Fraser A Armstrong

University of Oxford, Oxford, England, United Kingdom

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Publications (208)1250.26 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The class of [NiFe]-hydrogenases comprises oxygen-sensitive periplasmic (PH) and oxygen-tolerant membrane-bound (MBH) enzymes. For three PHs and four MBHs from six bacterial species, structural features of the nickel-iron active site of hydrogen turnover and of the iron-sulfur clusters functioning in electron transfer were determined using X-ray absorption spectroscopy (XAS). Fe-XAS indicated surplus oxidized iron and a lower number of ~2.7Å Fe-Fe distances plus additional shorter and longer distances in the oxidized MBHs compared to the oxidized PHs. This supported a double-oxidized and modified proximal FeS cluster in all MBHs with an apparent trimer-plus-monomer arrangement of its four iron atoms, in agreement with crystal data showing a [4Fe3S] cluster instead of a [4Fe4S] cubane as in the PHs. Ni-XAS indicated coordination of the nickel by the thiol group sulfurs of four conserved cysteines and at least one iron-oxygen bond in both MBH and PH proteins. Structural differences of the oxidized inactive [NiFe] cofactor of MBHs in the Ni-B state compared to PHs in the Ni-A state included a ~0.05Å longer Ni-O bond, a two times larger spread of the Ni-S bond lengths, and a ~0.1Å shorter Ni-Fe distance. The modified proximal [4Fe3S] cluster, weaker binding of the Ni-Fe bridging oxygen species, and an altered localization of reduced oxygen species at the active site may each contribute to O2 tolerance.
    Biochimica et biophysica acta. 10/2014;
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    ABSTRACT: We present a photocathode assembly for the visible-light-driven selective reduction of CO2 to CO at potentials below the thermodynamic equilibrium in the dark. The photoelectrode comprises a porous p-type semiconducting NiO electrode modified with the visible-light-responsive organic dye P1 and the reversible CO2 cycling enzyme carbon monoxide dehydrogenase. The direct electrochemistry of the enzymatic electrocatalyst on NiO shows that in the dark the electrocatalytic behavior is rectified toward CO oxidation, with the reactivity being governed by the carrier availability at the semiconductor-catalyst interface.
    Journal of the American Chemical Society 09/2014; 136(39):13518. · 10.68 Impact Factor
  • Andreas Bachmeier, Bonnie J. Murphy, Fraser A. Armstrong
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    ABSTRACT: The enzyme flavocytochrome c3 (fcc3), which catalyzes hydrogenation across a C═C double bond (fumarate to succinate), is used to carry out the fuel-forming reaction in an artificial photosynthesis system. When immobilized on dye-sensitized TiO2 nanoparticles, fcc3 catalyzes visible-light-driven succinate production in aqueous suspension. Solar-to-chemical conversion using neutral water as the oxidant is achieved with a photoelectrochemical cell comprising an fcc3-modified indium tin oxide cathode linked to a cobalt phosphate-modified BiVO4 photoanode. The results reinforce new directions in the area of artificial photosynthesis, in particular for solar-energy-driven synthesis of organic chemicals and commodities, moving away from simple fuels as target molecules.
    Journal of the American Chemical Society 09/2014; 136(37):12876. · 10.68 Impact Factor
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    ABSTRACT: Under anaerobic conditions, Escherichia coli can carry out a mixed-acid fermentation that ultimately produces molecular hydrogen. The enzyme directly responsible for hydrogen production is the membrane-bound formate hydrogenlyase (FHL) complex, which links formate oxidation to proton reduction and has evolutionary links to Complex I, the NADH:quinone oxidoreductase. Although the genetics, maturation, and some biochemistry of FHL are understood, the protein complex has never been isolated in an intact form to allow biochemical analysis. In this work, genetic tools are reported that allow the facile isolation of FHL in a single chromatographic step. The core complex is shown to comprise HycE (a [NiFe] hydrogenase component termed Hyd-3), FdhF (the molybdenum-dependent formate dehydrogenase-H), and three iron-sulfur proteins: HycB, HycF, and HycG. A proportion of this core complex remains associated with HycC and HycD, which are polytopic integral membrane proteins believed to anchor the core complex to the cytoplasmic side of the membrane. As isolated, the FHL complex retains formate hydrogenlyase activity in vitro. Protein film electrochemistry experiments on Hyd-3 demonstrate that it has a unique ability among [NiFe] hydrogenases to catalyze production of H2 even at high partial pressures of H2. Understanding and harnessing the activity of the FHL complex is critical to advancing future biohydrogen research efforts.
