A theoretical study on the expression of enzymic activity in reverse micelles

Abstract

The present work deals with a theoretical model of catalysis by enzymes entrapped in reverse micelles. Three aspects of the enzyme-reverse-micelle system have been considered: structure, dynamics and enzyme distribution and catalysis in reverse micelles. A proposed structural model of reverse micelles [El Seoud (1984) in Reverse Micelles (Luisi, P. L. and Straub, B. E., eds.), p. 81, Plenum Press, New York] consists of three domains: surfactant apolar tails, bound water and free water. Dynamics are based on a dynamic equilibrium of association-dissociation that lead one to consider the dispersed polar phase as a pseudocontinuous phase [Luisi, Giomini, Pileni and Robinson (1988) Biochim, Biophys. Acta 947, 207-246]. Enzyme is distributed among the reverse-micelle domains and it expresses a catalytic constant for each one of them. The overall activity is calculated taking into account the volume in which enzyme is solubilized, and expressed as a function of the whole volume (V). The characteristic parameters of reverse micelles, .omega.0 (= [H2O]/[surfactant]) and .lambda. (= % water, v/v), were investigated as modulators of enzyme activity. Three basic patterns of modulation of .omega.0 were found depending on which domain the enzyme expressed the highest catalytic constant. Combinations of those basic patterns lead to other modulation types that can be found experimentally, such as superactivation. Other combinations predict behaviour patterns not described to data, such as superinhibition. Dependence of catalytic activity on .lambda. was only stated at .omega.0 values around a critical value, which coincides with the appearance of free water.