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Figure 17.15 Reactions of the active site of [NiFe]-hydrogenases with small molecules that can
poison Pt active sites, showing likely structures for the product species. Inhibition of many [NiFe]-
hydrogenases by O
2
, CO, and sulfides is reversible, while hydrogenases from Ralstonia even
oxidize H
2
in the presence of these molecules (for a review, see Vincent et al. [2007]).
It has become apparent that some hydrogenases are not inhibited in this way by
O
2
. Certain microorganisms, such as the Knallgas bacteria Ralstonia, are suited to uti-
lizing trace H
2
as an energy source in aerobic environments (see Fig. 17.2), and their
hydrogenases must therefore remain catalytically active in air [Burgdorf et al., 2005].
Oxidation of H
2
in air presents interesting challenges: synthetic FeS-containing
clusters that are able to bind H
2
tend to be sensitive to both O
2
and moisture, while
conventional precious metal H
2
fuel cell catalysts are reactive to both H
2
and O
2
.
Electrochemical methods have been used to address these questions for the NiFe
membrane-bound hydrogenases from Ralstonia species that also adsorb readily on a
graphite electrode [Vincent et al., 2005] (for a review, see Vincent et al. [2007]).
Figure 17.16 shows the effect of increasing levels of O
2
on the activity of a mem-
brane-bound [NiFe]-hydrogenase from the heavy-metal-resistant strain R. metallidur-
ans CH34. Activity is measured as the electrocatalytic current response for a film of the
enzyme adsorbed directly on a PGE RDE at close to 1 bar H
2
. Introduction of 5 mbar O
2
has very little effect on the catalytic current, and even at 230 mbar O
2
, above the level in
air, substantial activity (about 50%) remains. The extremely high selectivity of this
enzyme for H
2
has been utilized in a fuel cell operating on low levels of H
2
in air
(see Section 17.4.3) [Vincent et al., 2006]. The origin of the O
2
tolerance remains
unclear: the Fourier transform infrared (FTIR) spectrum for the R. metallidurans
CH34 hydrogenase shows the same pattern of CO and CN
2
ligands at the active
site as that of O
2
-sensitive [NiFe]-hydrogenases, suggesting that O
2
-tolerance must
arise from subtle changes further away from the active site [Vincent et al., 2005].
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