Activity and stability of hyperthermostable cellulases and xylanases in ionic liquids

The biochemical conversion route in the utilization of biomass has required intensive development of processing methods for reducing recalcitrance of cellulose to enzymatic hydrolysis. Since discovering hydrophilic ionic liquids (ILs) are efficient lignocellulose solvents, considerable efforts have been made to demonstrate their potential applications in cellulose hydrolysis. However, most of the commercial lignocellulose hydrolyzing enzymes are largely deactivated or inhibited in the presence of even low concentrations of ILs. The link found between enzyme thermostability and IL tolerance leads to the consideration that hyperthermostable enzymes are good candidates to be used together with ILs. Hyperthermostable cellulases and xylanases that are highly tolerant to aqueous solutions of ILs show resistance to competitive inhibition by IL cations and have lower amounts of structural motifs (loop and helix structures) that are prone to denaturation by IL anions. In addition, they have a net negative protein surface charge that contributes to repulsion of anions. This review provides recent developments in understanding the behaviour of hyperthermostable cellulases and xylanases in ILs by elucidating protein-IL interactions. It also outlines the current research demonstrating the potential of rational enzyme engineering to improve enzyme tolerance to ILs based on comprehension of IL-induced deactivation mechanisms.