    Proceedings of the National Academy of Sciences of the United States of America. 08/2014;
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    ABSTRACT: Cyanide reacts rapidly with [NiFe]-hydrogenases (hydrogenase-1 and hydrogenase-2 from Escherichia coli) under mild oxidizing conditions, inhibiting the electrocatalytic oxidation of hydrogen as recorded by protein film electrochemistry. Electrochemical, EPR, and FTIR measurements show that the final enzyme product, formed within a second (even under 100% H2), is the resting state known as Ni-B, which contains a hydroxido-bridged species, Ni(III)-μ(OH)-Fe(II), at the active site. "Cyanide inhibition" is easily reversed because it is simply the reductive activation of Ni-B. This paper brings back into focus an observation originally made in the 1940s that cyanide inhibits microbial H2 oxidation and addresses the interesting mechanism by which cyanide promotes the formation of Ni-B. As a much stronger nucleophile than hydroxide, cyanide binds more rapidly and promotes oxidation of Ni(II) to Ni(III); however, it is quickly replaced by hydroxide which is a far superior bridging ligand.
    Journal of the American Chemical Society 07/2014; · 10.68 Impact Factor
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    ABSTRACT: An oxygen-tolerant respiratory [NiFe]-hydrogenase is proven to be a four-electron hydrogen/oxygen oxidoreductase, catalyzing the reaction 2 H2 + O2 = 2 H2O, equivalent to hydrogen combustion, over a sustained period without inactivating. At least 86% of the H2O produced by Escherichia coli hydrogenase-1 exposed to a mixture of 90% H2 and 10% O2 is accounted for by a direct four-electron pathway, whereas up to 14% arises from slower side reactions proceeding via superoxide and hydrogen peroxide. The direct pathway is assigned to O2 reduction at the [NiFe] active site, whereas the side reactions are an unavoidable consequence of the presence of low-potential relay centers that release electrons derived from H2 oxidation. The oxidase activity is too slow to be useful in removing O2 from the bacterial periplasm; instead, the four-electron reduction of molecular oxygen to harmless water ensures that the active site survives to catalyze sustained hydrogen oxidation.
    Proceedings of the National Academy of Sciences 04/2014; · 9.81 Impact Factor
  • Suzannah V Hexter, Thomas F Esterle, Fraser A Armstrong
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    ABSTRACT: Despite being so large, many enzymes are not only excellent electrocatalysts - making possible chemical transformations under almost reversible conditions - but they also facilitate our understanding of electrocatalysis by allowing complex processes to be dissected systematically. The electrocatalytic voltammograms obtained for enzymes attached to an electrode expose fundamental aspects of electrocatalysis that can be addressed in ways that are not available to conventional molecular or surface electrocatalysts. The roles of individual components, each characterisable by diffraction or spectroscopy, can be tested and optimised by genetic engineering. Importantly, unlike small-molecule electrocatalysts (RMM < 1000) that are structurally well-defined but invariably altered by being attached to a surface, the enzyme is a giant, multi-component assembly in which the active site is buried and relatively insensitive to the presence of the electrode and solvent interface. A central assertion is that for a given driving force (electrode potential) a true catalyst has no influence on the direction of the reaction; consequently, 'catalytic bias', i.e. the common observation that an enzyme or indeed any electrocatalyst operates preferentially in one direction, must arise from secondary effects beyond the elementary catalytic cycle. This Perspective highlights and extends a general model for electrocatalysis by surface-confined enzymes, and explains how two secondary effects control the bias: (i) the electrode potential at which electrons enter or leave the catalytic cycle; (ii) potential-dependent interconversions between states of the catalyst differing in catalytic activity due to changes in the composition and arrangements of atoms. The model, which is easily applied to enzymes that have been studied recently, highlights important considerations for understanding and developing surface-confined electrocatalysts.
