Fraser A. Armstrong

University of Oxford, Oxford, England, United Kingdom

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Publications (240)1852.01 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Despite extensive studies on [NiFe]-hydrogenases, the mechanism by which these enzymes produce and activate H2 so efficiently remains unclear. A well-known EPR-active state produced under H2 and known as Ni-C is assigned as a Ni(III)-Fe(II) species with a hydrido ligand in the bridging position between the two metals. It has long been known that low-temperature photolysis of Ni-C yields distinctive EPR-active states, collectively termed Ni-L, that are attributed to migration of the bridging-H species as a proton; however, Ni-L has mainly been regarded as an artifact with no mechanistic relevance. It is now demonstrated, based on EPR and infrared spectroscopic studies, that the Ni-C to Ni-L interconversion in Hydrogenase-1 (Hyd-1) from Escherichia coli is a pH-dependent process that proceeds readily in the dark-proton migration from Ni-C being favored as the pH is increased. The persistence of Ni-L in Hyd-1 must relate to unassigned differences in proton affinities of metal and adjacent amino acid sites, although the unusually high reduction potentials of the adjacent Fe-S centers in this O2-tolerant hydrogenase might also be a contributory factor, impeding elementary electron transfer off the [NiFe] site after proton departure. The results provide compelling evidence that Ni-L is a true, albeit elusive, catalytic intermediate of [NiFe]-hydrogenases.
    Journal of the American Chemical Society 06/2015; DOI:10.1021/jacs.5b03182 · 11.44 Impact Factor
  • Andreas Bachmeier, Fraser Armstrong
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    ABSTRACT: Metalloenzymes such as hydrogenases and carbon monoxide dehydrogenase can be attached to light-harvesting agents to produce informative photocatalytic systems of varying intricacy. Systematic studies yield important insight into mechanistic and design principles of artificial photosynthesis - one route to future renewable energy conversion, and the unconventional experiments reveal interesting new criteria for the catalytic performance of metals in biology. Recent advances are interpreted in terms of the importance of enzyme active centres that have evolved to perform fast and efficient catalysis using abundant elements, along with the ability of enzymes to trap photo-generated electrons by virtue of having receding, buried relay centres with low reorganisation energies. Copyright © 2015. Published by Elsevier Ltd.
    Current Opinion in Chemical Biology 04/2015; 25. DOI:10.1016/j.cbpa.2015.01.001 · 7.65 Impact Factor
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    ABSTRACT: Formaldehyde (HCHO), a strong electrophile and a rapid and reversible inhibitor of hydrogen production by [FeFe]-hydrogenases, is used to identify the point in the catalytic cycle at which a highly reactive metal-hydrido species is formed. Investigations of the reaction of Chlamydomonas reinhardtii [FeFe]-hydrogenase with formaldehyde using pulsed-EPR techniques including electron-nuclear double resonance spectroscopy establish that formaldehyde binds close to the active site. Density functional theory calculations support an inhibited super-reduced state having a short Fe-(13)C bond in the 2Fe subsite. The adduct forms when HCHO is available to compete with H(+) transfer to a vacant, nucleophilic Fe site: had H(+) transfer already occurred, the reaction of HCHO with the Fe-hydrido species would lead to methanol, release of which is not detected. Instead, Fe-bound formaldehyde is a metal-hydrido mimic, a locked, inhibited form analogous to that in which two electrons and only one proton have transferred to the H-cluster. The results provide strong support for a mechanism in which the fastest pathway for H2 evolution involves two consecutive proton transfer steps to the H-cluster following transfer of a second electron to the active site.
    Journal of the American Chemical Society 04/2015; 137(16). DOI:10.1021/ja513074m · 11.44 Impact Factor
  • From Molecules to Materials - Pathways to Artificial Photosynthesis, Edited by Elena A. Rozhkova, Katsuhiko Ariga, 01/2015: chapter 4: pages 99-123; Springer International Publishing., ISBN: 978-3-319-13799-5
  • Lang Xu, Fraser A. Armstrong
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    ABSTRACT: The performance characteristics of simple enzyme-based membrane-less hydrogen fuel cells running on non-explosive H2-rich air mixtures have been established using an adjustable test bed that allows multiple unit cells to operate in series or parallel. Recent advances with ‘3D’ electrodes constructed from compacted porous carbon loaded with hydrogenase (anode) and bilirubin oxidase (cathode) have been extended in order to scale up fuel cell power to useful levels. One result is an appealing ‘classroom’ demonstration of a model house containing small electronic devices powered by H2 mixed with a small amount of air. The 3D electrodes work by greatly increasing catalyst loading (at both the anode and cathode) and selectively restricting the access of O2 (relative to H2) to enzymes embedded in pores at the anode. The latter property raises the possibility of using standard hydrogenases that are not O2-tolerant: however, experiments with such an enzyme reveal good short-term performance due to restricted O2 access, but low long-term stability because the root cause of O2 sensitivity has not been addressed. Hydrogenases that are truly O2 tolerant must therefore remain the major focus of any future enzyme-based hydrogen fuel cell technology.
    RSC Advances 12/2014; 5(5). DOI:10.1039/C4RA13565B · 3.71 Impact Factor
  • [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 (BBA) - Bioenergetics 10/2014; 1847(2). DOI:10.1016/j.bbabio.2014.06.011 · 4.83 Impact Factor
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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. DOI:10.1021/ja506998b · 11.44 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. DOI:10.1021/ja507733j · 11.44 Impact Factor
  • [Show abstract] [Hide abstract]
    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 08/2014; 111(38). DOI:10.1073/pnas.1407927111 · 9.81 Impact Factor
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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; 136(29). DOI:10.1021/ja504942h · 11.44 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; 111(18). DOI:10.1073/pnas.1322393111 · 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; 16(24). DOI:10.1039/c3cp55230f · 4.20 Impact Factor
  • Source
    Mehmet Can, Fraser A Armstrong, Stephen W Ragsdale
    Chemical Reviews 02/2014; 114(8). DOI:10.1021/cr400461p · 45.66 Impact Factor
  • Bonnie J. Murphy, Frank Sargent, Fraser A. Armstrong
    Energy & Environmental Science 01/2014; 7(4):1426. DOI:10.1039/c3ee43652g · 15.49 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. DOI:10.1007/978-1-62703-794-5_6 · 1.29 Impact Factor
  • Vincent C-C Wang, Stephen W Ragsdale, Fraser A Armstrong
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    ABSTRACT: Carbon monoxide dehydrogenases (CODH) play an important role in utilizing carbon monoxide (CO) or carbon dioxide (CO2) in the metabolism of some microorganisms. Two distinctly different types of CODH are distinguished by the elements constituting the active site. A Mo-Cu containing CODH is found in some aerobic organisms, whereas a Ni-Fe containing CODH (henceforth simply Ni-CODH) is found in some anaerobes. Two members of the simplest class (IV) of Ni-CODH behave as efficient, reversible electrocatalysts of CO2/CO interconversion when adsorbed on a graphite electrode. Their intense electroactivity sets an important benchmark for the standard of performance at which synthetic molecular and material electrocatalysts comprised of suitably attired abundant first-row transition elements must be able to operate. Investigations of CODHs by protein film electrochemistry (PFE) reveal how the enzymes respond to the variable electrode potential that can drive CO2/CO interconversion in each direction, and identify the potential thresholds at which different small molecules, both substrates and inhibitors, enter or leave the catalytic cycle. Experiments carried out on a much larger (Class III) enzyme CODH/ACS, in which CODH is complexed tightly with acetyl-CoA synthase, show that some of these characteristics are retained, albeit with much slower rates of interfacial electron transfer, attributable to the difficulty in making good electronic contact at the electrode. The PFE results complement and clarify investigations made using spectroscopic investigations.
  • Source
  • Fraser A. Armstrong
    Interface focus: a theme supplement of Journal of the Royal Society interface 10/2013; 3(5). DOI:10.1098/rsfs.2013.0039 · 3.12 Impact Factor
  • Source
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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; 135(40). DOI:10.1021/ja4042675 · 11.44 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; 14(14). DOI:10.1002/cbic.201300270 · 3.06 Impact Factor

