A novel exo-cellulase from white spotted longhorn beetle (Anoplophora malasiaca)

Wood feeding insects depends heavily on the secretion of a combination of cellulases, mainly endoglucanases and other glucanases such as exoglucanases and xylanases, for efficient digestion of the cellulosic materials. To date, although a high number of endoglucanases have been found in xytophagous insects, little is known about exoglucanases encoded in the genome of these insects. Here we report the identification and isolation of an exoglucanase, designated as AmCel-5B, from the white spotted longhorn beetle, Anoplophora malasiaca. The optimal condition of enzymatic activity was found to be 50 °C and pH 4.0. Interestingly, this enzyme is not only exhibited exo-β-glucanase activity, but also with obvious endo-β-glucanase activity. Furthermore, this enzyme is unique in that, although it recognizes Avicel, evidenced as an exo-β-glucanase, it cannot recognize oligosaccharides smaller than cellohexaose. This may explain why longhorn beetle can well digest hard "living" wood, which contains primarily rigid long fibers. Although it is known that metal ions can enhance the activity of some cellulases, we further demonstrated that reducing agent could work synergistically with metal ions for significant activity enhancement of AmCel-5B. The discovery and investigation of an insect exoglucanase should lead to a greater understanding of the mechanism for efficient digestion of cellulosic materials by wood feeding insects, as well as facilitate their potential applications in the production of bioenergy and biomaterials from lignocellulosic biomass in the future.