Abstract
Ionic liquids (ILs) are organic salts molten at room temperature that can be used for a wide variety of applications. Many ILs, such as 1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]), have been shown to remove a significant fraction of the complex biopolymer lignin from biomass during pretreatment. Valorizing lignin via biological pathways (e.g. enzymes) holds promise but is limited by the low biocompatibility of many ILs used for pretreatment. The discovery of thermostable enzymes and the application of enzyme engineering techniques have yielded biocatalysts capable of withstanding high concentrations of ILs. Converting lignin from a waste product to value-added chemicals is vital to the success of future cellulosic biorefineries. To that end, we screened the activity of the lignolytic enzyme laccase from a hyperthermophilic bacterium (Thermus thermophilus) in aqueous [C2C1Im][OAc]. Despite the thermophilicity (Topt > 90 oC) of this laccase, significant activity loss (>50%) was observed in only 2% (w/v) [C2C1Im][OAc]. Kinetics studies show that the IL can bind to the free enzyme and the enzyme-substrate complex. Docking simulations suggest that the cation favors binding to a region close to the active site. We then used a rational design strategy to improve the activity of the laccase in [C2C1Im][OAc]. A total of 8 single amino acid mutations were made; however, there were no significant improvements in the activity of the mutants in [C2C1Im][OAc] compared to the wild type. The results of this study shed light on the complex nature of enzyme-IL interactions and the challenges faced when designing a biological lignin valorization strategy.
Highlights
Lignin is a complex biopolymer that makes up ∼30% of the terrestrial plant biomass on earth (Weng and Chapple, 2010; Doherty et al, 2011)
To determine the biocompatibility of [C2C1Im][OAc], we screened the activity of T. thermophilus (TtL) in a range of Ionic liquids (ILs) concentrations in water
A 50% reduction of TtL initial velocity in buffer was seen in 2% [C2C1Im][OAc], in contrast with recent studies of thermophilic enzymes in [C2C1Im][OAc]
Summary
Lignin is a complex biopolymer that makes up ∼30% of the terrestrial plant biomass on earth (Weng and Chapple, 2010; Doherty et al, 2011). Several lignolytic pathways have been identified in biomassdegrading fungi and bacteria These include the heme peroxidases (versatile, lignin, and manganese), laccases, and the recently discovered NAD or glutathione-dependent enzymes from the soil bacterium Sphingobium SYK-6 (Shi et al, 2012; Munk et al, 2015; Varmana et al, 2016). The application of these enzymes in an industrial setting has been limited by several factors, including the high cost of enzyme production, the poor solubility of lignin in a biocompatible solvent, and low selectivity of lignin-derived monomers by the biocatalyst (Bugg et al, 2011; Brown and Chang, 2014). This study expands our understanding of the effects ILs can have on laccases and highlights some of the challenges faced when designing a biological lignin valorization strategy
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