    Physical Chemistry Chemical Physics 02/2014; · 4.20 Impact Factor
  • Mehmet Can, Fraser A Armstrong, Stephen W Ragsdale
    Chemical Reviews 02/2014; · 41.30 Impact Factor
  • Rhiannon M Evans, Fraser A Armstrong
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    ABSTRACT: Protein film electrochemistry is a technique which allows the direct control of redox-active enzymes, providing particularly detailed information on their catalytic properties. The enzyme is deposited onto a working electrode tip, and through control of the applied potential the enzyme activity is monitored as electrical current, allowing for direct study of inherent activity as electrons are transferred to and from the enzyme redox center(s). No mediators are used. Because the only enzyme present in the experiment is bound at the electrode surface, gaseous and liquid phase inhibitors can be introduced and removed whilst the enzyme remains in situ. Potential control means that kinetics and thermodynamics are explored simultaneously; the kinetics of a reaction can be studied as a function of potential. Steady-state catalytic rates are observed directly as current (for a given potential) and non-steady-state rates (such as interconversions between different forms of the enzyme) are observed from the change in current with time. The more active the enzyme, the higher the current and the better the signal-to-noise. In this chapter we outline the practical aspects of PFE for studying electroactive enzymes, using the Escherichia coli [NiFe]-hydrogenase 1 (Hyd-1) as an example.
    Methods in molecular biology (Clifton, N.J.) 01/2014; 1122:73-94. · 1.29 Impact Factor
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  • Fraser A. Armstrong
    Interface focus: a theme supplement of Journal of the Royal Society interface 10/2013; 3(5). · 2.21 Impact Factor
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    ABSTRACT: The most efficient catalysts for solar fuel production should operate close to reversible potentials, yet possess a bias for the fuel-forming direction. Protein film electrochemical studies of Ni-containing carbon monoxide dehydrogenase and [NiFeSe]-hydrogenase, each a reversible electrocatalyst, show that the electronic state of the electrode strongly biases the direction of electrocatalysis of CO2/CO and H(+)/H2 interconversions. Attached to graphite electrodes, these enzymes show high activities for both oxidation and reduction, but there is a marked shift in bias, in favor of CO2 or H(+) reduction, when the respective enzymes are attached instead to n-type semiconductor electrodes constructed from CdS and TiO2 nanoparticles. This catalytic rectification effect can arise for a reversible electrocatalyst attached to a semiconductor electrode if the electrode transforms between semiconductor- and metallic-like behavior across the same narrow potential range (<0.25 V) that the electrocatalytic current switches between oxidation and reduction.
    Journal of the American Chemical Society 09/2013; · 10.68 Impact Factor
  • Vincent C-C Wang, Stephen W Ragsdale, Fraser A Armstrong
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    ABSTRACT: Carbon monoxide dehydrogenases (CODHs) catalyse the reversible conversion between CO and CO2 . Several small molecules or ions are inhibitors and probes for different oxidation states of the unusual [Ni-4 Fe-4 S] cluster that forms the active site. The actions of these small probes on two enzymes-CODH ICh and CODH IICh -produced by Carboxydothermus hydrogenoformans have been studied by protein film voltammetry to compare their behaviour and to establish general characteristics. Whereas CODH ICh is, so far, the better studied of the two isozymes in terms of its electrocatalytic properties, it is CODH IICh that has been characterised by X-ray crystallography. The two isozymes, which share 58.3 % sequence identity and 73.9 % sequence similarity, show similar patterns of behaviour with regard to selective inhibition of CO2 reduction by CO (product) and cyanate, potent and selective inhibition of CO oxidation by cyanide, and the action of sulfide, which promotes oxidative inactivation of the enzyme. For both isozymes, rates of binding of substrate analogues CN(-) (for CO) and NCO(-) (for CO2 ) are orders of magnitude lower than turnover, a feature that is clearly revealed through hysteresis of cyclic voltammetry. Inhibition by CN(-) and CO is much stronger for CODH IICh than for CODH ICh, a property that has relevance for applying these enzymes as model catalysts in solar-driven CO2 reduction.
