Evaluation of steady-state kinetic parameters for enzymes solubilized in water-in-oil microemulsion systems

Abstract

1. Equations are derived for the steady-state kinetics of substrate conversion by enzymes confined within the water-droplets of water-in-oil microemulsion systems. 2. Water-soluble substrates initially confined within droplets that do not contain enzyme are assumed to be converted into product only after they enter enzyme-containing droplets via the interdroplet exchange process. 3. Hyperbolic (Michaelis-Menten) kinetics are predicted when the substrate concentration is varied in microemulsions of fixed composition. Both k(cat.) and K(m) are predicted to be dependent on the size and concentration of the water-droplets in the microemulsion. 4. The predicted behavior is shown to be supported by published experimental data. A physical interpretation of the form of the rate equation is presented. 5. The rate equation for an oil-soluble substrate was derived assuming a pseudo-two-phase (oil + water) model for the microemulsion. Both k(cat.) and K(m) are shown to be independent of Φ(aq). K(m) is larger than the aqueous solution value by a factor approximately equal to the oil/water partition coefficient of the substrate. The validity of the rate equation is confirmed by published data.