The catalytic activities are dependent on the size of the micelles i.e. on the W 0
parameter. Consequently, the optimally sized micelles allow the achievement of maximal
kcat values. A constant value of W 0 corresponds to a distribution of various sizes of
micelles, which transforms the kcat by an average value (Kabanov et al., 1988). At lower
water content the enzymes often present a lower kinetic affinity for the substrate
(Barbaric and Luisi, 1981). When water-insoluble substrates are used, these substrates’
molecules are localised in the organic phase (oil phase) and to a great extent within the
micellar phase.
Distribution ModelsThe absence of specific and readily feasible techniques to determine directly the chemical
species in contact with the enzyme imply different kinetic treatments for enzymes in
different microenvironments. The models reported in the literature have the common
objective of explaining the kinetic behavior of biocatalysts but they have very different
approaches, including the distribution of enzymes, water, substrates, etc.
Eicke et al. (1976), proposed a model for solubilisátes with limited solubility in
isooctane that is based on the collision of two micelles accomplished by a deformation
during the compression phase. The compression of surfactant molecules induces an
opening-channel through which each of the molecules of solubilisátes has to diffuse from
one micelle to the other.
Maestro and Walde (1992) developed a simple diffusion model to describe the ( -
chymotrypsin activity in reversed micelles. The authors considered two consecutive
diffusion steps: the intermicellar diffusion (between micelles containing enzyme and
micelles with substrate) and the intramicellar diffusion. However, this theoretical
approach presented very low correlations with experimental data and did not consider the
substrate partitioning.
Verhaert et al. (1990) combined features from two approaches to conceive their
model. The first one is the diffusion and collision of substrate-filled and enzyme-filled
reverse micelles. Sequentially, the exchange of components allows the enzymatic
conversion in the reverse micelles. The authors raised the problem of real substrate
concentrations for microencapsulated enzymes and distinguished between overall and
water phase substrate concentrations to explain enzyme kinetics based on the pseudo
micellar phases composition. In addition, they considered an exchange rate of substrates
between filled and non-filled micelles. The deviations of kinetic parameters determined
in reversed micelles from the aqueous phase values were shown to depend strongly on the
concentration of reversed micelles, the intermicellar exchange rates and the volume
fraction of water.
Other models were also developed to explain the differences verified for reversed
micellar systems kinetics based on size distribution of reversed micelles (Kabanov et al.,
1988), partitioning of the enzyme between sub-phases (Bru et al., 1989; Yang and
Russel, 1995) and partitioning of substrates among micellar constituents (Khmelnitsky et
al., 1990b). These models helped to explain the catalysis in reversed micelles although
only from the theoretical point of view since they do not advance to a quantitative
definition of concentrations.
Reversed micellar bioreaction systems 203