Investigations of the Efficient Electrocatalytic Interconversions of Carbon Dioxide and Carbon Monoxide by Nickel-Containing Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 82

CODH II Ch is due, at least in part, to CO being a much stronger product inhibitor of
CO 2 reduction for that isozyme [ 19 ]. Features of the complex waveshape are
explained in terms of a model that takes into consideration the potential at which
electrons enter and leave the enzyme (via the D-cluster) and potential-dependent
interconversions between different catalytic states [ 47 ]. The potential of the center
that serves as the electron entry/exit site is an important determinant of catalytic bias,
and maximum reversibility is obtained when this value matches that of the reaction
being catalyzed. For CODH I Ch and CODH II Ch , where the obvious electron entry/
exit site is the D-cluster, it is noteworthy that the reduction potential of the D-cluster
has not been determined. However, a rationale for the observation of reversible
electrocatalysis in terms of the model shows that this value must lie close to -0.5 V.
At high potentials, both CODH I Ch and CODH II Ch become inactivated as the
active site transforms into the C ox state: this is seen particularly clearly for CODH
II Ch which shows a re-activation 'peak' at approximately -0.2 V [ 19 ]. Generally,
inactivation is slow and its rate is independent of potential (consistent with the rate
being determined by a chemical process rather than electron transfer) whereas
re-activation is fast and controlled by electron transfer, hence the sharp transition.
Before proceeding further, it is worthwhile drawing comparisons with
conventional methods of measuring activity. The standard procedure for determin-
ing the activity of CO oxidation by Ni-CODH is to measure the color change
that occurs when colorless oxidized methyl-viologen (MV 2+ ) is reduced to the
blue reduced MV + radical by electron transfer from CO-reduced CODH [ 48 - 51 ].
Measuring CO 2 reduction is much more problematic because it is necessary to use a
fast electron donor that is more reducing than CO: it is obviously difficult to
generate and stabilize such a donor in aqueous solution. Both V max and K M are
sensitive to the driving force (electrode potential) and this fact is easy to see from
cyclic voltammograms measured for a range of substrate concentrations [ 52 ].
4.2 The Electrocatalytic Voltammograms of Class III
Enzymes
Figure 5b shows the same kind of experiment carried out with a film of CODH/
ACS Mt on a PGE electrode. This enzyme is much larger than CODH I Ch or II Ch , the
two CODH subunits being flanked by two ACS subunits (Figure 2 ). In contrast to
the smaller enzymes, the cyclic voltammogram of CODH/ACS Mt shows no CO 2
reduction, even when no CO is present (100 % CO 2 ); instead the scan reveals a CO
oxidation current that increases slowly as the potential is raised, indicative of
sluggish electron transfer. The CO oxidation current at 0 V is about two orders of
magnitude smaller than that typically observed for CODH I Ch and CODH II Ch
under similar conditions. The sluggish electron transfer may reflect the much larger
size of CODH/ACS Mt as the flanking ACS subunits could prevent the D-cluster
from achieving such a close approach to the electrode surface. What is not clear
though is why CO 2 reduction is not observed, when it is fully expected in view of
the physiological function of the enzyme. One possibility is that electrons cannot