Electrocatalysis for Fuel Cells at Enzyme-Modified Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 607

Figure 17.12 Direct electrocatalytic oxidation of D -fructose at a glassy carbon electrode
painted with a paste of Ketjen black particles modified with D -fructose dehydrogenase from a
Gluconobacter species. The enzyme incorporates an additional heme center allowing direct
electron transfer from the electrode to the flavin active site. Cyclic voltammograms were
recorded at a scan rate of 20 mV s 21 and at 25+2 8C and pH 5.0. Reproduced by permission
of the PCCP Owner Societies, from Kamitaka et al., 2007.
17.3.2 Oxidation of H 2
17.3.2.1 Electrocatalysis of H 2 Oxidation by Hydrogenases Hydrogenase
enzymes oxidize H 2 or reduce H þ at bimetallic iron (FeFe), or nickel and iron (NiFe)
catalytic centers that are deeply buried within the protein. While most transition-metal
electrodes catalyze H 2 oxidation via homolytic cleavage, the bimetallic active sites cat-
alyze heterolytic cleavage, stabilizing the resulting hydride intermediate [Cammack
et al., 2001]. The crystal structure of an [NiFe]-hydrogenase is represented in
Fig. 17.13, with the two protein subunits shown in thin gray lines and the metallo-
clusters shown as spheres [Fontecilla-Camps et al., 2007]. The NiFe active site is coor-
dinated to the protein by bridging and terminal cysteine residues, and it is not possible
to extract the intact catalytic center. As shown in Fig. 17.1, the iron is additionally
coordinated by CO and CN 2 ligands (assigned through infrared spectroscopic
studies), which are unusual in biology. These ligands presumably play a key role in
tuning the electronic and spin states of the metals. Fast electron tunneling through
the enzyme is facilitated by a chain of iron - sulfur clusters spaced at approximately
1 nm intervals and connecting the surface of the protein to the buried catalytic site.
Proton transport is accommodated by a network of charged amino acid side chains
and ordered water molecules, and hydrophobic channels, identified by the trapping
of xenon atoms within the protein when crystals are held under a pressure of xenon,
may be important in regulating gas access to the catalytic site (for a review, see
Fontecilla-Camps et al. [2007]).
Similar structural features are present in the [FeFe]-hydrogenases (FeS electron
relay centers, CO and CN-ligands at the active site, and channels for gas access),