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

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. 107, 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).
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