Environmental Engineering Reference
In-Depth Information
The fact that the catalytic activity is observed as current and measured across a
range of potentials means that each enzyme produces a spectrum-like fingerprint of
its characteristics, but PFE rarely allows determination of absolute steady-state
catalytic rates because the electroactive coverage is generally unknown. In order to
determine the electroactive coverage of an enzyme it is necessary to detect
so-called 'non-turnover' signals due to stoichiometric electron exchanges with
relay centers such as FeS clusters. These signals are peak-like in each scan direction
and the electroactive coverage is estimated from the areas under the peaks.
Turnover frequency
k
cat
is then calculated from the current (
i
) using the relationship
i
¼
k
cat
nFA
Γ
ð
3
Þ
where
n
is the number of electrons transferred,
F
is the Faraday constant,
A
is
the electrode area, and
Γ
is the electroactive coverage (equation
3
). As a guide, it
is considered very difficult
to observe a non-turnover signal
if
Γ
is below
10
12
moles cm
2
. Consequently, with
A
0.03 cm
2
, a current of 10
¼
μ
A corre-
sponds to a lower limit for
k
cat
in the region of 2000 s
1
.
The difficulty in determining absolute steady-state activities contrasts with the
ease by which transient reaction rates are measured at a constant potential. The rate
of a change in activity is measured from the time course of the current following a
perturbation such as potential step or injection of reagent. The amplitude of the
change is proportional to catalytic activity and electroactive coverage, but the rate
should be independent of these variables. The chronoamperometry experiment
examines, directly, the rate of change of rate - a measurement that is equivalent
to recording the second derivative of a normal kinetic plot of concentration
versus
time.
4 Carbon Monoxide Dehydrogenases as Electrocatalysts
Based upon the structure of CODH II
Ch
, it is clear from Figure
2
that the D-cluster
should be a feasible entry/exit point for electrons, and the enzyme should be
electroactive when attached to a rough electrode surface at which productive
orientations are statistically likely.
4.1 The Electrocatalytic Voltammograms of Class IV
Enzymes
As shown in Figure
5a
, a cyclic voltammogram of CODH I
Ch
adsorbed on a PGE
electrode displays reversible electrocatalysis: under a gas mixture of composition
50 % CO/50 % CO
2
, the trace cuts through the potential axis at the expected
equilibrium potential and a detectable net current in each direction is easily
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