Structure, dynamics, and activity of xylanase solvated in binary mixtures of ionic liquid and water

We have discovered that a family 11 xylanase from Trichoderma longibrachiatum maintains significant activity in low concentrations of the ionic liquids (IL) 1-ethyl-3-methyl-imidazolium acetate ([EMIM][OAc]) or 1-ethyl-3-methyl-imidazolium ethyl sulfate ([EMIM][EtSO4]) in water. In order to understand the mechanisms by which the ionic liquids affect the activity of xylanase, we conducted molecular dynamics simulations of the enzyme in various concentrations of the cosolvent. The simulations show that higher concentrations of ionic liquid correlate with less deviation from the starting crystallographic structure. Dynamic motion of the protein is severely dampened by even the lowest tested concentrations of ionic liquid as measured by root-mean-square fluctuation. Principal component analysis shows that the characteristics of the main modes of enzyme motion are greatly affected by the choice of solvent. Cations become kinetically trapped in the binding pocket, allowing them to act as a competitive inhibitor to the natural substrate. Dynamic light scattering and kinetic studies evaluated the stability of the enzyme in the new solvents. These studies indicate that likely factors in the loss of enzyme activity for this xylanase are the dampening of dynamic motion and kinetic trapping of cations in the binding pocket as opposed to the denaturing of the protein.