Characterization of supercharged cellulase activity and stability in ionic liquids

Abstract

Ionic liquids (ILs) have many potential benefits in biochemical processes, including improved substrate or product solubility, increased enzyme selectivity, and higher yield. Varying ion substituents allow ILs to be tuned or optimized to achieve a specific goal. Unfortunately, optimization based on a single design criterion can have undesirable side effects on other process components. For example, hydrophilic ILs capable of efficiently dissolving biomass often inhibit enzymatic activity during hydrolysis. A panel of nine different aqueous ILs was selected for this study to systematically assess which factors contribute to the loss of enzyme activity. The activity of endoglucanase E1 from Acidothermus cellulolyticus steadily decreased in higher concentrations of ILs, especially in the presence of the common cellulose dissolving solvent 1-ethyl-3-methylimidazolium acetate. The impact of most other ILs could be rationalized via the Hofmeister series. Enzyme behavior was further probed by rationally modifying the surface charge of E1. Variants were computationally designed to have positively or negatively charged surfaces and assessed for activity in ILs. Surprisingly, positive supercharging maintained wild type activity levels in ILs, while negative supercharging drastically reduced activity. Discrepancy between stability and activity measurements for some ILs indicated active site inhibition or other unique inactivation mechanisms might be crucial components to consider in future studies.