Publication
550
J. Electroanal.
Chem., 521 (1-2),1 -7, 2002
DOI: 10.1016/S0022-0728(02)00657-5 |
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Kinetic
control by the substrate and/or the cosubstrate
in electrochemically monitored redox enzymatic
homogeneous systems. Catalytic responses in
cyclic voltammetry
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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 a homogeneous redox
enzymatic system 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 concentrations
of enzyme, substrate and cosubstrate and to the
scan rate, is systematically analyzed in the
framework of cyclic voltammetry. 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
reaction layer 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.
The plateau current is not proportional to the
substrate concentration but, at best, to its
square root. In the second case, a peak-shaped
curve is obtained with a peak current proportional
to the substrate concentration and square root
of the scan rate. 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. |