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. & 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 pseudo-continuous phase [Luisi, Giomini, Pileni & 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 theta (= % water, v/v), were investigated as modulators of enzymic activity. Three basic patterns of modulation by 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 date, such as superinhibition. Dependence of catalytic activity on theta was only stated at omega 0 values around a critical value, which coincides with the appearance of free water.

Influence of the hydrophobicity of lipase isoenzymes from Candida rugosa on its hydrolytic activity in reverse micelles

“…The profile was smoother for the longest chain-length (pNPL) ester; the most hydrophobic substrate. Our results could be fitted to a model in which an enzyme presents its highest activity in the bound water (water involved in the solvation of the surfactant headgroups and their counterions) or in the surfactant apolar tail domains of the micellar system, according to Bru et al [28]. This behaviour is consistent with the intrinsic nature of lipases, which act at the interphases.…”

Positive cooperativity in substrate binding of human prostatic acid phosphatase entrapped in AOT–isooctane–water reverse micelles

“…The facts that proteins entrapped within reverse micelles retain activity make them good model systems for understanding the properties of proteins in a cell-like environment [17][18][19][20][21][22][23][24][25][26][27][28]. The reverse micellar system based upon the use of anionic surfactant AOT (sodium bis-(2-ethylhexyl)-sulfosuccinate) is well known [17][18][19][20][21][22][23][24][25][26][27][28]. AOT-isooctane-water micelles are characterized by a relatively narrow distribution of their dimensions.…”

Reverse micelles in organic solvents: a medium for the biotechnological use of extreme halophilic enzymes at low salt concentration

“…The structure of water molecules within the micelle is determined by micellar size; if w 0 increases, the amount of "free" water (water molecules not bound to the surfactant polar heads), relative to the amount of "bound" water, also increases (Luisi et al 1988). Therefore, micellar size may affect the local aqueous environment, which in turn may affect the catalytic properties of the enzyme in that environment (Bru et al 1989). Other studies suggest that water has a uniform structure within the interior of the reverse micelle (Novaki and El Seoud 1998).…”