ISBN: 978-83-60043-10-3 - eurobic9

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Eurobic9, 2-6 September, 2008, Wrocław, Poland F. Armstrong KL9. Oxygen, the Trojan Horse for Hydrogenases Department of Chemistry, Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK e-mail: Fraser.armstrong@chem.ox.ac.uk, Telephone +44 1865 272647/287182, Fax +44 1865 287182 Hydrogenases are attracting great interest because they offer inspiration and even practical solutions to developing a hydrogen economy that would help remove the world’s dependence on fossil fuels. The binuclear active sites of [NiFe]- or [FeFe]-hydrogenases have activities that compare with platinum, that is, they behave as essentially reversible electrocatalysts requiring only the smallest of overpotentials. Future technologies ranging from ‘renewable’ hydrogen production, based either on ‘biohydrogen’ or electrolysis, to efficient hydrogen oxidation in low-temperature fuel cells without platinum catalysts –may be based upon microorganisms containing these enzymes or upon man-made catalysts that are functional analogues of their active sites. These fragile catalytic centres, all of which contain low-spin Fe coordinated mainly by non-protein ligands CO and CN � , are deeply buried within the protein. They resemble organometallic compounds and may be among the earliest catalysts; hence it is not surprising that they are inactivated by O2. Like H2 and the competitive inhibitor CO, O2 enters the protein and coordinates to the metal centres by synergic bonding involving a combination of σ-donor and π-back donation; however, a major difference is that O2 withdraws rather than provides electrons and in the process is likely to attack the active site through reactive oxygen intermediates. We have defined ‘O2-tolerance as the capability of a hydrogenase to function in the presence of O2 (not merely to resist degradation on the bench) and we are investigating three different ways in which hydrogenases can (and may have evolved to survive) the attack of O2 – the ‘Trojan Horse’. Firstly, and most widely acknowledged, is that O2 might be prevented from reaching the active site because it is excluded from ‘gas channels’ through the protein. Secondly, the rate and extent of damage caused by O2 reduction may be minimized by certain electronic properties of the active site. Thirdly, the damage from O2 attack may be repaired extremely rapidly, thus allowing the enzyme to resume normal catalysis. All of these options are being investigated by protein film voltammetry and some interesting new results are surfacing. Recent references - Investigating and Exploiting the Electrocatalytic Properties of Hydrogenases K. A. Vincent, A. Parkin and F. A. Armstrong. Chemical Reviews. <strong>10</strong>7, 4366-4413 (2007). - Enzymatic Oxidation of H2 in Atmospheric O2: The Electrochemistry of Energy Generation from Trace H2 by - Aerobic Microorganisms J. A. Cracknell, K. A. Vincent, M. Ludwig, O. Lenz, B. Friedrich and F. A. Armstrong. J. Amer. Chem. Soc. 130, 424-425 (2008). - Hydrogen Production under Aerobic Conditions by Membrane-bound Hydrogenases from Ralstonia species. G. Goldet, A.Wait, J. A. Cracknell, B. Friedrich, M. Ludwig, O. Lenz and F. A. Armstrong. J. Amer. Chem. Soc. 130, in press (2008). - The difference a Se makes? Oxygen-tolerant hydrogen production by the [NiFeSe]-hydrogenase from Desulfomicrobium baculatum. A. Parkin, G. Goldet, C. Cavazza, J. C. Fontecilla-Camps and F. A. Armstrong. J. Amer. Chem. Soc. 130, in press (2008). _____________________________________________________________________ 28
Eurobic9, 2-6 September, 2008, Wrocław, Poland KL<strong>10</strong>. Preparing Chiral Vanadium Catalysts based on the Vanadium Haloperoxidases C.J. Schneider a , G. Zampella b , L. De Gioia b , V.L. Pecoraro a a Department of Chemistry, University of Michigan, Ann Arbor, MI USA email: vlpec@umich.edu b Universita degli Studi di Milano Biocca, Italia Vanadium Haloperoxidases catalyze the halogenation of organic substrates using hydrogen peroxide and halides. In addition, these vanadium containing enzymes can stereoselectively convert sulfides to sulfoxides. The active center contains a single V(V) which is thought to bind peroxide prior to oxidation of halide or sulfur. Protonation of this vanadium peroxo species has been proposed as a critical step in the catalytic mechanism. We will describe experiments using small molecule models (VO(O2)Heida and VO(O2)noreida) for the reactivity of the Vanadium Haloperoxidases that have been useful in developing the presently accepted mechanism for this enzyme. Furthermore, we will present recent studies using DFT calculations that provide insight into the site of active site protonation and the role of active site residue positioning in the catalytic process. _____________________________________________________________________ 29
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ISBN: 978-83-60043-10-3 - eurobic9

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