Enzyme catalysis in uni- and bi-continuous microemulsions: Dependence of kinetics on substrate partitioning

The kinetics of enzymatic conversion in a microemulsion have been investigated. Racemic 3-methylcyclohexanone was oxidized by horse-liver alcohol dehydrogenase (HLADH, E.C. 1.1.1.1.) using a coupled substrate-coenzyme regenerating cycle in a sodium bis-(2-ethylhexyl) sulphosuccinate (AOT)-isooctane-buffer microemulsion. Initial enzyme activity was measured as a function of the oil volume fraction in the range 0 ≤ Φ ≤ 0.83 for a constant surfactant concentration. The change in composition is followed by a change in microstructure from oil-in-water (O/W) to water-in-oil (W/O) via a bicontinuous structure as determined by self-diffusion measurements using the pulsed-gradient NMR technique. The variation of the initial rate with composition is well described by modifying the rate equation, valid in pure buffer, by simply taking into account the partitioning of the substrates between the polar and apolar microdomains in the structured solvent. Also the enzyme stability was investigated at various compositions of the microemulsion. The stability was found to increase with increasing Φ.