    ChemBioChem 09/2013; · 3.74 Impact Factor
  • Lang Xu, Fraser A. Armstrong
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    ABSTRACT: The unusual ability of O2-tolerant hydrogenases (H2ase) to produce electricity from a H2–air mixture (when used as the anodic electrocatalyst in a simple, membrane-less fuel cell) is investigated with the aim of establishing a strategy for raising volume power density, the measure of importance for miniature devices. Compacted mesoporous carbon electrodes provide a simple and inexpensive method for obtaining a large increase in productive enzyme loading, greatly increasing current densities and stability. Operated under a 78% H2–22% air mixture at 25 °C, typical current densities at a stationary H2ase anode and bilirubin oxidase cathode are 4.60 ± 0.32 mA cm−2 and 1.23 ± 0.12 mA cm−2, respectively. The power limitation due to low O2 concentration is addressed by re-proportioning the cathode/anode area ratio to balance the cathodic and anodic currents. At room temperature, the maximum power density of the fuel cell with an anode/cathode (A/C) ratio of 1:3 (1A/3C) is 1.67 ± 0.24 mW cm−2 (per anode area) or 0.42 ± 0.06 mW cm−2 (per total area). Good prospects for stability are demonstrated by the fact that 90% of the power is retained after continuously working for 24 h, and more than half of the power is retained after one week of non-stop operation. Using an even weaker O2 mixture (89% H2, 11% air) the 1A/3C cell gives over 0.8 mW cm−2 (anode) or 0.2 mW cm−2 (total electrode area). The results demonstrate the feasibility of membrane-less hydrogen–air fuel cells delivering volume power densities well in excess of 1 mW cm−3.
    Energy & Environmental Science 05/2013; · 11.65 Impact Factor
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    ABSTRACT: "Hyd-1", produced by Escherichia coli , exemplifies a special class of [NiFe]-hydrogenase that can sustain high catalytic H(2) oxidation activity in the presence of O(2)-an intruder that normally incapacitates the sulfur- and electron-rich active site. The mechanism of "O(2) tolerance" involves a critical role for the Fe-S clusters of the electron relay, which is to ensure the availability-for immediate transfer back to the active site-of all of the electrons required to reduce an attacking O(2) molecule completely to harmless H(2)O. The unique [4Fe-3S] cluster proximal to the active site is crucial because it can rapidly transfer two of the electrons needed. Here we investigate and establish the equally crucial role of the high potential medial [3Fe-4S] cluster, located >20 Å from the active site. A variant, P242C, in which the medial [3Fe-4S] cluster is replaced by a [4Fe-4S] cluster, is unable to sustain steady-state H(2) oxidation activity in 1% O(2). The [3Fe-4S] cluster is essential only for the first stage of complete O(2) reduction, ensuring the supply of all three electrons needed to form the oxidized inactive state "Ni-B" or "Ready" (Ni(III)-OH). Potentiometric titrations show that Ni-B is easily reduced (E(m) ≈ +0.1 V at pH 6.0); this final stage of the O(2)-tolerance mechanism regenerates active enzyme, effectively completing a competitive four-electron oxidase cycle and is fast regardless of alterations at the proximal or medial clusters. As a consequence of all these factors, the enzyme's response to O(2), viewed by its electrocatalytic activity in protein film electrochemistry (PFE) experiments, is merely to exhibit attenuated steady-state H(2) oxidation activity; thus, O(2) behaves like a reversible inhibitor rather than an agent that effectively causes irreversible inactivation. The data consolidate a rich picture of the versatile role of Fe-S clusters in electron relays and suggest that Hyd-1 can function as a proficient hydrogen oxidase.
