sphere of the active-site pocket. Surprisingly, the choice of host enzyme was found to have a dramatic impact on reactivity. Moreover, the study shows that synthetic manipulations of the active-site can significantly increase inhibitor tolerance, as compared to native [FeFe] hydrogenase, while retaining the enzyme's native capacity for reversible catalysis.

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.

Trapping an Oxidized and Protonated Intermediate of the [FeFe]-Hydrogenase Cofactor under Mildly Reducing Conditions

The H-cluster is the catalytic cofactor of [FeFe]-hydrogenase, a metalloenzyme that catalyzes the formation of dihydrogen (H₂). The catalytic diiron site of the H-cluster carries two cyanide and three carbon monoxide ligands, making it an excellent target for IR spectroscopy. In previous work, we identified an oxidized and protonated H-cluster species, whose IR signature differs from that of the oxidized resting state (Hox) by a small but distinct shift to higher frequencies. This "blue shift" was explained by a protonation at the [4Fe-4S] subcomplex of the H-cluster. The novel species, denoted HoxH, was preferentially accumulated at low pH and in the presence of the exogenous reductant sodium dithionite (NaDT). When HoxH was reacted with H₂, the hydride state (Hhyd) was formed, a key intermediate of [FeFe]-hydrogenase turnover. A recent publication revisited our protocol for the accumulation of HoxH in wild-type [FeFe]-hydrogenase, concluding that inhibition by NaDT decay products rather than cofactor protonation causes the spectroscopic "blue shift". Here, we demonstrate that HoxH formation does not require the presence of NaDT (or its decay products), but accumulates also with the milder reductants tris(2-carboxyethyl)phosphine, dithiothreitol, or ascorbic acid, in particular at low pH. Our data consistently suggest that HoxH is accumulated when deprotonation of the H-cluster is impaired, thereby preventing the regain of the oxidized resting state Hox in the catalytic cycle.

Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines

Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by [FeFe]-hydrogenases, which contain a pendant amine positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base. These 1,5-diaza-3,7-diphosphacyclooctanes, now often referred to as "P₂N₂" ligands, have profound effects on the reactivity of many catalysts. The resulting [Ni(P^R₂N^R'₂)₂]²⁺ complexes are electrocatalysts for both the oxidation and production of H₂. Achieving the optimal benefit of the pendant amine requires that it has suitable basicity and is properly positioned relative to the metal center. In addition to the catalytic efficacy demonstrated with [Ni(P^R₂N^R'₂)₂]²⁺ complexes for the oxidation and production of H₂, catalysts with diphosphine ligands containing pendant amines have also been demonstrated for several metals for many different reactions, both in solution and immobilized on surfaces. The impact of pendant amines in catalyst design continues to expand.

Reversible down-regulation and up-regulation of catalytic activity of poly(N-isopropylacrylamide)-anchored gold nanoparticles

Regulating catalytic activity plays an important role in further optimizing and developing multifunctional catalysts with high selectivity and high activity. Reversible dual regulation of catalytic activity has always been a challenging task. Here, we prepared poly(N-isopropylacrylamide)-anchored gold nanoparticles (AuNP@CDs-Azo-PNIPAM) through host-guest interaction of cyclodextrin capped gold nanoparticles (AuNP@CDs) and azobenzene-terminated poly(N-isopropylacrylamide) (Azo-PNIPAM). Azo-PNIPAM as thermal and light responsive ligand allows reversible dual regulation of catalytic activity. When the temperature is higher than the lowest critical solution temperature, the PNIPAM chain shrinks rapidly, increasing the steric hindrance around AuNPs and reducing the catalytic activity. Under ultraviolet light irradiation,cis-azobenzene disassembles from cyclodextrin and the number of surface active sites of AuNPs increases, which improves the catalytic activity. The reaction rate of UV irradiation is almost 1.3 times that of visible light irradiation. This work provides a simple and effective strategy for the construction of reversible catalysts.

Electrochemical Studies of CO2 -Reducing Metalloenzymes

Only two enzymes are capable of directly reducing CO₂ : CO dehydrogenase, which produces CO at a [NiFe₄ S₄ ] active site, and formate dehydrogenase, which produces formate at a mononuclear W or Mo active site. Both metalloenzymes are very rapid, energy-efficient and specific in terms of product. They have been connected to electrodes with two different objectives. A series of studies used protein film electrochemistry to learn about different aspects of the mechanism of these enzymes (reactivity with substrates, inhibitors…). Another series focused on taking advantage of the catalytic performance of these enzymes to build biotechnological devices, from CO₂ -reducing electrodes to full photochemical devices performing artificial photosynthesis. Here, we review all these works.

