Publication
551
J. Electroanal.
Chem., 521 (1-2),8 -15, 2002
DOI: 10.1016/S0022-0728(02)00658-7 |
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Kinetic
control by the substrate and the cosubstrate
in electrochemically monitored redox enzymatic
immobilized systems. Catalytic responses in
cyclic voltammetry and steady state techniques
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Benoît
Limoges, Jacques Moiroux and Jean-Michel Savéant
Laboratoire d'Electrochimie Moléculaire
de l'Université Denis Diderot (Paris 7),
UMR CNRS 7591, 2 place Jussieu, Tour 44–45,
75251 Paris Cedex 05, France
The catalytic response of an immobilized redox
enzyme connected to the electrode by a freely
diffusing mediator (cosubstrate) may be kinetically
controlled by the substrate and/or the cosubstrate.
How the electrochemical responses are related
to the rate constants, to the amount of enzyme
on the electrode, to the substrate and cosubstrate
concentration and to the mass transport parameter
is systematically analyzed in the framework of
cyclic voltammetry and steady state techniques
(SST). Because of its frequent occurrence in
practice, emphasis is put on the case of a fast
enzymatic process, as compared to the diffusion
of the cosubstrate, provision being made for
Michaelis–Menten behavior for both substrate
and cosubstrate. Within this framework, two situations
of particular interest are discussed, namely
the case of a negligible consumption of the substrate
in the enzyme coating and the opposite case where
the consumption of the substrate is so important
that its diffusion toward the electrode controls
the current. In the first case, plateau-shaped
responses, independent of scan rate, are obtained
in cyclic voltammetry, and likewise mass transport
independent waves, in SST. In the second case,
a curve exhibiting a sharp, discontinuous peak
is obtained in cyclic voltammetry with a peak
current proportional to the substrate concentration
and square root of the scan rate. A discontinuity
also appears in SST between the rising portion
of the wave and the plateau current while the
current is likewise proportional to substrate
concentration. The combination of these two regimes
accounts for a biphasic variation of the electrochemical
signal with the substrate concentration. The
relationships between the electrochemical responses
and the kinetic characteristics of the enzymatic
reaction form the bases of procedures for ascertaining
the mechanism and measuring the key rate constants.
In this connection, strategies for determining
Michaelis–Menten characteristics of both
the substrate and cosubstrate reactions are discussed. |