(225g) Molecular Dynamics Studies of the Structure and Dynamics of An Endoglucanase Cel9A In Ionic Liquid Solutions

Molecular Dynamics Studies of the Structure
and Dynamics of an Endoglucanase Cel9A in Ionic Liquid Solutions

Hanbin Liu, Supratim Datta, Blake A Simmons and Ken Sale

Joint BioEnergy Institute, Emeryville, CA 94608 and Sandia
National laboratories, Livermore, CA

Ionic liquids (ILs) are currently under intense study as solvents
and as additives to macromolecules and have recently show great promise as a pretreatment
of lignocellulosic biomass for the production of biofuels. Pretreatment of
biomass is followed by saccharification using a cocktail of cellulase enzymes (glycoside
hydrolases) that catalyze the breaking of the b1-4
glycosidic bonds of the cellulose, which was liberated from the biomass by the
ionic liquid, to glucose. This process means that these cellulases must be
tolerant of the ionic liquid, making the study of the structures and dynamics
of cellulases in the present of ionic liquids of great current interests in
biofuels research, especially in industrially relevant processes in which
biomass pretreatment is consolidated with the saccharification step
(simultaneous pretreatment and saccharification). In this work, we investigated
the origins of the tolerance of a thermophilic family 9 cellulase (Cel9A) to
ionic liquids. Cel9A is an endoglucanase that also displays exoglucanase
activity and has been experimentally shown to be functional in bioprocessing relevant
concentrations of the ionic liquid 1-ethyl-2methyl imidazolium acetate [C2mim][OAc].
Using all atom molecular dynamics simulations of Cel9A over a wide range of [C2mim][OAc]
concentrations (weight range from 0% IL to 50% IL), we show that the cel9A
structure is stable in [C2mim][OAc] over the entire 80ns simulation at these IL
concentrations. We also show that, due to the slow diffusion of [C2mim][OAc] and
interactions between [C2mim][OAc] and Cel9A, [C2mim][OAc] stabilizes the
dynamics of Cel9A. During our simulations, [C2mim][OAc] diffused into the
active site of the enzyme and interacts directly with the two catalytic
residues, indicating that [C2mim][OAc] may also promote enzymatic activity by
altering either the dynamics of the overall active site, the dynamics of the
catalytic residues or the electrostatic environment of the active site; we are
currently investigating these possibilities.