Reversible or Irreversible Catalysis of H+/H2 Conversion by FeFe Hydrogenases

Studies of molecular catalysts traditionally aim at understanding how a certain mechanism allows the reaction to be fast. A distinct question, which has only recently received attention in the case of bidirectional molecular catalysts, is how much thermodynamic driving force is required to achieve fast catalysis in either direction of the reaction. "Reversible" catalysts are bidirectional catalysts that work either way in response to even a small departure from equilibrium and thus do not waste input free energy as heat; conversely, "irreversible" catalysts require a large driving force to proceed at an appreciable rate [Fourmond et al. Nat. Rev. Chem. 2021, 5, 348-360]. Numerous mechanistic rationales for these contrasting behaviors have been proposed. To understand the determinants of catalytic (ir)reversibility, we examined the steady-state, direct electron transfer voltammetry of a particular FeFe hydrogenase, from Thermoanaerobacter mathranii, which is very unusual in that it irreversibly catalyzes H₂ oxidation and production: a large overpotential is required for the reaction to proceed in either direction [Land et al. Chem. Sci. 2020, 11, 12789-12801]. In contrast to previous hypotheses, we demonstrate that in this particular enzyme catalytic irreversibility can be explained without invoking slow interfacial electron transfer or variations in the mechanism: the observed kinetics is fully consistent with the same catalytic pathway being used in both directions of the reaction.

Inhibition of [FeFe]-hydrogenase by formaldehyde: proposed mechanism and reactivity of FeFe alkyl complexes

The mechanism for inhibition of [FeFe]-hydrogenases by formaldehyde is examined with model complexes. Key findings: (i) CH2 donated by formaldehyde covalently link Fe and the amine cofactor, blocking the active site and (ii) the resulting Fe-alkyl is a versatile electrophilic alkylating agent. Solutions of Fe2[(μ-SCH2)2NH](CO)4(PMe3)2 (1) react with a mixture of HBF4 and CH2O to give three isomers of [Fe2[(μ-SCH2)2NCH2](CO)4(PMe3)2]+ ([2]+). X-ray crystallography verified the NCH2Fe linkage to an octahedral Fe(ii) site. Although [2]+ is stereochemically rigid on the NMR timescale, spin-saturation transfer experiments implicate reversible dissociation of the Fe-CH2 bond, allowing interchange of all three diastereoisomers. Using 13CH2O, the methylenation begins with formation of [Fe2[(μ-SCH2)2N13CH2OH](CO)4(PMe3)2]+. Protonation converts this hydroxymethyl derivative to [2]+, concomitant with 13C-labelling of all three methylene groups. The Fe-CH2N bond in [2]+ is electrophilic: PPh3, hydroxide, and hydride give, respectively, the phosphonium [Fe2[(μ-SCH2)2NCH2PPh3](CO)4(PMe3)2]+, 1, and the methylamine Fe2[(μ-SCH2)2NCH3](CO)4(PMe3)2. The reaction of [Fe2[(μ-SCH2)2NH](CN)2(CO)4]2- with CH2O/HBF4 gave [Fe2[(μ-SCH2)2NCH2CN](CN)(CO)5]- ([4]-), the result of reductive elimination from [Fe2[(μ-SCH2)2NCH2](CN)2(CO)4]-. The phosphine derivative [Fe2[(μ-SCH2)2NCH2CN](CN)(CO)4(PPh3)]- ([5]-) was characterized crystallographically.

Voltage-Driven Molecular Catalysis of Electrochemical Reactions

Heterogeneous electrocatalysis and molecular redox catalysis have developed over several decades as two distinct ways to facilitate charge-transfer processes essential for energy conversion and storage. Whereas electrocatalytic reactions are driven by the applied voltage, molecular catalytic processes are driven by the difference between standard potentials of the catalyst and the reactant. Here, we demonstrate that the rate of electron transfer between a dissolved reactant and a molecular catalyst immobilized directly on the surface of a carbon nanoelectrode is governed by combination of chemical driving force and electrostatic potential drop across the double layer. DFT calculations show that varying the applied voltage alters the potential drop between the surface-bound and dissolved redox species. These results suggest a new route for designing next-generation hybrid molecular/electrocatalysts.