    Journal of the American Chemical Society 02/2013; · 10.68 Impact Factor
  • Fraser A Armstrong
    Science 02/2013; 339(6120):658-9. · 31.20 Impact Factor
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    ABSTRACT: Several small molecules and ions, notably carbon monoxide, cyanide, cyanate, and hydrogen sulfide, are potent inhibitors of Ni-containing carbon monoxide dehydrogenases (Ni-CODH) that catalyze very rapid, efficient redox interconversions of CO(2) and CO. Protein film electrochemistry, which probes the dependence of steady-state catalytic rate over a wide potential range, reveals how these inhibitors target particular oxidation levels of Ni-CODH relating to intermediates (C(ox), C(red1), and C(red2)) that have been established for the active site. The following properties are thus established: (1) CO suppresses CO(2) reduction (CO is a product inhibitor), but its binding affinity decreases as the potential becomes more negative. (2) Cyanide totally inhibits CO oxidation, but its effect on CO(2) reduction is limited to a narrow potential region (between -0.5 and -0.6 V), below which CO(2) reduction activity is restored. (3) Cyanate is a strong inhibitor of CO(2) reduction but inhibits CO oxidation only within a narrow potential range just above the CO(2)/CO thermodynamic potential-EPR spectra confirm that cyanate binds selectively to C(red2). (4) Hydrogen sulfide (H(2)S/HS(-)) inhibits CO oxidation but not CO(2) reduction-the complex on/off characteristics are consistent with it binding at the same oxidation level as C(ox) and forming a modified version of this inactive state rather than reacting directly with C(red1). The results provide a new perspective on the properties of different catalytic intermediates of Ni-CODH-uniting and clarifying many previous investigations.
    Journal of the American Chemical Society 01/2013; · 10.68 Impact Factor
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    ABSTRACT: A policy case is made for a global project on artificial photosynthesis including its scientific justification, potential governance structure and funding mechanisms.
    Energy & Environmental Science 01/2013; · 11.65 Impact Factor
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    ABSTRACT: We report the 3.3 Å resolution structure of dimeric membrane-bound O(2)-tolerant hydrogenase 1 from Escherichia coli in a 2:1 complex with its physiological partner, cytochrome b. From the short distance between distal [Fe(4)S(4)] clusters, we predict rapid transfer of H(2)-derived electrons between hydrogenase heterodimers. Thus, under low O(2) levels, a functional active site in one heterodimer can reductively reactivate its O(2)-exposed counterpart in the other. Hydrogenase 1 is maximally expressed during fermentation, when electron acceptors are scarce. These conditions are achieved in the lower part of the host's intestinal tract when E. coli is soon to be excreted and undergo an anaerobic-to-aerobic metabolic transition. The apparent paradox of having an O(2)-tolerant hydrogenase expressed under anoxia makes sense if the enzyme functions to keep intracellular O(2) levels low by reducing it to water, protecting O(2)-sensitive enzymes during the transition. Cytochrome b's main role may be anchoring the hydrogenase to the membrane.
    Structure 12/2012; · 5.99 Impact Factor
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Publication Stats

3k Citations
1,250.26 Total Impact Points

Institutions

  • 1988–2014
    • University of Oxford
      • • Department of Chemistry
      • • Inorganic Chemistry Laboratory
      Oxford, England, United Kingdom
    • University of Sussex
      Brighton, England, United Kingdom
  • 2012
    • University Joseph Fourier - Grenoble 1
      • Institut de Biologie Structurale
      Grenoble, Rhone-Alpes, France
  • 2011
    • University of Cambridge
      • Department of Chemistry
      Cambridge, ENG, United Kingdom
  • 2008–2011
    • Humboldt-Universität zu Berlin
      • Department of Biology
      Berlin, Land Berlin, Germany
  • 2010
    • University of Illinois, Urbana-Champaign
      Urbana, Illinois, United States
  • 2009
    • Atomic Energy and Alternative Energies Commission
      • Jean-Pierre Ebel Institue of Structural Biology (IBS)
      Gif-sur-Yvette, Ile-de-France, France
  • 2002–2007
    • Leiden University
      Leyden, South Holland, Netherlands
  • 2006
    • Oxford College
      Oxford, Ohio, United States
    • Monash University (Australia)
      • School of Chemistry, Clayton
      Melbourne, Victoria, Australia
  • 1991–2002
    • University of California, Irvine
      • • Department of Molecular Biology and Biochemistry
      • • Department of Chemistry
      Irvine, CA, United States
  • 2000
    • University of Kansas
      Lawrence, Kansas, United States
  • 1989–2000
    • University of East Anglia
      • School of Biological Sciences
      Norwich, ENG, United Kingdom
  • 1993
    • University of Southern California
      • Department of Chemistry
      Los Angeles, California, United States