Metagenomic cellulases highly tolerant towards the presence of ionic liquids--linking thermostability and halotolerance

Abstract

Cellulose is an important renewable resource for the production of bioethanol and other valuable compounds. Several ionic liquids (ILs) have been described to dissolve water-insoluble cellulose and/or wood. Therefore, ILs would provide a suitable reaction medium for the enzymatic hydrolysis of cellulose if cellulases were active and stable in the presence of high IL concentrations. For the discovery of novel bacterial enzymes with elevated stability in ILs, metagenomic libraries from three different hydrolytic communities (i.e. an enrichment culture inoculated with an extract of the shipworm Teredo navalis, a biogas plant sample and elephant faeces) were constructed and screened. Altogether, 14 cellulolytic clones were identified and subsequently assayed in the presence of six different ILs. The most promising enzymes, CelA2, CelA3 (both derived from the biogas plant) and CelA84 (derived from elephant faeces), showed high activities (up to 6.4 U/mg) in the presence of 30% (v/v) ILs. As these enzymes were moderately thermophilic and halotolerant, they retained 40% to 80% relative activity after 34 days in 4 M NaCl, and they were benchmarked with two thermostable enzymes, CelA from Thermotoga maritima and Cel5K from a metagenome library derived from Avachinsky crater in Kamchatka. These enzymes also exhibited high activity (up to 11.1 U/mg) in aqueous IL solutions (30% (v/v)). Some of the enzymes furthermore exhibited remarkable stability in 60% (v/v) IL. After 4 days, CelA3 and Cel5K retained up to 79% and 100% of their activity, respectively. Altogether, the obtained data suggest that IL tolerance appears to correlate with thermophilicity and halotolerance.