Micellar solubilization of biopolymers in organic solvents. Activity and conformation of lysozyme in isooctane reverse micelles.

Abstract

Lysozyme solubilized in reverse micelles of bis(2-ethylhexyl) sodium sulfosuccinate in isooctane containing as little as 0.8% water (v/v) has been shown to be active. The amount of enzymatic activity depends on the water content and the pH of the reverse micellar system and under optimum conditions (i.e. pH 7.7 with 1.2% water) is 90% of the activity in water. The dependence of lysozyme activity on pH in reverse micelles is different than that in water, with the entire pH profile shifted 2 to 3 pH units higher in reverse micelles. Moreover, maximum enzyme activity is not found at the highest water contents tested (i.e. 1.6% and 2.0% water), but instead at 1.2% water. The Km for the N-acetylglucosamine oligomers used as substrate is 0.1 mM in reverse micelles (compared to 0.01 mM in water) when the concentration of substrate is referred to the water pools. Spectroscopic studies (CD, fluorescence, and UV absorbance) indicate that the conformation of lysozyme is significantly different in reverse micelles compared to water. In particular, CD studies indicate that the helical content of lysozyme changes from approximately 34% in water to approximately 48% in reverse micelles. Conformation and activity data are qualitatively correlated to the anomalous character of water in the reverse micelles. In particular, this may induce a stronger hydrogen bonding within the lysozyme which would in turn increase both the pKa of certain amino acid residues and the helical content of the macromolecule.