(133e) Comparison of Ionic Liquid-Tolerant Glycoside Hydrolases Using Molecular Dynamics
Glycoside hydrolases are a class of enzyme that catalyzes the hydrolysis of bonds between glycosides such as cellulose and xylose. This reaction is particularly important for the conversion of biomass into value-added chemicals. One problem for the application of enzymes toward the degradation of cellulosic biomass is the recalcitrance of the highly structured cellulose structure and its entwinement with hemicellulose and lignin. Certain ionic liquids (IL), which are organic salts that are liquid at temperatures below 100° C, have the rare ability to solvate cellulosic biomass to help processes overcome this limitation. By discovering enzymes that can tolerate the presence of ILs, new processes can be developed that take advantage of the unique properties of ILs.
Datta et al.  studied the relative activity of three different cellulases in various concentrations of the IL 1-ethyl-3-methylimidazolium acetate [EMIM][OAC]. While the structures of the enzymes appear similar, their tolerance of IL varied greatly. The cellulase from Pyrococcus horikoshii was tolerant of up to 20% (v/v) IL, the cellulase from Thermatoga maritima was moderately tolerant of the same concentration, and the cellulase from Trichoderma viride was completely deactivated. It has been suggested that enzymes from halophilic and thermophilic organisms will tend to have adaptations that allow the enzyme to also tolerate organic salts such as ILs.
We have conducted molecular dynamics simulations to compare the structure and modes of motion for these enzymes in mixtures of IL and water. Preliminary results show the IL did not induce unfolding of the enzyme on the time scale of hundreds of nanoseconds. Therefore, some other factor is likely influencing the activity of these enzymes. Principle component analysis suggests that in each of the systems the presence of IL affects the slow modes of motion. The effects are varied, but they are more exaggerated for higher concentrations of IL and the more IL-intolerant enzymes. The dynamic fluctuations of the enzymes were measured by root mean square fluctuation. Higher concentrations of IL correspond to lower magnitude fluctuations. At higher temperatures with high concentrations of IL, we observe differences across the sequence in the relative magnitude of fluctuations as well. We tracked the evolution of solvent-protein interactions over the simulations. Areas of strong IL interaction tend to correspond with those that display changes in fluctuation. If these simulations are indicative of the behavior of other glycoside hydrolases in binary mixtures of IL and water, we can use molecular dynamics to predict retention or loss of activity of an enzyme in such solvents by applying tests such as those mentioned herein. We have gained molecular-level insight into the effects of ILs on these cellulases and which properties of the enzyme lead to its deactivation in IL.
 S. Datta, B. Holmes and J. I. Park, Green Chem. 2010, 12, 338.