Rhamnolipid enhanced beta-glucosidase from Paenibacillus sp. LLZ1 for in situ lignocellulose saccharification in ionic liquids

Low β-glucosidase activity and stability are challenging limitations for the in situ saccharification of lignocellulose pretreated by ionic liquids. The addition of 0.1% (v/v) rhamnolipid, an ionic biosurfactant, was found to enhance the activity of β-glucosidase from Paenibacillus LLZ1 by 39% through a synergistic effect with 5% [Emim]OAc. The concentration of [Emim]OAc was further optimized to 2.5% (v/v) obtaining as high as 48% PsBGL activity increase. Fluorescence and near-UV circular dichroism spectroscopy analyses revealed looser protein structure and more hydrophilic microenvironment in the system of rhamnolipid and [Emim]OAc. Though ILs showed deactivation effect on cellulases, rhamnolipid was speculated to alleviate protein denaturation significantly. The reaction system was further applied in the hydrolysis of cellobiose and bagasse cellulose. The conversion was increased by 33% for cellobiose and by 62% for bagasse cellulose, demonstrating the application potential of the reaction system. The improvement was attributed to both reduced cellobiose inhibition and effective substrate solubilizing, and the latter was monitored by scanning electron microscopy to show the morphological change of substrates directly. This study proved the effect of surfactant in in situ enzymatic saccharification of lignocellulose and provided a practical enzymatic hydrolysis system.