Publication Stats

8k Citations
1,852.01 Total Impact Points


  • 1982–2015
    • University of Oxford
      • • Inorganic Chemistry Laboratory
      • • Department of Chemistry
      Oxford, England, United Kingdom
  • 2011
    • University of Cambridge
      • Department of Chemistry
      Cambridge, ENG, United Kingdom
  • 2006–2011
    • Humboldt-Universität zu Berlin
      • Department of Biology
      Berlin, Land Berlin, Germany
    • Oxford College
      Oxford, Ohio, United States
  • 1991–2010
    • University of California, Irvine
      • • Department of Chemistry
      • • Department of Molecular Biology and Biochemistry
      Irvine, California, United States
  • 2000–2007
    • Leiden University
      Leyden, South Holland, Netherlands
    • The University of Edinburgh
      • Institute of Cell Biology
      Edinburgh, Scotland, United Kingdom
  • 2004
    • University of Amsterdam
      Amsterdamo, North Holland, Netherlands
  • 2003
    • Boston University
      Boston, Massachusetts, United States
    • The Scripps Research Institute
      • Department of Cell and Molecular Biology
      لا هویا, California, United States
  • 2002
    • The University of Arizona
      • Department of Chemistry and Biochemistry (College of Science)
      Tucson, Arizona, 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
  • 1991–1993
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France